Metric Thread Size Chart: Dimensions, Pitch & Tap Drill Sizes

Contents

1 1. What Is a Metric Thread?
2 2. Metric Thread Terminology and Dimensions
3 3. Metric Coarse Thread Size Chart
4 4. Metric Fine Thread Size Chart
5 5. Metric Tap Drill Size Chart
6 6. Metric Thread Tolerance Classes
7 7. Metric Thread Dimensions and Calculation Formulas
8 8. Metric Thread Identification and Measurement
9 9. Metric Threads vs Inch Threads
10 10. Metric Thread Selection, Standards, and FAQ

Metric threads are among the most widely used fastening thread systems in industrial equipment, machinery, automobiles, piping components, construction products, and consumer goods. Standardized under the ISO metric screw thread system, they allow bolts, screws, nuts, and threaded components manufactured in different countries to fit together when their thread size, pitch, and tolerance class match.

A metric thread is normally identified by the letter M, followed by its nominal diameter in millimetres. For example, M10 represents a metric thread with a nominal major diameter of 10 mm. When the pitch is included, the designation may appear as M10 × 1.5, where 1.5 mm is the axial distance between adjacent thread crests. Additional information, such as 6H or 6g, may be added to define the tolerance class.

Although metric thread designations appear simple, selecting the correct thread requires more than checking the nominal diameter. Two threads can have the same major diameter but different pitches, making them incompatible. Coarse, fine, and extra-fine threads also have different mechanical characteristics and require different tap drill sizes.

A reliable metric thread size chart helps engineers, machinists, technicians, designers, and maintenance personnel quickly identify important dimensions, including:

Nominal and major diameter
Standard coarse or fine pitch
Pitch diameter
Minor diameter
Recommended tap drill size
Clearance-hole diameter
Common tolerance classes

This guide provides metric coarse and fine thread charts, tap drill recommendations, dimensional formulas, thread identification methods, tolerance information, and comparisons with inch-based thread systems. It can be used when designing threaded parts, selecting fasteners, preparing machining operations, or identifying an unknown metric thread during inspection and maintenance.

1. What Is a Metric Thread?

Metric Thread Size Chart: Dimensions, Pitch & Tap Drill Sizes

A metric thread is a standardized screw thread whose dimensions are expressed in millimetres. It is used on bolts, screws, nuts, threaded holes, fittings, machine components, and many other mechanical products. Metric threads are the dominant fastening thread system in most countries and are internationally standardized through the ISO metric screw thread series.

The basic profile of an ISO metric thread has a symmetrical V-shaped form with a 60-degree included angle. Both external threads, such as those on bolts, and internal threads, such as those inside nuts or tapped holes, use the same basic thread profile. However, their actual crest, root, and diameter dimensions are modified to provide clearance and allow the parts to assemble correctly.

Metric Thread Designation

A metric thread designation normally begins with the capital letter M, followed by its nominal diameter:

$\text{M10}$

In this designation:

M indicates an ISO metric thread.
10 represents a nominal major diameter of 10 mm.

When the thread pitch is specified, it is written after the nominal diameter:

$\text{M10} \times 1.5$

This means that the thread has:

A nominal diameter of 10 mm
A pitch of 1.5 mm
A 60-degree thread angle

For an external thread, the nominal diameter approximately corresponds to the outside diameter measured across the thread crests. For an internal thread, it represents the nominal size of the matching external thread rather than the directly measured hole diameter.

Coarse and Fine Metric Threads

Metric threads are available in coarse, fine, and extra-fine pitches. The nominal diameter alone is often sufficient when referring to a standard coarse thread because each diameter has a preferred coarse pitch.

For example:

M6 normally means M6 × 1.0
M8 normally means M8 × 1.25
M10 normally means M10 × 1.5
M12 normally means M12 × 1.75

The pitch should be stated when a fine or extra-fine thread is used:

M10 × 1.25 — fine thread
M10 × 1.0 — extra-fine thread
M12 × 1.5 — fine thread
M12 × 1.25 — extra-fine thread

Two threads with the same nominal diameter but different pitches are not compatible. An M10 × 1.5 bolt must not be forced into an M10 × 1.25 threaded hole, even though both have approximately the same outside diameter.

How to Read a Complete Metric Thread Designation

A complete metric thread designation can contain information about the diameter, pitch, tolerance class, thread direction, and number of starts.

Consider the following example:

$\text{M12} \times 1.75\text{-6g}$

The designation means:

Element	Meaning
M	ISO metric screw thread
12	Nominal diameter of 12 mm
1.75	Thread pitch of 1.75 mm
6	Tolerance grade
g	Fundamental deviation for an external thread

A typical internal thread may be designated:

$\text{M12} \times 1.75\text{-6H}$

Lowercase tolerance letters, such as g and h, are used for external threads. Uppercase letters, such as G and H, are used for internal threads.

A fitted thread pair may be written as:

$\text{M12} \times 1.75\text{-6H/6g}$

Here, 6H applies to the internal thread and 6g applies to the external thread.

Other possible designation details include:

LH for a left-hand thread
P to identify pitch in multi-start thread designations
A lead value for multi-start threads
A tolerance class for pitch and major or minor diameters

For example:

$\text{M20} \times 2.5\text{-LH}$

This identifies a left-hand M20 thread with a 2.5 mm pitch.

Metric Thread Standards

The ISO metric thread system is covered by several related standards rather than a single document. Important standards include:

ISO 68-1 — Basic profile for ISO general-purpose metric screw threads
ISO 261 — General-purpose metric screw thread plan
ISO 262 — Selected metric thread sizes for screws, bolts, and nuts
ISO 724 — Basic thread dimensions
ISO 965 — Tolerances and limits of size

These standards define the thread profile, preferred diameter-and-pitch combinations, basic dimensions, tolerance classes, and limits needed for interchangeable threaded components.

2. Metric Thread Terminology and Dimensions

Understanding metric threads requires more than knowing the nominal diameter and pitch. A thread is defined by several related diameters, angles, and axial dimensions that determine whether internal and external components will assemble and function correctly.

Major Diameter

The major diameter is the largest diameter of a straight thread.

For an external thread, it is measured across the thread crests and is commonly identified by the symbol

$d$ $d$ . For an internal thread, it is the largest diameter across the thread roots and is identified by

$D$ .

In an M12 thread, the basic major diameter is:

$d = D = 12.000\text{ mm}$

The actual external major diameter may be slightly smaller than 12 mm because of the applied tolerance class and manufacturing allowance.

Minor Diameter

The minor diameter is the smallest diameter of the thread.

For an external thread, it is measured across the thread roots and is represented by $d_3$

.
For an internal thread, it is measured across the thread crests and is represented by $D_1$

.

The minor diameter is important because it influences the thread’s cross-sectional area, tensile strength, tap drill selection, and resistance to stripping.

The internal minor diameter is close to the diameter produced by the tap drill before tapping, although the final dimension depends on the tap, material, thread percentage, and manufacturing process.

Pitch Diameter

The pitch diameter is an imaginary cylindrical diameter at which the width of the thread ridge equals the width of the adjacent thread groove. It is represented by:

$d_2$

for an external thread
$D_2$

for an internal thread

The pitch diameter is one of the most important dimensions controlling thread fit. Two components can have acceptable major and minor diameters but still fail to assemble correctly if their pitch diameters are outside the permitted tolerance limits.

Pitch diameter can be inspected using:

Thread micrometers
The three-wire measurement method
Thread plug gauges
Thread ring gauges
Optical or coordinate measuring equipment

Thread Pitch

The thread pitch, represented by

$P$ $P$ , is the axial distance between corresponding points on two adjacent threads. In the metric system, pitch is stated directly in millimetres.

For an M12 × 1.75 thread:

$P = 1.75\text{ mm}$

This means the axial distance from one thread crest to the next is 1.75 mm.

Metric pitch should not be confused with threads per inch, or TPI, used for many inch-based threads. The approximate relationship is:

$\text{TPI}=\frac{25.4}{P}$

For an M12 × 1.75 thread:

$\text{TPI}=\frac{25.4}{1.75}\approx 14.51$

This value is useful for comparison, but metric threads should always be specified using pitch in millimetres.

Lead

The lead is the axial distance a threaded component advances during one complete revolution.

For a single-start thread:

$L=P$

For a multi-start thread:

$L=nP$

where:

$L$

= lead
$n$

= number of thread starts
$P$

= pitch

Most standard metric fasteners use single-start threads, so their lead and pitch are equal. Multi-start threads are more commonly used in motion-control components where rapid axial travel is required.

Thread Angle

The ISO metric thread has a symmetrical included angle of:

$\alpha = 60^\circ$

Each flank is positioned at 30 degrees relative to a line perpendicular to the thread axis. The thread angle affects engagement, load transfer, friction, strength, and compatibility with mating parts.

A metric thread should not be assumed interchangeable with another thread system simply because both use a 60-degree angle. The diameter, pitch, thread profile, crest form, root form, and tolerance system must also match.

Crest, Root, and Flank

The main surfaces of a thread profile are:

Crest: The top surface joining the two flanks of a thread
Root: The bottom surface joining adjacent thread flanks
Flank: The angled surface connecting the crest and root

The basic profile is derived from a sharp V-shaped triangle. In manufactured threads, the crests and roots are truncated or rounded to prevent interference, reduce stress concentration, and provide the necessary clearance.

Fundamental Triangle Height

The theoretical height of the fundamental triangle is represented by

$H$ $H$ :

$H=\frac{\sqrt{3}}{2}P$

Therefore:

$H=0.866025P$

For an M12 × 1.75 thread:

$H=0.866025 \times 1.75=1.5155\text{ mm}$

The complete sharp-V height is not used as the actual thread depth because the crests and roots are truncated.

Basic Metric Thread Diameter Formulas

For the ISO basic profile, the commonly used basic dimensions are:

$d_2=D_2=d-0.649519P$

$D_1=d-1.082532P$

$d_3=d-1.226869P$

where:

$d$

= nominal major diameter
$d_2$

= basic external pitch diameter
$D_2$

= basic internal pitch diameter
$D_1$

= basic internal minor diameter
$d_3$

= basic external minor diameter
$P$

= thread pitch

These are basic profile dimensions, not the final maximum and minimum manufacturing limits. Actual acceptable dimensions must be determined from the applicable ISO tolerance class.

Example: Basic Dimensions of M12 × 1.75

For an M12 × 1.75 thread:

$d=12.000\text{ mm}$

$P=1.750\text{ mm}$

Basic pitch diameter:

$d_2=D_2=12-(0.649519 \times 1.75)$

$d_2=D_2\approx10.864\text{ mm}$

Basic internal minor diameter:

$D_1=12-(1.082532 \times 1.75)$

$D_1\approx10.106\text{ mm}$

Basic external minor diameter:

$d_3=12-(1.226869 \times 1.75)$

$d_3\approx9.853\text{ mm}$

The basic dimensions are therefore:

Dimension	Symbol	Basic value
Nominal major diameter	$d$ $d$ , $D$ $D$	12.000 mm
Pitch	$P$ $P$	1.750 mm
Pitch diameter	$d_2$ $d_{2}$ , $D_2$ $D_{2}$	10.864 mm
Internal minor diameter	$D_1$ $D_{1}$	10.106 mm
External minor diameter	$d_3$ $d_{3}$	9.853 mm
Thread angle	$\alpha$ $α$	60°

These calculated values describe the ISO basic profile. Production drawings and inspection procedures must also specify the relevant tolerance class, such as 6H for the internal thread or 6g for the external thread.

3. Metric Coarse Thread Size Chart

The metric coarse thread series is the preferred choice for most general-purpose fastening applications. For each nominal diameter, ISO standards assign a standard coarse pitch. For example, the standard coarse pitch for M8 is 1.25 mm, while the standard coarse pitch for M12 is 1.75 mm.

Because the coarse pitch is implied, a thread may be identified simply as M12. However, writing the full designation—M12 × 1.75—helps prevent confusion during manufacturing, purchasing, and inspection.

Advantages of Metric Coarse Threads

Compared with fine threads, metric coarse threads generally provide:

Faster assembly and disassembly
Better resistance to thread damage
Greater tolerance of dirt and surface contamination
Lower risk of cross-threading
Better performance in soft or brittle materials
Greater resistance to stripping when thread engagement is limited
Easier tapping and lower tap-breakage risk

Coarse threads are widely used for structural fasteners, machinery, equipment frames, automotive components, general maintenance, and threaded connections in cast materials.

Metric Coarse Thread Size Chart: M1 to M24

The table below lists the basic dimensions of commonly used ISO metric coarse threads. Dimensions are theoretical basic-profile values. Actual production dimensions depend on the specified tolerance class.

Thread size	Coarse pitch, P (mm)	Basic major diameter (mm)	Basic pitch diameter (mm)	Basic internal minor diameter (mm)	Approx. tap drill (mm)
M1 × 0.25	0.25	1.000	0.838	0.729	0.75
M1.2 × 0.25	0.25	1.200	1.038	0.929	0.95
M1.4 × 0.30	0.30	1.400	1.205	1.075	1.10
M1.6 × 0.35	0.35	1.600	1.373	1.221	1.25
M1.8 × 0.35	0.35	1.800	1.573	1.421	1.45
M2 × 0.40	0.40	2.000	1.740	1.567	1.60
M2.2 × 0.45	0.45	2.200	1.908	1.713	1.75
M2.5 × 0.45	0.45	2.500	2.208	2.013	2.05
M3 × 0.50	0.50	3.000	2.675	2.459	2.50
M3.5 × 0.60	0.60	3.500	3.110	2.851	2.90
M4 × 0.70	0.70	4.000	3.545	3.242	3.30
M4.5 × 0.75	0.75	4.500	4.013	3.688	3.75
M5 × 0.80	0.80	5.000	4.480	4.134	4.20
M5.5 × 0.90	0.90	5.500	4.915	4.526	4.60
M6 × 1.00	1.00	6.000	5.350	4.917	5.00
M7 × 1.00	1.00	7.000	6.350	5.917	6.00
M8 × 1.25	1.25	8.000	7.188	6.647	6.80
M9 × 1.25	1.25	9.000	8.188	7.647	7.80
M10 × 1.50	1.50	10.000	9.026	8.376	8.50
M11 × 1.50	1.50	11.000	10.026	9.376	9.50
M12 × 1.75	1.75	12.000	10.864	10.106	10.20
M14 × 2.00	2.00	14.000	12.701	11.835	12.00
M16 × 2.00	2.00	16.000	14.701	13.835	14.00
M18 × 2.50	2.50	18.000	16.376	15.294	15.50
M20 × 2.50	2.50	20.000	18.376	17.294	17.50
M22 × 2.50	2.50	22.000	20.376	19.294	19.50
M24 × 3.00	3.00	24.000	22.051	20.752	21.00

Metric Coarse Thread Size Chart: M27 to M100

Large metric threads are commonly found on heavy machinery, industrial equipment, structural assemblies, pressure equipment, large valves, and rotating equipment.

Thread size	Coarse pitch, P (mm)	Basic pitch diameter (mm)	Basic internal minor diameter (mm)	Approx. tap drill (mm)
M27 × 3.00	3.00	25.051	23.752	24.00
M30 × 3.50	3.50	27.727	26.211	26.50
M33 × 3.50	3.50	30.727	29.211	29.50
M36 × 4.00	4.00	33.402	31.670	32.00
M39 × 4.00	4.00	36.402	34.670	35.00
M42 × 4.50	4.50	39.077	37.129	37.50
M45 × 4.50	4.50	42.077	40.129	40.50
M48 × 5.00	5.00	44.752	42.587	43.00
M52 × 5.00	5.00	48.752	46.587	47.00
M56 × 5.50	5.50	52.428	50.046	50.50
M60 × 5.50	5.50	56.428	54.046	54.50
M64 × 6.00	6.00	60.103	57.505	58.00
M68 × 6.00	6.00	64.103	61.505	62.00
M72 × 6.00	6.00	68.103	65.505	66.00
M76 × 6.00	6.00	72.103	69.505	70.00
M80 × 6.00	6.00	76.103	73.505	74.00
M85 × 6.00	6.00	81.103	78.505	79.00
M90 × 6.00	6.00	86.103	83.505	84.00
M95 × 6.00	6.00	91.103	88.505	89.00
M100 × 6.00	6.00	96.103	93.505	94.00

The approximate tap drill values in these tables are primarily based on:

$\text{Tap drill diameter} \approx d-P$

where

$d$ is the nominal thread diameter and

$P$ $P$ is the pitch. The closest practical drill size is then selected.

This simplified method is suitable for preliminary selection and many ordinary cutting-tap applications. Final drill selection should also consider the required thread engagement, workpiece material, tap geometry, hole depth, plating allowance, and whether a cutting or forming tap will be used.

Example: M10 Coarse Thread

The standard coarse thread for M10 is:

$\text{M10} \times 1.5$

Its approximate tap drill diameter is:

$10-1.5=8.5\text{ mm}$

Therefore, an 8.5 mm drill is commonly used before cutting an M10 × 1.5 internal thread.

The basic pitch diameter is:

$d_2=10-(0.649519 \times 1.5)$

$d_2\approx9.026\text{ mm}$

The basic internal minor diameter is:

$D_1=10-(1.082532 \times 1.5)$

$D_1\approx8.376\text{ mm}$

The calculated basic minor diameter and recommended tap drill are not identical. The tap drill defines the initial hole, while the tap creates the final thread profile. Material behavior, drill accuracy and thread percentage affect the resulting internal minor diameter.

4. Metric Fine Thread Size Chart

Metric fine threads have a smaller pitch than the standard coarse thread assigned to the same nominal diameter. For example, the standard coarse M12 thread has a pitch of 1.75 mm, while common fine versions include M12 × 1.5 and M12 × 1.25.

The complete diameter and pitch must always be stated for a fine thread:

$\text{M12} \times 1.5$

Writing only M12 normally indicates the standard M12 × 1.75 coarse thread and may result in the wrong fastener, tap, die, or threaded component being selected.

Advantages of Metric Fine Threads

Fine threads can provide several advantages:

Greater tensile stress area for the same nominal diameter
Smaller axial movement per revolution
More precise adjustment
Better suitability for thin-walled components
Greater control of clamping force
Larger minor diameter and stronger bolt core
Lower tendency to loosen under some vibration conditions

Fine threads are commonly used in automotive assemblies, aerospace equipment, precision instruments, hydraulic components, adjustment mechanisms, bearing locknuts, and applications with limited thread depth.

However, fine threads are more sensitive to dirt, galling, cross-threading, impact damage, and manufacturing errors. They may also strip more easily in soft materials unless sufficient engagement is provided.

Common Metric Fine Thread Size Chart: M2 to M20

Thread size	Pitch (mm)	Basic pitch diameter (mm)	Basic internal minor diameter (mm)	Approx. tap drill (mm)
M2 × 0.25	0.25	1.838	1.729	1.75
M2.5 × 0.35	0.35	2.273	2.121	2.15
M3 × 0.35	0.35	2.773	2.621	2.65
M3.5 × 0.35	0.35	3.273	3.121	3.15
M4 × 0.50	0.50	3.675	3.459	3.50
M4.5 × 0.50	0.50	4.175	3.959	4.00
M5 × 0.50	0.50	4.675	4.459	4.50
M6 × 0.75	0.75	5.513	5.188	5.25
M7 × 0.75	0.75	6.513	6.188	6.25
M8 × 1.00	1.00	7.350	6.917	7.00
M8 × 0.75	0.75	7.513	7.188	7.25
M9 × 1.00	1.00	8.350	7.917	8.00
M9 × 0.75	0.75	8.513	8.188	8.25
M10 × 1.25	1.25	9.188	8.647	8.80
M10 × 1.00	1.00	9.350	8.917	9.00
M10 × 0.75	0.75	9.513	9.188	9.25
M11 × 1.00	1.00	10.350	9.917	10.00
M12 × 1.50	1.50	11.026	10.376	10.50
M12 × 1.25	1.25	11.188	10.647	10.80
M12 × 1.00	1.00	11.350	10.917	11.00
M14 × 1.50	1.50	13.026	12.376	12.50
M14 × 1.25	1.25	13.188	12.647	12.80
M14 × 1.00	1.00	13.350	12.917	13.00
M16 × 1.50	1.50	15.026	14.376	14.50
M16 × 1.00	1.00	15.350	14.917	15.00
M18 × 2.00	2.00	16.701	15.835	16.00
M18 × 1.50	1.50	17.026	16.376	16.50
M18 × 1.00	1.00	17.350	16.917	17.00
M20 × 2.00	2.00	18.701	17.835	18.00
M20 × 1.50	1.50	19.026	18.376	18.50
M20 × 1.00	1.00	19.350	18.917	19.00

Common Metric Fine Thread Size Chart: M22 to M100

Thread size	Common fine pitches (mm)	Typical tap drills (mm)
M22	2.0, 1.5, 1.0	20.0, 20.5, 21.0
M24	2.0, 1.5, 1.0	22.0, 22.5, 23.0
M27	2.0, 1.5, 1.0	25.0, 25.5, 26.0
M30	3.0, 2.0, 1.5, 1.0	27.0, 28.0, 28.5, 29.0
M33	3.0, 2.0, 1.5	30.0, 31.0, 31.5
M36	3.0, 2.0, 1.5	33.0, 34.0, 34.5
M39	3.0, 2.0, 1.5	36.0, 37.0, 37.5
M42	4.0, 3.0, 2.0, 1.5	38.0, 39.0, 40.0, 40.5
M45	4.0, 3.0, 2.0, 1.5	41.0, 42.0, 43.0, 43.5
M48	4.0, 3.0, 2.0, 1.5	44.0, 45.0, 46.0, 46.5
M52	4.0, 3.0, 2.0, 1.5	48.0, 49.0, 50.0, 50.5
M56	4.0, 3.0, 2.0, 1.5	52.0, 53.0, 54.0, 54.5
M60	4.0, 3.0, 2.0, 1.5	56.0, 57.0, 58.0, 58.5
M64	4.0, 3.0, 2.0	60.0, 61.0, 62.0
M68	4.0, 3.0, 2.0	64.0, 65.0, 66.0
M72	4.0, 3.0, 2.0	68.0, 69.0, 70.0
M76	4.0, 3.0, 2.0	72.0, 73.0, 74.0
M80	4.0, 3.0, 2.0	76.0, 77.0, 78.0
M85	4.0, 3.0, 2.0	81.0, 82.0, 83.0
M90	4.0, 3.0, 2.0	86.0, 87.0, 88.0
M95	4.0, 3.0, 2.0	91.0, 92.0, 93.0
M100	4.0, 3.0, 2.0	96.0, 97.0, 98.0

Not every diameter-and-pitch combination is equally preferred or readily available. The selected combination should be confirmed against the applicable ISO standard, product specification, and supplier availability.

Coarse and Fine Thread Comparison

Nominal size	Coarse thread	Fine thread	Extra-fine thread
M6	M6 × 1.0	M6 × 0.75	M6 × 0.5
M8	M8 × 1.25	M8 × 1.0	M8 × 0.75
M10	M10 × 1.5	M10 × 1.25	M10 × 1.0
M12	M12 × 1.75	M12 × 1.5	M12 × 1.25
M14	M14 × 2.0	M14 × 1.5	M14 × 1.25
M16	M16 × 2.0	M16 × 1.5	M16 × 1.0
M18	M18 × 2.5	M18 × 2.0	M18 × 1.5
M20	M20 × 2.5	M20 × 2.0	M20 × 1.5
M24	M24 × 3.0	M24 × 2.0	M24 × 1.5
M30	M30 × 3.5	M30 × 3.0	M30 × 2.0

Example: M12 Coarse vs Fine Threads

Three common M12 thread options are:

Designation	Pitch	Approx. tap drill	Threads per 25.4 mm
M12 × 1.75	1.75 mm	10.2 mm	14.51
M12 × 1.5	1.50 mm	10.5 mm	16.93
M12 × 1.25	1.25 mm	10.8 mm	20.32

All three have the same nominal major diameter, but they have different pitches, minor diameters, tap drill requirements, and thread geometry. They are therefore not interchangeable.

A thread should always be verified by checking both its nominal diameter and pitch. Selecting a component based only on the measured outside diameter is one of the most common causes of thread mismatch and assembly damage.

5. Metric Tap Drill Size Chart

Metric Tap Drill Size Chart

A tap drill is used to create the initial hole before cutting an internal thread. Selecting the correct drill diameter is essential because it determines how much material remains for the tap to form the thread.

A hole that is too small creates excessive thread engagement and increases tapping torque. This can cause tap breakage, poor surface finish, dimensional errors, and galling. A hole that is too large reduces thread engagement and may weaken the connection or increase the risk of thread stripping.

Basic Metric Tap Drill Formula

A convenient rule for estimating the tap drill diameter for an ISO metric thread is:

$D_{\text{drill}}\approx d-P$

where:

$D_{\text{drill}}$

= approximate tap drill diameter
$d$

= nominal thread diameter
$P$

= thread pitch

For an M10 × 1.5 thread:

$D_{\text{drill}}\approx10-1.5=8.5\text{ mm}$

Therefore, an 8.5 mm drill is commonly selected before tapping an M10 × 1.5 internal thread.

This rule provides a practical starting point for many general-purpose cutting-tap operations. It does not calculate the exact theoretical minor diameter or account for all manufacturing conditions.

Metric Coarse Thread Tap Drill Size Chart

Thread size	Pitch (mm)	Approx. calculated drill (mm)	Common tap drill (mm)
M1 × 0.25	0.25	0.75	0.75
M1.2 × 0.25	0.25	0.95	0.95
M1.4 × 0.30	0.30	1.10	1.10
M1.6 × 0.35	0.35	1.25	1.25
M1.8 × 0.35	0.35	1.45	1.45
M2 × 0.40	0.40	1.60	1.60
M2.2 × 0.45	0.45	1.75	1.75
M2.5 × 0.45	0.45	2.05	2.05
M3 × 0.50	0.50	2.50	2.50
M3.5 × 0.60	0.60	2.90	2.90
M4 × 0.70	0.70	3.30	3.30
M4.5 × 0.75	0.75	3.75	3.75
M5 × 0.80	0.80	4.20	4.20
M6 × 1.00	1.00	5.00	5.00
M7 × 1.00	1.00	6.00	6.00
M8 × 1.25	1.25	6.75	6.80
M9 × 1.25	1.25	7.75	7.80
M10 × 1.50	1.50	8.50	8.50
M11 × 1.50	1.50	9.50	9.50
M12 × 1.75	1.75	10.25	10.20
M14 × 2.00	2.00	12.00	12.00
M16 × 2.00	2.00	14.00	14.00
M18 × 2.50	2.50	15.50	15.50
M20 × 2.50	2.50	17.50	17.50
M22 × 2.50	2.50	19.50	19.50
M24 × 3.00	3.00	21.00	21.00
M27 × 3.00	3.00	24.00	24.00
M30 × 3.50	3.50	26.50	26.50
M33 × 3.50	3.50	29.50	29.50
M36 × 4.00	4.00	32.00	32.00
M39 × 4.00	4.00	35.00	35.00
M42 × 4.50	4.50	37.50	37.50
M45 × 4.50	4.50	40.50	40.50
M48 × 5.00	5.00	43.00	43.00
M52 × 5.00	5.00	47.00	47.00
M56 × 5.50	5.50	50.50	50.50
M60 × 5.50	5.50	54.50	54.50
M64 × 6.00	6.00	58.00	58.00

Metric Fine Thread Tap Drill Size Chart

Thread size	Pitch (mm)	Approx. calculated drill (mm)	Common tap drill (mm)
M3 × 0.35	0.35	2.65	2.65
M4 × 0.50	0.50	3.50	3.50
M5 × 0.50	0.50	4.50	4.50
M6 × 0.75	0.75	5.25	5.25
M6 × 0.50	0.50	5.50	5.50
M8 × 1.00	1.00	7.00	7.00
M8 × 0.75	0.75	7.25	7.25
M10 × 1.25	1.25	8.75	8.80
M10 × 1.00	1.00	9.00	9.00
M10 × 0.75	0.75	9.25	9.25
M12 × 1.50	1.50	10.50	10.50
M12 × 1.25	1.25	10.75	10.80
M12 × 1.00	1.00	11.00	11.00
M14 × 1.50	1.50	12.50	12.50
M14 × 1.25	1.25	12.75	12.80
M14 × 1.00	1.00	13.00	13.00
M16 × 1.50	1.50	14.50	14.50
M16 × 1.00	1.00	15.00	15.00
M18 × 2.00	2.00	16.00	16.00
M18 × 1.50	1.50	16.50	16.50
M18 × 1.00	1.00	17.00	17.00
M20 × 2.00	2.00	18.00	18.00
M20 × 1.50	1.50	18.50	18.50
M20 × 1.00	1.00	19.00	19.00
M22 × 2.00	2.00	20.00	20.00
M22 × 1.50	1.50	20.50	20.50
M24 × 2.00	2.00	22.00	22.00
M24 × 1.50	1.50	22.50	22.50
M27 × 2.00	2.00	25.00	25.00
M27 × 1.50	1.50	25.50	25.50
M30 × 3.00	3.00	27.00	27.00
M30 × 2.00	2.00	28.00	28.00
M30 × 1.50	1.50	28.50	28.50
M36 × 3.00	3.00	33.00	33.00
M36 × 2.00	2.00	34.00	34.00
M42 × 3.00	3.00	39.00	39.00
M42 × 2.00	2.00	40.00	40.00
M48 × 3.00	3.00	45.00	45.00
M48 × 2.00	2.00	46.00	46.00

Cutting Taps and Forming Taps

The correct drill size also depends on how the internal thread will be produced.

Cutting taps

A cutting tap removes material to create the thread flanks. Chips are produced and must be controlled or evacuated from the hole.

Common cutting tap types include:

Straight-flute taps
Spiral-point taps
Spiral-flute taps
Pipe and special-form taps

The nominal-diameter-minus-pitch rule is primarily used as a preliminary drill-size estimate for cutting taps.

Forming taps

A forming tap, also called a roll tap, creates the internal thread by plastically displacing material instead of cutting it. No chips are produced, but the process requires a larger starting hole because material flows inward to form the thread crests.

For this reason, a cutting-tap drill chart should not be used for a forming tap. The correct hole diameter must be obtained from the tap manufacturer’s recommendations for the specific:

Thread size and tolerance
Workpiece material
Required thread percentage
Lubrication system
Machine and tapping speed

Forming taps are generally suitable for ductile materials such as aluminum, copper alloys, low-carbon steel, and some stainless steels. They are usually unsuitable for brittle materials that cannot plastically deform without cracking.

Thread Engagement Percentage

Thread engagement percentage describes how closely the manufactured internal thread approaches the theoretical full thread height. A higher percentage does not necessarily produce a proportionally stronger connection.

Increasing thread engagement:

Reduces the initial hole diameter
Increases tapping torque
Increases tool wear
Raises the risk of tap breakage
Makes chip evacuation more difficult

A moderate thread engagement is often sufficient for general applications, but the required value depends on the material, design load, thread length, and applicable engineering specification.

The drill diameter should therefore be selected using manufacturer data or a validated machining standard when working with:

High-strength or safety-critical fasteners
Difficult-to-machine stainless steels
Nickel-based alloys
Titanium
Very small threads
Deep blind holes
Thin-walled components
Plated or coated threads

Tap Drill Selection Example

Suppose an M12 × 1.75 internal thread is required in a steel component.

Using the simplified formula:

$D_{\text{drill}}=12-1.75=10.25\text{ mm}$

A commonly available 10.2 mm drill may be selected for general tapping. However, the final choice should be checked against the tap supplier’s recommendations. Depending on the required thread engagement and material, a nearby size such as 10.2, 10.3, or another specified drill may be appropriate.

After drilling and tapping, the internal thread should be verified using a suitable GO/NO-GO thread plug gauge, especially when the thread must meet a specified ISO tolerance class.

6. Metric Thread Tolerance Classes

A thread cannot be manufactured to one exact theoretical dimension. Every machining process produces dimensional variation, so acceptable upper and lower limits must be specified.

The ISO metric tolerance system controls this variation using a combination of:

A tolerance grade, represented by a number
A tolerance position, represented by a letter

A typical external metric thread is designated:

$\text{M12} \times 1.75\text{-6g}$

A typical internal thread is designated:

$\text{M12} \times 1.75\text{-6H}$

The number and letter together form the thread tolerance class.

Tolerance Grade

The tolerance grade controls the width of the tolerance zone. It indicates how much dimensional variation is permitted.

Common metric thread grades include:

Grade 4 — relatively narrow tolerance
Grade 5 — narrow tolerance
Grade 6 — medium, general-purpose tolerance
Grade 7 — wider tolerance
Grade 8 — relatively wide tolerance

A smaller grade number generally indicates a narrower tolerance zone. However, the permissible dimensional deviation must be obtained from the applicable ISO tolerance tables because it also depends on the nominal diameter, pitch, and length of thread engagement.

Grade 6 is widely used for ordinary commercial fasteners and general engineering components.

Tolerance Position

The tolerance position determines where the tolerance zone lies relative to the basic thread profile. It is expressed by a letter:

Uppercase letters apply to internal threads.
Lowercase letters apply to external threads.

Common internal-thread positions include:

Common external-thread positions include:

The H and h positions have zero fundamental deviation at the basic profile boundary. The g position provides allowance between the external and internal threads, helping ensure assembly and accommodating common manufacturing conditions.

Common Metric Thread Tolerance Classes

Tolerance class	Thread type	Typical use
4H	Internal	Close-tolerance internal threads
5H	Internal	Accurate internal threads
6H	Internal	General-purpose tapped holes and nuts
7H	Internal	Larger manufacturing variation or special conditions
5g6g	External	Relatively accurate external threads with allowance
6g	External	General-purpose bolts and screws
6h	External	External threads with zero fundamental deviation
8g	External	Coarser manufacturing tolerance or selected special applications

The appropriate class must be selected from the relevant design standard. A tighter class should not be chosen automatically because it increases machining and inspection costs and can make assembly more sensitive to surface damage and contamination.

Understanding 6H and 6g

The combination 6H/6g is one of the most common general-purpose metric thread fits.

In this combination:

6H applies to the internal thread.
6g applies to the external thread.
6 indicates a medium tolerance grade.
H defines the internal-thread tolerance position.
g defines the external-thread tolerance position.

A complete designation may be written as:

$\text{M12} \times 1.75\text{-6H/6g}$

This specifies an M12 thread with a 1.75 mm pitch, a 6H internal tolerance, and a 6g external tolerance.

On individual component drawings, only the applicable tolerance is normally stated:

Nut or tapped hole: M12 × 1.75–6H
Bolt or external thread: M12 × 1.75–6g

Separate Pitch- and Major-Diameter Tolerances

Some external thread designations contain two tolerance classes, such as:

$\text{M12} \times 1.75\text{-5g6g}$

For an external thread:

The first class applies to the pitch diameter.
The second class applies to the major diameter.

Therefore, 5g6g means:

Pitch diameter: 5g
Major diameter: 6g

If only one class is stated, such as 6g, it applies according to the ISO designation rules to the controlled external-thread diameters.

An internal thread may also use separate tolerance classes. In that case, the classes apply to the pitch diameter and minor diameter in the order defined by the standard.

Short, Normal, and Long Engagement Lengths

ISO metric thread tolerances also depend on the length of thread engagement. Engagement lengths are grouped into three general categories:

S — short engagement
N — normal engagement
L — long engagement

Normal engagement is generally assumed when no special engagement category is identified. The limits associated with a tolerance grade may change with the engagement-length group because longer threads are more difficult to manufacture and assemble without accumulated lead and form errors.

Thread engagement length should not be confused with the number of fully formed threads or the depth of a drilled hole. Blind holes require additional depth for the drill point, tap chamfer, and chip clearance.

Allowance and Clearance

Allowance is the intentional difference between mating thread dimensions at maximum material condition. It provides space between the internal and external threads.

Adequate clearance may be necessary to accommodate:

Manufacturing variation
Surface coatings
Plating thickness
Oxide layers
Thermal expansion
Contamination
Assembly conditions
Minor thread damage

If both mating threads are produced too close to the basic profile without accounting for coating or process variation, interference may occur even though their nominal diameter and pitch match.

Effect of Coatings and Plating

Coatings applied to a thread change its effective dimensions. On an external thread, plating increases the material thickness on the flanks and can increase the functional pitch diameter. On an internal thread, coating reduces the available space.

This effect is especially important for:

Zinc plating
Nickel plating
Chromium plating
Galvanizing
Anodizing
Paint and polymer coatings
Dry-film lubricants

The required allowance depends on coating thickness and thread geometry. The drawing or purchasing specification should clearly state whether the thread tolerance applies before or after coating.

Hot-dip galvanized threads often require specially coordinated limits because the coating is much thicker than ordinary electroplating.

Selecting a Metric Thread Tolerance Class

The following factors should be considered when selecting a tolerance class:

Required ease of assembly
Expected production process
Thread size and pitch
Engagement length
Workpiece material
Operating temperature
Coating or plating thickness
Dirt and corrosion exposure
Required positional accuracy
Inspection capability
Cost of manufacturing

For ordinary machinery and commercial fasteners, 6H internal threads with 6g external threads are widely used. More precise classes may be necessary for adjustment mechanisms, gauges, precision equipment, or tightly controlled assemblies.

Wider tolerances may be appropriate where ease of assembly, large component size, coating thickness, or difficult manufacturing conditions are more important than precise positioning.

Metric Thread Inspection

Thread tolerances are commonly verified with limit gauges.

For an internal thread:

The GO plug gauge must enter to the required depth.
The NO-GO plug gauge must not enter beyond the limit permitted by the inspection standard.

For an external thread:

The GO ring gauge checks the maximum-material functional condition.
The NO-GO ring or snap gauge checks the opposing limit.

Other inspection methods include:

Thread micrometers
Three-wire measurements
Optical comparators
Coordinate measuring machines
Toolmaker’s microscopes
Electronic thread-measuring systems

A standard caliper can help identify the approximate major diameter, but it cannot fully verify pitch diameter, thread form, lead, flank angle, or compliance with a tolerance class.

Tolerance Designation Examples

Designation	Meaning
M8 × 1.25–6H	M8 coarse internal thread, tolerance class 6H
M8 × 1.25–6g	M8 coarse external thread, tolerance class 6g
M10 × 1–6H	M10 fine internal thread with 1 mm pitch
M10 × 1–6g	M10 fine external thread with 1 mm pitch
M12 × 1.75–6H/6g	Mating internal and external general-purpose threads
M16 × 1.5–5H	Fine internal thread with a relatively narrow tolerance
M20 × 2–5g6g	Fine external thread with separate pitch- and major-diameter classes

Thread tolerance selection should ultimately be based on the applicable edition of ISO 965, the component standard, and the engineering requirements of the assembly.

7. Metric Thread Dimensions and Calculation Formulas

ISO metric threads use a symmetrical 60-degree profile defined from a theoretical sharp V-shaped triangle. The fundamental profile provides the basis for calculating the major diameter, pitch diameter, minor diameter, thread height, and other important dimensions.

These formulas calculate basic dimensions only. They do not include tolerance allowances, manufacturing deviations, plating thickness, crest rounding, or other modifications. Final production limits must be obtained from the applicable ISO tolerance tables.

Metric Thread Symbols

The ISO system uses capital letters for internal-thread dimensions and lowercase letters for external-thread dimensions.

Symbol	Description
$P$ $P$	Thread pitch
$d$ $d$	Basic major diameter of external thread
$D$ $D$	Basic major diameter of internal thread
$d_2$ $d_{2}$	Basic pitch diameter of external thread
$D_2$ $D_{2}$	Basic pitch diameter of internal thread
$d_3$ $d_{3}$	Basic minor diameter of external thread
$D_1$ $D_{1}$	Basic minor diameter of internal thread
$H$ $H$	Height of the fundamental triangle
$L$ $L$	Thread lead
$n$ $n$	Number of thread starts
$\alpha$ $α$	Included thread angle

For the basic ISO metric thread:

$d=D$

and:

$d_2=D_2$

The actual internal and external thread limits differ after the relevant tolerance classes are applied.

Fundamental Triangle Height

The fundamental triangle is the theoretical sharp V-profile from which the ISO metric thread form is developed.

Its height is:

$H=\frac{\sqrt{3}}{2}P$

or:

$H=0.866025P$

For a thread with a pitch of 1.5 mm:

$H=0.866025\times1.5$

$H=1.299\text{ mm}$

The actual thread depth is less than

$H$ $H$ because the crests and roots are truncated or rounded.

Basic Pitch Diameter

The basic pitch diameter of an external or internal thread is calculated using:

$d_2=D_2=d-0.649519P$

The coefficient

$0.649519$ $0.649519$ is equivalent to:

$\frac{3}{4}H$

Therefore:

$d_2=D_2=d-\frac{3}{4}H$

The pitch diameter is the imaginary diameter at which the thread-ridge width equals the thread-groove width. It is one of the most important dimensions controlling thread fit and assembly.

Basic Internal Minor Diameter

The basic minor diameter of an internal thread is:

$D_1=d-1.082532P$

This can also be written as:

$D_1=d-\frac{5}{4}H$

The internal minor diameter is measured across the thread crests inside a nut or tapped hole. It is related to, but not necessarily identical to, the initial tap drill diameter.

Basic External Minor Diameter

For the ISO basic profile, the basic external minor diameter is commonly calculated as:

$d_3=d-1.226869P$

This basic value represents the diameter across the external thread roots. Actual external-thread root dimensions depend on the applied tolerance, manufacturing method, and root form.

Basic Thread Depth

The radial depth of the basic external thread profile can be estimated as:

$h_3=\frac{d-d_3}{2}$

Substituting the external minor-diameter formula:

$h_3=0.613435P$

For internal threads, the radial distance from the basic major diameter to the basic minor diameter is:

$h_1=\frac{D-D_1}{2}$

Therefore:

$h_1=0.541266P$

These values differ because the internal and external crest and root truncations are not identical.

Pitch, Lead, and Number of Starts

The pitch is the axial distance between corresponding points on adjacent threads. The lead is the axial distance traveled during one complete revolution.

For a single-start thread:

$L=P$

For a multi-start thread:

$L=nP$

where

$n$ is the number of starts.

For example, a double-start thread with a pitch of 2 mm has a lead of:

$L=2\times2=4\text{ mm}$

The threaded component therefore advances 4 mm per complete revolution.

Standard metric bolts and screws normally use single-start threads. Multi-start threads are more common in lead screws, adjustment devices, actuators, closures, and mechanisms requiring faster linear movement.

Threads per Unit Length

Metric threads are specified by pitch rather than threads per inch. However, the approximate TPI value can be calculated for comparison:

$\text{TPI}=\frac{25.4}{P}$

For an M10 × 1.5 thread:

$\text{TPI}=\frac{25.4}{1.5}$

$\text{TPI}\approx16.93$

This does not make M10 × 1.5 interchangeable with a 17-TPI inch thread. The nominal diameter, exact pitch, thread form, and tolerance system would still differ.

Approximate Tap Drill Diameter

For a general-purpose cutting tap, the approximate tap drill diameter can be calculated using:

$D_{\text{drill}}\approx d-P$

For M16 × 2:

$D_{\text{drill}}\approx16-2=14\text{ mm}$

This formula is intended for preliminary selection. The actual drill size should be confirmed using tap-manufacturer data, particularly for forming taps, difficult materials, precision threads, and safety-critical components.

Approximate Bolt Tensile Stress Area

The tensile stress area represents the effective cross-sectional area used when estimating the tensile capacity of a threaded fastener. For an ISO metric thread, it may be approximated by:

$A_s=\frac{\pi}{4}\left(d-0.9382P\right)^2$

where:

$A_s$

= tensile stress area in square millimetres
$d$

= nominal diameter in millimetres
$P$

= thread pitch in millimetres

For M12 × 1.75:

$A_s=\frac{\pi}{4}\left(12-0.9382\times1.75\right)^2$

$A_s\approx84.3\text{ mm}^2$

This value is commonly used in bolt load, proof load, and tensile strength calculations. The applicable fastener standard should be consulted for official tabulated stress areas and mechanical-property requirements.

Worked Example: M12 × 1.75

Consider a standard coarse M12 thread:

$d=12.000\text{ mm}$

$P=1.750\text{ mm}$

Step 1: Fundamental triangle height

$H=0.866025\times1.75$

$H=1.5155\text{ mm}$

Step 2: Basic pitch diameter

$d_2=D_2=12-(0.649519\times1.75)$

$d_2=D_2=10.8633\text{ mm}$

Rounded to three decimal places:

$d_2=D_2\approx10.863\text{ mm}$

Step 3: Basic internal minor diameter

$D_1=12-(1.082532\times1.75)$

$D_1=10.1056\text{ mm}$

$D_1\approx10.106\text{ mm}$

Step 4: Basic external minor diameter

$d_3=12-(1.226869\times1.75)$

$d_3=9.85298\text{ mm}$

$d_3\approx9.853\text{ mm}$

Step 5: Approximate tap drill

$D_{\text{drill}}\approx12-1.75$

$D_{\text{drill}}\approx10.25\text{ mm}$

A commonly selected standard drill is approximately 10.2 mm, although the final size depends on the required thread percentage and tapping conditions.

Step 6: Tensile stress area

$A_s=\frac{\pi}{4}\left(12-0.9382\times1.75\right)^2$

$A_s\approx84.3\text{ mm}^2$

M12 × 1.75 Basic-Dimension Summary

Parameter	Symbol	Calculated value
Nominal major diameter	$d$ $d$ , $D$ $D$	12.000 mm
Pitch	$P$ $P$	1.750 mm
Thread angle	$\alpha$ $α$	60°
Fundamental triangle height	$H$ $H$	1.516 mm
Basic pitch diameter	$d_2$ $d_{2}$ , $D_2$ $D_{2}$	10.863 mm
Basic internal minor diameter	$D_1$ $D_{1}$	10.106 mm
Basic external minor diameter	$d_3$ $d_{3}$	9.853 mm
Approximate tap drill	$D_{\text{drill}}$ $D_{drill}$	10.2–10.25 mm
Tensile stress area	$A_s$ $A_{s}$	84.3 mm²

These dimensions describe the theoretical basic profile. They must not be used as final manufacturing limits without applying the specified thread tolerance class.

8. Metric Thread Identification and Measurement

Correct thread identification requires both the nominal diameter and pitch to be determined. Measuring only the outside diameter is insufficient because a single nominal diameter can be produced with several coarse, fine, and extra-fine pitches.

For example, all of the following are M12 threads:

M12 × 1.75
M12 × 1.5
M12 × 1.25
M12 × 1.0

They have approximately the same major diameter but are not interchangeable.

Tools for Identifying Metric Threads

Common inspection and identification tools include:

Vernier, dial, or digital caliper
Metric thread-pitch gauge
Thread micrometer
Micrometer and thread wires
GO/NO-GO plug gauges
GO/NO-GO ring gauges
Optical comparator
Coordinate measuring machine
Known reference nut or bolt
Metric thread size chart

A caliper and pitch gauge are normally sufficient for preliminary field identification. Precision inspection requires calibrated thread-measuring equipment or limit gauges.

Step 1: Determine Whether the Thread Is External or Internal

An external thread is found on a:

Bolt
Screw
Stud
Shaft
Male fitting

An internal thread is found inside a:

Nut
Tapped hole
Female fitting
Threaded housing

This distinction determines which diameter can be measured directly and which inspection tools should be used.

Step 2: Measure the Major Diameter

For an external thread, use calipers to measure across the thread crests. Hold the caliper perpendicular to the thread axis and avoid applying excessive pressure.

The measured diameter may be slightly smaller than the nominal size because of:

External-thread tolerance
Plating allowance
Wear
Crest truncation
Manufacturing variation
Surface damage

An external thread measuring approximately 9.8 to 10.0 mm may be an M10 thread, but its pitch must still be checked.

For an internal thread, direct measurement of the major diameter is more difficult because it lies near the roots. Internal thread identification usually relies on the measured opening, pitch gauge, known mating parts, or plug gauges.

Step 3: Measure the Thread Pitch

A metric thread-pitch gauge contains blades marked with pitches such as:

0.5 mm
0.7 mm
0.8 mm
1.0 mm
1.25 mm
1.5 mm
1.75 mm
2.0 mm

Place a gauge blade against the thread and observe whether its teeth fully match the thread grooves. The correct blade should fit without visible gaps or rocking.

For example, if a thread measures approximately 12 mm in diameter and the 1.75 mm gauge matches, it is likely:

$\text{M12} \times 1.75$

If the 1.5 mm gauge matches instead, it is likely:

$\text{M12} \times 1.5$

Measuring Pitch Without a Pitch Gauge

If a pitch gauge is unavailable, measure the axial distance across several thread intervals and divide by the number of intervals.

For example, suppose the distance from the first crest to the eleventh crest is 15 mm. There are ten pitch intervals between eleven crests:

$P=\frac{15}{10}$

$P=1.5\text{ mm}$

Measuring across multiple intervals reduces the effect of caliper resolution and positioning error.

A common mistake is to count crests instead of intervals. Eleven crests contain ten pitch intervals, not eleven.

Step 4: Compare the Results with a Metric Thread Chart

After measuring the major diameter and pitch, compare both values with an ISO metric thread chart.

Approx. measured diameter	Measured pitch	Likely designation
5 mm	0.8 mm	M5 × 0.8
6 mm	1.0 mm	M6 × 1.0
8 mm	1.25 mm	M8 × 1.25
8 mm	1.0 mm	M8 × 1.0
10 mm	1.5 mm	M10 × 1.5
10 mm	1.25 mm	M10 × 1.25
12 mm	1.75 mm	M12 × 1.75
12 mm	1.5 mm	M12 × 1.5
16 mm	2.0 mm	M16 × 2.0
20 mm	2.5 mm	M20 × 2.5

These measurements identify the nominal size and pitch but do not establish the tolerance class.

Step 5: Check the Thread Direction

Most metric threads are right-hand threads. A right-hand thread tightens when turned clockwise when viewed from the end of the fastener.

A left-hand thread tightens counterclockwise and is normally marked with LH:

$\text{M16} \times 2\text{-LH}$

Left-hand threads are used where normal rotation might loosen a right-hand connection or where the design requires opposite movement.

They may be found in:

Rotating shafts
Turnbuckles
Bicycle components
Gas fittings
Adjustment mechanisms
Paired right- and left-hand assemblies

Step 6: Inspect the Thread Form and Condition

Before attempting assembly, inspect the thread for:

Flattened or damaged crests
Burrs
Corrosion
Galling
Cross-threading
Coating buildup
Dirt or metal particles
Incomplete threads
Taper or deformation

A damaged metric thread can produce misleading measurements and may not accept a mating component even when its nominal size and pitch are correct.

Measuring Pitch Diameter with a Thread Micrometer

A thread micrometer uses a V-shaped anvil and conical spindle to contact the thread flanks. It provides a direct or comparative measurement related to pitch diameter.

The measuring contacts must be appropriate for the thread pitch. The instrument should be calibrated with the correct standard before use.

Thread micrometers are useful for:

Production inspection
Machined external threads
Process adjustment
Comparing components
Verifying pitch-diameter limits

They do not replace functional gauges when the inspection plan requires GO/NO-GO gauging.

Three-Wire Measurement Method

The three-wire method is a precise technique for measuring the pitch diameter of an external thread. Three calibrated wires are placed in the thread grooves:

Two wires are positioned on one side.
One wire is positioned on the opposite side.
A micrometer measures over the wires.

The measured value over the wires is then related to the pitch diameter using the appropriate thread geometry.

For a symmetrical 60-degree thread, the theoretical best wire size is:

$W_{\text{best}}=\frac{P}{2\cos30^\circ}$

Because:

$2\cos30^\circ=1.73205$

the formula becomes:

$W_{\text{best}}\approx0.57735P$

For a thread with a pitch of 1.5 mm:

$W_{\text{best}}=0.57735\times1.5$

$W_{\text{best}}\approx0.866\text{ mm}$

The closest available calibrated thread wire is selected. Exact calculations may require corrections for wire diameter, helix angle, measurement force, and other inspection conditions.

GO/NO-GO Gauges

Limit gauges provide a fast method for checking whether a manufactured thread falls within specified functional limits.

Internal threads

An internal thread is commonly inspected using a thread plug gauge:

The GO gauge should enter through the required length of the thread.
The NO-GO gauge should not enter beyond the amount permitted by the applicable inspection standard.

External threads

An external thread is commonly inspected using a thread ring gauge or thread snap gauge:

The GO gauge checks the functional maximum-material limit.
The NO-GO gauge checks the opposing dimensional limit.

GO/NO-GO gauges provide an acceptance result but do not normally give the actual numerical pitch diameter.

Identifying Metric and Inch Threads

Metric threads can sometimes be confused with UNC, UNF, or other inch threads because their diameters and pitches may appear similar.

Examples of close but incompatible sizes include:

Metric thread	Approx. inch thread	Main issue
M5 × 0.8	No. 10-32 UNF	Diameter and pitch are close but different
M6 × 1.0	1/4-20 UNC	Different diameter and pitch
M8 × 1.25	5/16-18 UNC	Similar diameter but different pitch
M10 × 1.5	3/8-16 UNC	Close size, but not interchangeable
M12 × 1.75	1/2-13 UNC	Different diameter and pitch

Never test an uncertain thread by applying high torque. A mismatched fastener may initially engage for one or two turns before binding and damaging both components.

Metric Thread Identification Procedure

A practical identification sequence is:

Clean the thread.
Determine whether it is internal or external.
Measure the approximate major diameter.
Measure the pitch with a metric pitch gauge.
Check whether the thread is right-hand or left-hand.
Compare the measurements with a metric thread chart.
Inspect the thread form and condition.
Confirm the size with a known reference component or gauge.
Verify the tolerance class if dimensional compliance is required.

A mating nut or bolt may help with preliminary identification, but it should not be forced. Proper inspection gauges should be used for critical components, pressure-containing equipment, lifting devices, rotating machinery, or other safety-related assemblies.

9. Metric Threads vs Inch Threads

Metric and inch threads serve the same basic purpose: they allow external and internal threaded components to assemble, carry loads, and create removable mechanical connections. However, they use different units, designations, pitch systems, and standard size series.

The most common inch-based fastening threads are the Unified thread series:

UNC — Unified National Coarse
UNF — Unified National Fine
UNEF — Unified National Extra Fine

Metric threads and Unified threads both normally use a symmetrical 60-degree thread angle. This similarity does not make them interchangeable because their diameters and pitches are based on different size systems.

Metric and Unified Thread Designations

A metric thread is designated by its nominal diameter and pitch in millimetres:

$\text{M12} \times 1.75$

This means:

Metric thread system
12 mm nominal diameter
1.75 mm pitch

A Unified thread is designated by its nominal diameter in inches and its number of threads per inch:

$\frac{1}{2}\text{-13 UNC}$

This means:

1/2-inch nominal diameter
13 threads per inch
Unified National Coarse series

Metric vs Inch Thread Comparison

Feature	ISO metric thread	Unified inch thread
Primary standards system	ISO	ASME/ANSI
Diameter units	Millimetres	Inches or numbered screw sizes
Pitch designation	Millimetres per thread	Threads per inch
Typical designation	M12 × 1.75	1/2-13 UNC
Standard coarse series	Metric coarse	UNC
Standard fine series	Metric fine	UNF
Extra-fine series	Metric extra-fine	UNEF
Basic thread angle	60°	60°
Common tolerance example	6H/6g	2B/2A

Pitch and Threads per Inch

Metric pitch expresses the distance from one thread crest to the next. A smaller metric pitch means the threads are closer together.

Unified threads use threads per inch. A higher TPI value means the threads are closer together.

The conversion formulas are:

$\text{TPI}=\frac{25.4}{P}$

and:

$P=\frac{25.4}{\text{TPI}}$

where:

$P$

= pitch in millimetres
TPI = threads per inch
25.4 = millimetres per inch

For M10 × 1.5:

$\text{TPI}=\frac{25.4}{1.5}=16.93$

The nearest whole-number value is approximately 17 TPI. However, this does not mean M10 × 1.5 is compatible with any 17-TPI inch thread.

For a 1/2-13 UNC thread:

$P=\frac{25.4}{13}=1.954\text{ mm}$

The pitch is therefore approximately 1.954 mm, not 2.0 mm.

Metric-to-Inch Thread Comparison Chart

The following table compares common metric threads with inch threads having relatively similar nominal diameters. These pairs are provided for identification only and are not interchangeable.

Metric thread	Metric diameter (mm)	Metric pitch (mm)	Similar inch thread	Inch diameter (mm)	Inch pitch (mm)
M3 × 0.5	3.000	0.500	No. 4-48 UNF	2.845	0.529
M4 × 0.7	4.000	0.700	No. 8-36 UNF	4.166	0.706
M5 × 0.8	5.000	0.800	No. 10-32 UNF	4.826	0.794
M6 × 1.0	6.000	1.000	1/4-20 UNC	6.350	1.270
M8 × 1.25	8.000	1.250	5/16-18 UNC	7.938	1.411
M10 × 1.5	10.000	1.500	3/8-16 UNC	9.525	1.588
M12 × 1.75	12.000	1.750	1/2-13 UNC	12.700	1.954
M14 × 2.0	14.000	2.000	9/16-12 UNC	14.288	2.117
M16 × 2.0	16.000	2.000	5/8-11 UNC	15.875	2.309
M20 × 2.5	20.000	2.500	3/4-10 UNC	19.050	2.540
M24 × 3.0	24.000	3.000	1-8 UNC	25.400	3.175

Some pairs are close enough to begin engaging for one or two turns. Continued tightening can damage the thread flanks, create a false torque reading, or permanently lock the components together.

Metric Tolerances vs Unified Thread Classes

Metric threads commonly use tolerance classes such as:

6H for internal threads
6g for external threads
6H/6g for a general-purpose mating pair

Unified threads use classes such as:

1A, 2A, and 3A for external threads
1B, 2B, and 3B for internal threads

The letter identifies whether the thread is external or internal:

A — external Unified thread
B — internal Unified thread

The numerical classes indicate different levels of fit and tolerance. A metric 6g thread is not a direct dimensional equivalent of a Unified 2A thread. Each must be evaluated using its own standard and tolerance tables.

Metric Threads vs BSP Threads

BSP threads are common in piping, pneumatic equipment, hydraulic components, instrumentation, and process systems. They should not be confused with metric fastening threads.

Important differences include:

Feature	ISO metric thread	BSP thread
Typical purpose	Fastening and mechanical assembly	Pipe and pressure connections
Thread angle	60°	55°
Size designation	Nominal diameter in millimetres	Nominal pipe size in inches
Pitch format	Millimetres	Threads per inch
Common forms	Parallel	Parallel and tapered
Examples	M12 × 1.5	G1/4, R1/4

A G1/4 BSPP thread does not have a major diameter of 1/4 inch. BSP designations are based on historical nominal pipe bore sizes, so direct measurement must be compared with a BSP chart.

Metric Threads vs NPT Threads

NPT is a 60-degree tapered pipe thread system commonly used in North America. Although the thread angle matches the ISO metric angle, NPT differs in several important ways:

NPT threads are tapered.
Size designations are based on nominal pipe sizes.
Pitch is specified in threads per inch.
Sealing occurs through thread interference, often with sealant.
The profile and dimensional standards differ from ISO metric threads.

A straight metric thread must not be substituted for an NPT connection. Even when the measured diameters appear similar, the connection may leak, seize, or fail mechanically.

How to Avoid Metric and Inch Thread Mismatch

Before assembling an unidentified thread:

Measure the major diameter.
Determine whether the diameter is metric or inch-based.
Measure the pitch using the correct pitch gauge.
Check the thread angle and profile if necessary.
Determine whether the thread is straight or tapered.
Compare the results with the appropriate standard chart.
Confirm the thread direction.
Test only by hand without applying force.
Use a proper gauge for critical applications.

The fastener should rotate smoothly through several turns by hand. Early resistance, wobbling, excessive looseness, or binding indicates a possible mismatch or damaged thread.

10. Metric Thread Selection, Standards, and FAQ

Selecting a metric thread involves more than choosing a nominal diameter. The designer must also consider pitch, tolerance class, engagement length, material strength, operating load, vibration, corrosion, coating, manufacturing method, and maintenance requirements.

Choosing Between Coarse and Fine Metric Threads

Coarse threads are normally preferred for general-purpose applications because they are widely available and relatively resistant to handling damage.

Choose a coarse thread when:

Rapid assembly is required.
The component will be assembled frequently.
The environment contains dirt or corrosion.
The thread is produced in a soft or brittle material.
The connection may experience rough handling.
Standard commercial fasteners are preferred.
Cross-threading resistance is important.

Fine threads may be preferred when:

A larger tensile stress area is required within the same nominal size.
Precise axial adjustment is needed.
The available wall thickness is limited.
A smaller movement per revolution is desirable.
The application uses established automotive or aerospace specifications.
The design requires a finer control of preload.

Fine threads should not automatically be considered better for vibration. Resistance to self-loosening depends on preload, joint stiffness, external loading, friction, fastener geometry, and the locking method—not pitch alone.

Thread Selection for Soft Materials

Materials such as aluminum, magnesium, cast iron, and engineering plastics may require longer thread engagement than high-strength steel.

Possible design approaches include:

Increasing engagement length
Using a coarse pitch
Installing threaded inserts
Increasing the nominal diameter
Reducing assembly torque
Using a flange or through-bolt arrangement
Controlling repeated assembly cycles

A fine thread provides more engaged threads over the same axial length, but its shallower profile does not automatically prevent stripping. The strength of the parent material and effective shear area must be evaluated.

Thread Selection for Thin-Walled Components

Fine threads are often suitable for thin walls because their smaller pitch allows more complete threads within a limited axial thickness. However, the wall must still provide sufficient radial and axial strength.

For sheet metal or very thin sections, ordinary tapped threads may be unsuitable. Alternatives include:

Rivet nuts
Weld nuts
Clinch nuts
Thread-forming screws
Threaded inserts
Backing nuts

Thread Selection for Vibration

A properly preloaded bolted joint relies on clamping force to prevent relative movement between the joined parts. If preload is lost, either coarse or fine threads can loosen.

For vibration-sensitive joints, consider:

Correct tightening torque
Direct tension measurement
Controlled lubrication
Locknuts
Mechanical locking devices
Thread-locking compounds
Locking-wire systems
Joint stiffness
Fatigue-resistant fastener design
Inspection and retightening procedures

Spring washers alone should not be assumed to solve all vibration-related loosening problems.

Thread Selection for High-Temperature Service

At elevated temperatures, thread behavior may be affected by:

Thermal expansion
Loss of material strength
Oxidation
Creep
Lubricant degradation
Galling
Differential expansion between materials

Material compatibility, surface treatment, lubricant selection, preload loss, and the applicable high-temperature design code should be considered.

Thread Selection for Stainless Steel

Stainless steel threads can gall or cold-weld during assembly, particularly when similar stainless-steel grades are used for both mating components.

Ways to reduce galling include:

Using an appropriate anti-seize compound
Reducing installation speed
Avoiding excessive torque
Keeping the threads clean
Selecting compatible material combinations
Applying suitable coatings
Using rolled threads with a good surface finish
Preventing misalignment

Lubrication changes the relationship between torque and bolt preload. Tightening values must therefore correspond to the actual lubrication and coating condition.

Important ISO Metric Thread Standards

The ISO metric screw thread system is defined through several related standards.

Standard	Primary subject
ISO 68-1	Basic profile of ISO general-purpose metric screw threads
ISO 261	General-purpose metric screw thread plan
ISO 262	Selected sizes for screws, bolts, and nuts
ISO 724	Basic dimensions of metric threads
ISO 965 series	Tolerances, deviations, limits, and gauging principles
ISO 1502	Gauges and gauging for general-purpose metric screw threads
ISO 5408	Vocabulary for screw threads
ISO 225	Symbols and descriptions of dimensions for fasteners
ISO 898-1	Mechanical properties of carbon- and alloy-steel fasteners
ISO 898-2	Mechanical properties of nuts made from carbon and alloy steel

The applicable standard should be confirmed on the engineering drawing, purchase specification, or product standard. Thread geometry alone does not define the fastener’s material, strength class, dimensions, finish, or mechanical performance.

Metric Thread Drawing Callouts

A general-purpose external coarse thread may be specified as:

$\text{M16} \times 2\text{-6g}$

An internal thread may be specified as:

$\text{M16} \times 2\text{-6H}$

A fine left-hand thread may be specified as:

$\text{M16} \times 1.5\text{-6g-LH}$

A drawing should also specify relevant requirements such as:

Threaded length
Hole depth
Through or blind hole
Chamfer
Thread direction
Surface finish
Coating or plating
Inspection requirements
Thread condition before or after coating

Common Metric Thread Selection Mistakes

Frequent mistakes include:

Measuring diameter without measuring pitch
Assuming every M10 thread uses a 1.5 mm pitch
Confusing pitch with lead
Mixing metric and inch fasteners
Using cutting-tap drill sizes for forming taps
Ignoring the tolerance class
Failing to account for coating thickness
Specifying excessive thread engagement
Using a pipe thread as a fastening thread
Applying torque values for the wrong lubrication condition
Forcing a fastener that binds during hand assembly

Frequently Asked Questions

What does M10 mean?

M10 identifies an ISO metric thread with a nominal major diameter of 10 mm. If no pitch is stated, M10 normally refers to the standard coarse pitch of 1.5 mm.

Therefore:

$\text{M10}=\text{M10} \times 1.5$

when the standard coarse series is intended.

What does M10 × 1.25 mean?

It identifies a metric thread with:

10 mm nominal diameter
1.25 mm pitch
60-degree thread angle

M10 × 1.25 is a fine thread and is not interchangeable with the standard M10 × 1.5 coarse thread.

What does M12 × 1.75–6H mean?

It identifies:

ISO metric thread
12 mm nominal diameter
1.75 mm pitch
Internal thread
Tolerance class 6H

What is the standard pitch for M8?

The standard coarse pitch for M8 is 1.25 mm:

$\text{M8} \times 1.25$ $M8 \times 1.25$

Common fine versions include M8 × 1.0 and M8 × 0.75.

What is the tap drill size for M8 × 1.25?

Using the approximate formula:

$D_{\text{drill}}=8-1.25=6.75\text{ mm}$

A 6.8 mm drill is commonly used for general-purpose cutting-tap applications.

What is the tap drill size for M10 × 1.5?

$D_{\text{drill}}=10-1.5=8.5\text{ mm}$

The commonly used tap drill is 8.5 mm.

Are M10 and 3/8-inch threads interchangeable?

No. M10 has a nominal diameter of 10 mm, while a 3/8-inch thread has a nominal diameter of 9.525 mm. Their pitches also differ.

For example:

M10 × 1.5 has a pitch of 1.5 mm.
3/8-16 UNC has a pitch of approximately 1.588 mm.

They may appear similar but should not be assembled together.

Are metric threads always 60 degrees?

General-purpose ISO metric screw threads use a 60-degree included angle. However, not every thread measured in millimetres is necessarily an ISO general-purpose metric thread. Special thread forms can use different profiles and must be identified through their applicable standards.

Is a smaller metric pitch a finer thread?

Yes. For the same nominal diameter, a smaller pitch means the thread crests are closer together.

For M12:

1.75 mm is the standard coarse pitch.
1.5 mm is finer.
1.25 mm is finer again.
1.0 mm is an extra-fine option.

Can a caliper identify a metric thread?

A caliper can estimate the major diameter, but it cannot fully identify the thread. The pitch must also be measured using a thread-pitch gauge or another suitable method.

A caliper also cannot confirm the tolerance class or functional acceptability of the thread.

What is the difference between 6H and 6g?

6H is a common tolerance class for internal metric threads.
6g is a common tolerance class for external metric threads.

They are frequently paired for general-purpose assemblies.

How much thread engagement is required?

The required engagement depends on:

Fastener strength
Parent material strength
Nominal diameter
Pitch
Applied load
Failure mode
Safety factor

An engagement length equal to one nominal diameter is sometimes used as an initial rule for steel components of comparable strength, but it is not universally sufficient. Softer materials commonly require greater engagement or a threaded insert. The final length should be verified by engineering calculation or the applicable design standard.

Should the thread be measured before or after coating?

That depends on the engineering specification. Because coating changes the effective thread dimensions, the drawing or purchasing document should clearly state whether the tolerance applies before or after coating.

Can metric fasteners be reused?

Reuse depends on the fastener type, service condition, loading history, applicable standard, and manufacturer’s instructions. Torque-to-yield fasteners, damaged threads, corroded fasteners, and highly loaded safety-critical bolts may require mandatory replacement.

Conclusion

A metric thread size chart provides a practical reference for selecting and identifying ISO metric threads, but correct selection requires both the nominal diameter and pitch. The designation M12, for example, normally indicates the standard M12 × 1.75 coarse thread, while M12 × 1.5 and M12 × 1.25 identify different fine-thread versions.

For general engineering work, coarse threads offer convenient assembly, broad availability, and good resistance to damage. Fine threads provide a larger tensile stress area and more precise axial adjustment but require greater care during manufacturing and assembly.

Tap drill values calculated from:

$D_{\text{drill}}\approx d-P$

are useful preliminary recommendations for cutting taps. Production applications should also consider material, thread engagement, tap type, tolerance class, coating, and tool-manufacturer recommendations.

For critical components, the final thread dimensions and acceptance limits should always be verified against the relevant ISO standard and inspected using calibrated thread gauges or suitable precision measurement equipment.

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