Thread Pitch Chart: Metric, UNC, UNF, BSP & NPT

Contents

1 1. What Is Thread Pitch?
2 2. Thread Pitch Terminology and Measurements
3 3. Metric Thread Pitch Chart
- 3.1 Metric Coarse and Fine Thread Pitch Chart
- 3.2 Metric Thread Designation Examples
4 4. UNC, UNF, and UNEF Thread Pitch Chart
- 4.1 UNC, UNF, and UNEF Thread Pitch Chart
- 4.2 Unified Thread Pitch in Millimeters
5 5. BSPP and BSPT Thread Pitch Chart
6 6. NPT and NPTF Thread Pitch Chart
7 7. Thread Pitch vs. Threads per Inch (TPI)
8 8. How to Measure and Identify Thread Pitch
9 9. How to Select the Correct Thread Pitch
10 Conclusion

Thread pitch is one of the most important dimensions used to identify and select threaded fasteners, fittings, pipes, and mechanical components. Even when two threads have the same nominal diameter, they cannot be connected correctly unless their pitch, thread profile, and other design characteristics are compatible.

Metric threads specify pitch as the distance between adjacent thread crests, measured in millimeters. Inch-based thread systems typically specify thread spacing as threads per inch (TPI). For example, an M10 × 1.5 metric thread has a nominal diameter of 10 mm and a pitch of 1.5 mm, while a 1/2-13 UNC thread has a nominal diameter of 1/2 inch and 13 threads per inch.

This article provides thread pitch charts for common metric and inch thread systems, including ISO metric, UNC, UNF, UNEF, BSP, NPT, and NPTF threads. It also explains how to measure thread pitch, convert between pitch and TPI, read thread designations, and select the correct thread for an application.

1. What Is Thread Pitch?

What Is Thread Pitch?

Thread pitch is the axial distance between corresponding points on two adjacent threads. It is normally measured from one thread crest to the next, parallel to the axis of the threaded component.

For metric threads, pitch is the axial distance between corresponding points on two adjacent threads:

Thread pitch = Distance between adjacent thread crests

In inch-based thread systems, pitch can be calculated from the number of threads per inch:

Pitch (inches) = 1 ÷ TPI

The equivalent metric pitch is:

Pitch (mm) = 25.4 ÷ TPI

For example, a 1/2-13 UNC thread has 13 threads per inch:

Pitch = 25.4 ÷ 13 = 1.954 mm

Therefore, the distance between adjacent threads is approximately 1.954 mm.

A smaller pitch means the threads are closer together and is generally described as a fine thread. A larger pitch means the threads are farther apart and is described as a coarse thread.

Coarse threads are commonly used for general assembly because they are easier to start, faster to install, and more tolerant of dirt and minor surface damage. Fine threads provide more threads within a given engagement length and allow finer adjustment, but they are easier to cross-thread and may be less suitable for soft or damaged materials.

Thread pitch alone does not determine compatibility. Two threads must also have matching nominal diameters, thread profiles, flank angles, diameters, and taper characteristics. For example, some BSP and NPT sizes may have similar pitches, but their different thread angles and forms mean they should not normally be connected.

2. Thread Pitch Terminology and Measurements

Thread Pitch Terminology and Measurements

Understanding the basic dimensions of a thread makes it easier to read a thread pitch chart and correctly identify an unknown thread.

Nominal Diameter

The nominal diameter is the size used to identify the thread. For a parallel external thread, it is generally related to the major diameter, although the actual measured diameter may be slightly smaller because of manufacturing tolerances.

Examples include:

M12 for a nominal 12 mm metric thread
1/2-13 UNC for a nominal 1/2-inch unified thread
1/2 NPT for a nominal pipe thread size

For pipe threads, the nominal size does not represent the actual outside diameter. A 1/2-inch NPT or BSP thread, for example, has an outside diameter considerably larger than 1/2 inch.

Pitch

Pitch is the axial distance from one thread crest to the corresponding point on the next thread.

Metric thread designations normally state this value in millimeters. In the designation M16 × 2, the pitch is 2 mm.

When the standard coarse pitch is used, it may be omitted. Therefore, M16 typically means M16 × 2. Fine and extra-fine pitches should be stated explicitly, such as M16 × 1.5 or M16 × 1.

Threads per Inch

Threads per inch, abbreviated as TPI, indicate how many complete thread cycles occur over an axial length of one inch.

In the designation 3/4-16 UNF:

3/4 inch is the nominal diameter.
16 is the number of threads per inch.
UNF identifies the Unified National Fine thread series.

TPI and metric pitch describe the same spacing in different ways. A high TPI value corresponds to a small pitch, while a low TPI value corresponds to a large pitch.

Major Diameter

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

On an external thread, it is measured across the thread crests.
On an internal thread, it is associated with the diameter at the thread roots.

A caliper can provide an approximate major-diameter measurement, but it is not sufficient by itself to confirm a thread standard.

Minor Diameter

The minor diameter is the smallest diameter of the thread form.

On an external thread, it is measured across the thread roots.
On an internal thread, it is measured across the thread crests.

This dimension affects the remaining material thickness and the tensile strength of a threaded component.

Pitch Diameter

The pitch diameter is the theoretical diameter at which the widths of the thread ridges and grooves are equal. It is a critical dimension for determining how an external thread fits with an internal thread.

Pitch diameter is not normally measured with a standard caliper. Accurate inspection may require thread micrometers, the three-wire method, calibrated thread gauges, or optical measuring equipment.

Thread Crest and Root

The crest is the top surface of a thread ridge, while the root is the bottom of the groove between adjacent threads. Their shapes vary according to the thread standard and may be flat, rounded, or truncated.

Thread Flank

The flank is the side surface connecting the crest and root. The angle and shape of the flanks determine the thread profile and influence load distribution, strength, sealing, and compatibility.

Thread Angle

The thread angle is the included angle between the two flanks of a thread profile.

Common values include:

ISO metric threads: 60°
Unified threads: 60°
NPT and NPTF threads: 60°
BSPP and BSPT threads: 55°

Matching pitch does not guarantee compatibility when the thread angles or profiles are different.

Lead

Lead is the axial distance that a threaded component advances during one complete revolution.

For a single-start thread:

Lead = Pitch

For a multiple-start thread:

Lead = Number of thread starts × Pitch

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

Lead = 2 × 2 mm = 4 mm

Measuring Thread Pitch

Thread pitch can be estimated by measuring across several complete pitch intervals:

Pitch = Measured length ÷ Number of pitch intervals

For example, if 10 pitch intervals measure 15 mm:

Pitch = 15 ÷ 10 = 1.5 mm

Always count the intervals between thread crests. The distance from the first crest to the sixth crest contains five pitch intervals.

Coarse and Fine Pitch

A nominal thread diameter may be available with several standard pitches:

Coarse pitch has greater spacing between adjacent threads.
Fine pitch has smaller spacing and more threads per unit length.
Extra-fine pitch has even smaller spacing for specialized applications.

For example, common M12 threads include:

Thread designation	Pitch	Classification
M12 × 1.75	1.75 mm	Coarse
M12 × 1.5	1.50 mm	Fine
M12 × 1.25	1.25 mm	Fine
M12 × 1.0	1.00 mm	Extra-fine

Parallel and Tapered Threads

Parallel threads maintain approximately the same diameter along their threaded length. Examples include ISO metric, UNC, UNF, UNEF, and BSPP threads.

Tapered threads gradually change in diameter along their length. NPT, NPTF, and BSPT are common tapered pipe threads. Their identification requires checking not only pitch and outside diameter but also taper, thread angle, and profile.

How Thread Pitch Is Measured

The quickest method is to place a thread pitch gauge against the threads and select the blade that fits the grooves without gaps. Separate gauges are commonly used for metric pitch and inch TPI.

Thread pitch can also be estimated by measuring across several threads:

$P = N L$

Where:

$P$ = thread pitch
$L$ = measured distance
$N$ = number of complete pitch intervals

Measuring across multiple intervals reduces error. For example, if the distance across ten pitch intervals is 15 mm:

$P = 1015 = 1.5 mm$

Accurate thread identification should combine pitch measurement with the major diameter, thread angle, profile, and whether the thread is parallel or tapered.

3. Metric Thread Pitch Chart

Metric Thread Pitch Chart

ISO metric threads are designated by the letter M, followed by the nominal diameter and thread pitch in millimeters.

The standard designation format is:

$M d \times P$

Where:

$d$ = nominal thread diameter in millimeters
$P$ = thread pitch in millimeters

For example, M10 × 1.5 indicates a 10 mm nominal diameter and a 1.5 mm pitch.

When the pitch is omitted, the designation normally refers to the standard coarse pitch. Therefore, M10 generally means M10 × 1.5. Fine and extra-fine pitches must be stated explicitly.

Metric Coarse and Fine Thread Pitch Chart

Nominal size	Standard coarse pitch	Common fine pitches
M1	0.25 mm	0.20 mm
M1.2	0.25 mm	0.20 mm
M1.4	0.30 mm	0.20 mm
M1.6	0.35 mm	0.20 mm
M1.8	0.35 mm	0.20 mm
M2	0.40 mm	0.25 mm
M2.2	0.45 mm	0.25 mm
M2.5	0.45 mm	0.35 mm
M3	0.50 mm	0.35 mm
M3.5	0.60 mm	0.35 mm
M4	0.70 mm	0.50 mm
M4.5	0.75 mm	0.50 mm
M5	0.80 mm	0.50 mm
M6	1.00 mm	0.75 mm, 0.50 mm
M7	1.00 mm	0.75 mm, 0.50 mm
M8	1.25 mm	1.00 mm, 0.75 mm, 0.50 mm
M10	1.50 mm	1.25 mm, 1.00 mm, 0.75 mm
M12	1.75 mm	1.50 mm, 1.25 mm, 1.00 mm
M14	2.00 mm	1.50 mm, 1.25 mm, 1.00 mm
M16	2.00 mm	1.50 mm, 1.00 mm
M18	2.50 mm	2.00 mm, 1.50 mm, 1.00 mm
M20	2.50 mm	2.00 mm, 1.50 mm, 1.00 mm
M22	2.50 mm	2.00 mm, 1.50 mm, 1.00 mm
M24	3.00 mm	2.00 mm, 1.50 mm, 1.00 mm
M27	3.00 mm	2.00 mm, 1.50 mm, 1.00 mm
M30	3.50 mm	3.00 mm, 2.00 mm, 1.50 mm
M33	3.50 mm	3.00 mm, 2.00 mm, 1.50 mm
M36	4.00 mm	3.00 mm, 2.00 mm, 1.50 mm
M39	4.00 mm	3.00 mm, 2.00 mm, 1.50 mm
M42	4.50 mm	4.00 mm, 3.00 mm, 2.00 mm, 1.50 mm
M45	4.50 mm	4.00 mm, 3.00 mm, 2.00 mm, 1.50 mm
M48	5.00 mm	4.00 mm, 3.00 mm, 2.00 mm, 1.50 mm
M52	5.00 mm	4.00 mm, 3.00 mm, 2.00 mm, 1.50 mm
M56	5.50 mm	4.00 mm, 3.00 mm, 2.00 mm, 1.50 mm
M60	5.50 mm	4.00 mm, 3.00 mm, 2.00 mm, 1.50 mm
M64	6.00 mm	4.00 mm, 3.00 mm, 2.00 mm, 1.50 mm
M68	6.00 mm	4.00 mm, 3.00 mm, 2.00 mm, 1.50 mm
M72	6.00 mm	4.00 mm, 3.00 mm, 2.00 mm, 1.50 mm
M76	6.00 mm	4.00 mm, 3.00 mm, 2.00 mm, 1.50 mm
M80	6.00 mm	4.00 mm, 3.00 mm, 2.00 mm, 1.50 mm

Not every pitch shown is equally common in commercial fasteners. Availability depends on the thread diameter, component type, material, strength class, and applicable manufacturing standard.

Metric Thread Designation Examples

Designation	Nominal diameter	Pitch	Description
M6	6 mm	1.00 mm	Standard coarse pitch
M8 × 1	8 mm	1.00 mm	Fine pitch
M10 × 1.25	10 mm	1.25 mm	Fine pitch
M12 × 1.75	12 mm	1.75 mm	Standard coarse pitch
M16 × 1.5	16 mm	1.50 mm	Fine pitch
M20 × 1	20 mm	1.00 mm	Extra-fine pitch

A metric bolt and nut must have the same nominal diameter and pitch. An M12 × 1.75 bolt cannot correctly engage an M12 × 1.5 nut even though both have the same nominal diameter.

4. UNC, UNF, and UNEF Thread Pitch Chart

The Unified Thread Standard is widely used for fasteners in the United States, Canada, and many international industries. Unified threads have a symmetrical 60-degree thread angle and specify thread spacing in threads per inch.

The three principal thread series are:

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

A typical unified thread designation is:

$21 -13 UNC$

This designation indicates:

Nominal diameter: 1/2 inch
Thread count: 13 TPI
Thread series: UNC

UNC, UNF, and UNEF Thread Pitch Chart

Nominal size	Major diameter	UNC	UNF	UNEF
No. 0	0.0600 in	—	80 TPI	—
No. 1	0.0730 in	64 TPI	72 TPI	—
No. 2	0.0860 in	56 TPI	64 TPI	—
No. 3	0.0990 in	48 TPI	56 TPI	—
No. 4	0.1120 in	40 TPI	48 TPI	—
No. 5	0.1250 in	40 TPI	44 TPI	—
No. 6	0.1380 in	32 TPI	40 TPI	—
No. 8	0.1640 in	32 TPI	36 TPI	—
No. 10	0.1900 in	24 TPI	32 TPI	—
No. 12	0.2160 in	24 TPI	28 TPI	32 TPI
1/4 in	0.2500 in	20 TPI	28 TPI	32 TPI
5/16 in	0.3125 in	18 TPI	24 TPI	32 TPI
3/8 in	0.3750 in	16 TPI	24 TPI	32 TPI
7/16 in	0.4375 in	14 TPI	20 TPI	28 TPI
1/2 in	0.5000 in	13 TPI	20 TPI	28 TPI
9/16 in	0.5625 in	12 TPI	18 TPI	24 TPI
5/8 in	0.6250 in	11 TPI	18 TPI	24 TPI
11/16 in	0.6875 in	—	16 TPI	24 TPI
3/4 in	0.7500 in	10 TPI	16 TPI	20 TPI
13/16 in	0.8125 in	—	16 TPI	20 TPI
7/8 in	0.8750 in	9 TPI	14 TPI	20 TPI
15/16 in	0.9375 in	—	14 TPI	20 TPI
1 in	1.0000 in	8 TPI	12 TPI	20 TPI
1-1/16 in	1.0625 in	—	12 TPI	18 TPI
1-1/8 in	1.1250 in	7 TPI	12 TPI	18 TPI
1-3/16 in	1.1875 in	—	12 TPI	18 TPI
1-1/4 in	1.2500 in	7 TPI	12 TPI	18 TPI
1-5/16 in	1.3125 in	—	12 TPI	18 TPI
1-3/8 in	1.3750 in	6 TPI	12 TPI	18 TPI
1-7/16 in	1.4375 in	—	12 TPI	18 TPI
1-1/2 in	1.5000 in	6 TPI	12 TPI	18 TPI
1-9/16 in	1.5625 in	—	12 TPI	18 TPI
1-5/8 in	1.6250 in	5.5 TPI	—	18 TPI
1-11/16 in	1.6875 in	—	—	18 TPI
1-3/4 in	1.7500 in	5 TPI	—	16 TPI
1-13/16 in	1.8125 in	—	—	16 TPI
1-7/8 in	1.8750 in	5 TPI	—	16 TPI
1-15/16 in	1.9375 in	—	—	16 TPI
2 in	2.0000 in	4.5 TPI	—	16 TPI

A dash indicates that the diameter-and-series combination is not normally included as a standard preferred thread size. Special constant-pitch unified threads may still be available under designations such as 8-UN, 12-UN, 16-UN, 20-UN, and 28-UN.

Unified Thread Pitch in Millimeters

The equivalent metric pitch is calculated using:

$P_{mm} = TPI 25.4$

TPI	Equivalent pitch
4	6.350 mm
4.5	5.644 mm
5	5.080 mm
5.5	4.618 mm
6	4.233 mm
7	3.629 mm
8	3.175 mm
9	2.822 mm
10	2.540 mm
11	2.309 mm
12	2.117 mm
13	1.954 mm
14	1.814 mm
16	1.588 mm
18	1.411 mm
20	1.270 mm
24	1.058 mm
28	0.907 mm
32	0.794 mm
36	0.706 mm
40	0.635 mm
48	0.529 mm
56	0.454 mm
64	0.397 mm
72	0.353 mm
80	0.318 mm

UNC threads are commonly selected for general fastening, soft materials, castings, and applications exposed to dirt or damage. UNF and UNEF threads provide smaller axial movement per revolution and may be preferred for adjustment, thin-wall components, and applications requiring finer clamping control.

5. BSPP and BSPT Thread Pitch Chart

BSPP and BSPT Thread Pitch Chart

BSP threads are pipe threads based on the Whitworth thread form. They have a 55-degree thread angle with rounded crests and roots.

The two principal BSP thread types are:

BSPP: British Standard Pipe Parallel, also designated with the letter G
BSPT: British Standard Pipe Taper, commonly designated with the letter R

BSPP threads remain parallel along their length. They do not normally create a pressure-tight seal on the threads alone. Sealing is generally provided by an O-ring, bonded washer, gasket, or metal sealing surface.

BSPT threads are tapered and can create a pressure-tight joint through thread interference, usually with the addition of an appropriate thread sealant.

Both BSPP and BSPT use the same basic pitch for a given nominal pipe size. However, they differ in diameter variation and sealing method.

BSPP and BSPT Thread Pitch Chart

Nominal BSP size	Threads per inch	Pitch	Approximate major diameter
1/16	28 TPI	0.907 mm	7.723 mm
1/8	28 TPI	0.907 mm	9.728 mm
1/4	19 TPI	1.337 mm	13.157 mm
3/8	19 TPI	1.337 mm	16.662 mm
1/2	14 TPI	1.814 mm	20.955 mm
5/8	14 TPI	1.814 mm	22.911 mm
3/4	14 TPI	1.814 mm	26.441 mm
7/8	14 TPI	1.814 mm	30.201 mm
1	11 TPI	2.309 mm	33.249 mm
1-1/8	11 TPI	2.309 mm	37.897 mm
1-1/4	11 TPI	2.309 mm	41.910 mm
1-1/2	11 TPI	2.309 mm	47.803 mm
1-3/4	11 TPI	2.309 mm	53.746 mm
2	11 TPI	2.309 mm	59.614 mm
2-1/4	11 TPI	2.309 mm	65.710 mm
2-1/2	11 TPI	2.309 mm	75.184 mm
2-3/4	11 TPI	2.309 mm	81.534 mm
3	11 TPI	2.309 mm	87.884 mm
3-1/2	11 TPI	2.309 mm	100.330 mm
4	11 TPI	2.309 mm	113.030 mm
4-1/2	11 TPI	2.309 mm	125.730 mm
5	11 TPI	2.309 mm	138.430 mm
5-1/2	11 TPI	2.309 mm	151.130 mm
6	11 TPI	2.309 mm	163.830 mm

The listed major diameters are nominal reference values. Actual measurements can vary because of thread tolerances and, for tapered threads, the point along the thread where the measurement is taken.

Common BSP Thread Designations

Designation	Meaning
G 1/2	1/2-inch BSPP parallel thread
G 1/2 A	BSPP external thread with tolerance class A
G 1/2 B	BSPP external thread with tolerance class B
Rp 1/2	Internal parallel pipe thread for a pressure-tight joint
Rc 1/2	Internal tapered pipe thread
R 1/2	External tapered BSPT thread

BSP Thread Size Is Nominal

The nominal BSP size does not equal the measured outside diameter. For example, a 1/2-inch BSP thread has an outside diameter of approximately 20.955 mm, rather than 12.7 mm.

This difference exists because BSP sizes were historically related to the approximate internal bore of the pipe. Therefore, both the outside diameter and pitch should be measured when identifying an unknown BSP thread.

BSP and NPT Are Not Interchangeable

Some BSP and NPT threads have similar diameters, but they should not be treated as interchangeable. The two systems use different thread forms:

Feature	BSP	NPT
Thread angle	55°	60°
Crest and root	Rounded	Flattened or truncated
Common 1/2-inch pitch	14 TPI	14 TPI
Common 3/4-inch pitch	14 TPI	14 TPI
Common 1-inch pitch	11 TPI	11.5 TPI

A BSP and NPT connection may appear to engage for a few turns, especially in certain sizes, but the mismatched profiles can produce poor engagement, thread damage, and leakage. Proper adapters should be used when connecting components from the two systems.

6. NPT and NPTF Thread Pitch Chart

NPT and NPTF are tapered pipe thread standards widely used in piping, instrumentation, hydraulic, pneumatic, and fluid-handling systems.

NPT: National Pipe Taper
NPTF: National Pipe Taper Fuel, also called Dryseal thread

Both thread types have a 60-degree thread angle and a nominal taper of 1:16 on diameter, equivalent to 3/4 inch per foot. Their diameters decrease toward the end of an external thread and increase toward the opening of an internal thread.

NPT joints normally require a suitable thread sealant or PTFE tape to fill the spiral leakage path between the mating threads. NPTF threads use controlled crest and root geometry to produce greater interference between mating threads. This design can create a pressure-tight dry seal when the threads are correctly manufactured and assembled, although sealants may still be specified for lubrication, corrosion protection, or additional sealing reliability.

NPT and NPTF Thread Pitch Chart

Nominal pipe size	Threads per inch	Pitch	Approximate outside diameter
1/16	27 TPI	0.941 mm	7.895 mm
1/8	27 TPI	0.941 mm	10.287 mm
1/4	18 TPI	1.411 mm	13.716 mm
3/8	18 TPI	1.411 mm	17.145 mm
1/2	14 TPI	1.814 mm	21.336 mm
3/4	14 TPI	1.814 mm	26.670 mm
1	11.5 TPI	2.209 mm	33.401 mm
1-1/4	11.5 TPI	2.209 mm	42.164 mm
1-1/2	11.5 TPI	2.209 mm	48.260 mm
2	11.5 TPI	2.209 mm	60.325 mm
2-1/2	8 TPI	3.175 mm	73.025 mm
3	8 TPI	3.175 mm	88.900 mm
3-1/2	8 TPI	3.175 mm	101.600 mm
4	8 TPI	3.175 mm	114.300 mm
5	8 TPI	3.175 mm	141.300 mm
6	8 TPI	3.175 mm	168.275 mm
8	8 TPI	3.175 mm	219.075 mm
10	8 TPI	3.175 mm	273.050 mm
12	8 TPI	3.175 mm	323.850 mm

The listed outside diameter represents the nominal major diameter at a defined reference or gauge plane. A caliper measurement taken at the end of a tapered male thread may be smaller. Manufacturing tolerances and the measurement location must therefore be considered during identification.

NPT Thread Designation Examples

Designation	Meaning
1/8-27 NPT	1/8-inch nominal size with 27 TPI
1/4-18 NPT	1/4-inch nominal size with 18 TPI
1/2-14 NPT	1/2-inch nominal size with 14 TPI
1-11.5 NPT	1-inch nominal size with 11.5 TPI
1/4-18 NPTF	1/4-inch Dryseal tapered thread with 18 TPI

The nominal pipe size is not the measured outside diameter. For example, a 1/2-inch NPT external thread has an approximate major diameter of 21.336 mm, not 12.7 mm.

NPT vs. NPTF Threads

NPT and NPTF threads have the same nominal sizes, taper rates, and TPI values. The main difference lies in the crest and root tolerances.

Feature	NPT	NPTF
Thread angle	60°	60°
Taper	1:16 on diameter	1:16 on diameter
Pitch series	Same	Same
Primary sealing principle	Thread interference plus sealant	Controlled crest-root interference
Sealant normally required	Yes	Not necessarily
Common applications	General piping and instrumentation	Fuel, hydraulic and pneumatic systems

An NPT component may appear to assemble with an NPTF component because their basic forms are similar. However, a mixed connection may not provide the intended dry-seal performance. The specified thread designation should always be followed for critical pressure applications.

NPT vs. BSPT Pitch Comparison

Nominal size	NPT pitch	BSPT pitch
1/8	27 TPI	28 TPI
1/4	18 TPI	19 TPI
3/8	18 TPI	19 TPI
1/2	14 TPI	14 TPI
3/4	14 TPI	14 TPI
1	11.5 TPI	11 TPI
1-1/4	11.5 TPI	11 TPI
1-1/2	11.5 TPI	11 TPI
2	11.5 TPI	11 TPI

Even when the pitch appears identical, as with the 1/2-inch and 3/4-inch sizes, NPT and BSPT threads remain incompatible because NPT uses a 60-degree thread angle while BSPT uses a 55-degree Whitworth profile.

7. Thread Pitch vs. Threads per Inch (TPI)

Thread pitch and TPI describe the same thread spacing using different measurement systems.

To calculate pitch in inches:

Pitch (inches) = 1 ÷ TPI

To convert TPI to metric pitch:

Pitch (mm) = 25.4 ÷ TPI

To convert metric pitch to TPI:

TPI = 25.4 ÷ Pitch (mm)

Example: Converting TPI to Metric Pitch

For a 1/2-13 UNC thread:

Pitch = 25.4 ÷ 13 = 1.954 mm

The distance between adjacent threads is approximately 1.954 mm.

Example: Converting Metric Pitch to TPI

For an M12 × 1.75 thread:

TPI = 25.4 ÷ 1.75 = 14.51 TPI

The equivalent thread spacing is approximately 14.51 threads per inch.

These converted values are used for comparison only. They do not make metric and inch threads interchangeable.

Thread Pitch and TPI Conversion Chart

TPI	Pitch in inches	Pitch in millimeters
4	0.2500 in	6.350 mm
4.5	0.2222 in	5.644 mm
5	0.2000 in	5.080 mm
5.5	0.1818 in	4.618 mm
6	0.1667 in	4.233 mm
7	0.1429 in	3.629 mm
8	0.1250 in	3.175 mm
9	0.1111 in	2.822 mm
10	0.1000 in	2.540 mm
11	0.0909 in	2.309 mm
11.5	0.0870 in	2.209 mm
12	0.0833 in	2.117 mm
13	0.0769 in	1.954 mm
14	0.0714 in	1.814 mm
16	0.0625 in	1.588 mm
18	0.0556 in	1.411 mm
19	0.0526 in	1.337 mm
20	0.0500 in	1.270 mm
24	0.0417 in	1.058 mm
27	0.0370 in	0.941 mm
28	0.0357 in	0.907 mm
32	0.0313 in	0.794 mm
36	0.0278 in	0.706 mm
40	0.0250 in	0.635 mm
44	0.0227 in	0.577 mm
48	0.0208 in	0.529 mm
56	0.0179 in	0.454 mm
64	0.0156 in	0.397 mm
72	0.0139 in	0.353 mm
80	0.0125 in	0.318 mm

Metric Pitch to Approximate TPI Chart

Metric pitch	Approximate TPI
0.20 mm	127.00
0.25 mm	101.60
0.30 mm	84.67
0.35 mm	72.57
0.40 mm	63.50
0.45 mm	56.44
0.50 mm	50.80
0.60 mm	42.33
0.70 mm	36.29
0.75 mm	33.87
0.80 mm	31.75
1.00 mm	25.40
1.25 mm	20.32
1.50 mm	16.93
1.75 mm	14.51
2.00 mm	12.70
2.50 mm	10.16
3.00 mm	8.47
3.50 mm	7.26
4.00 mm	6.35
4.50 mm	5.64
5.00 mm	5.08
5.50 mm	4.62
6.00 mm	4.23

These converted values are mathematical equivalents, not necessarily interchangeable standard thread sizes. For example, a 1.5 mm pitch corresponds to approximately 16.93 TPI, but this does not mean that an M10 × 1.5 thread can mate with a 17-TPI inch thread.

Conversion Example: TPI to Metric Pitch

For a 1/2-13 UNC thread:

$P_{mm} = 1325.4 = 1.954 mm$

The thread therefore has an axial spacing of approximately 1.954 mm between adjacent crests.

Conversion Example: Metric Pitch to TPI

For an M12 × 1.75 thread:

$TPI = 1.7525.4 = 14.51$

This thread has the equivalent spacing of approximately 14.51 threads per inch.

Pitch conversion is useful for identification and comparison, but compatibility also requires the same diameter, thread profile, angle, taper, and tolerance class.

8. How to Measure and Identify Thread Pitch

Correct thread identification requires more than measuring the outside diameter. The pitch, nominal diameter, thread profile, and taper must all be checked before selecting a mating component.

Clean the Thread

Remove dirt, corrosion, sealant, paint, and PTFE tape before taking measurements. Debris can prevent a pitch gauge from seating correctly and may cause inaccurate diameter readings.

Damaged first threads should not be used as the primary measurement area. Whenever possible, measure a clean and undamaged section farther from the thread end.

Determine Whether the Thread Is External or Internal

External threads are located on the outside of a bolt, screw, pipe, or fitting. Internal threads are located inside a nut, port, coupling, or threaded hole.

External threads are easier to measure directly with a caliper. Internal threads may require plug gauges, bore gauges, or comparison with a known mating component.

Measure the Major Diameter

Use a caliper to measure across the crests of an external thread. Take several measurements and avoid measuring only across a damaged or incomplete thread.

For an internal thread, measure the largest accessible diameter near the opening. This measurement is less precise than an external-thread measurement and should be used together with a pitch gauge and reference chart.

Metric fastener threads usually measure close to their nominal diameter. Pipe thread sizes, however, are nominal and do not match their measured outside diameters.

Measure the Pitch with a Thread Pitch Gauge

A thread pitch gauge is the most convenient tool for identifying pitch. The gauge contains multiple blades marked with metric pitch values or TPI values.

To use it:

Select a blade with a pitch that appears close to the thread.
Place the blade against the thread profile.
Check whether the teeth fit completely into the grooves.
Hold the assembly against a light source and look for gaps.
Try the adjacent gauge sizes to confirm the closest fit.

The correct blade should sit evenly across several threads without rocking, binding, or leaving visible gaps.

Metric and inch thread pitch gauges are not marked in the same way. A blade marked 1.5 normally represents a 1.5 mm metric pitch, while a blade marked 18 normally represents 18 TPI.

Measure Pitch Without a Gauge

If a thread pitch gauge is unavailable, the pitch can be estimated with a caliper or ruler by measuring the axial distance across several complete thread intervals. Measuring multiple intervals reduces the effect of small reading errors and provides a more reliable result than measuring only one thread.

For metric threads, use:

Pitch (mm) = Measured length (mm) ÷ Number of pitch intervals

For example, if 10 pitch intervals cover a distance of 15 mm:

Pitch = 15 ÷ 10 = 1.5 mm

The identified metric thread therefore has a pitch of approximately 1.5 mm.

For inch threads, measure over a known distance and calculate the number of threads per inch:

TPI = Number of pitch intervals ÷ Measured length (inches)

For example, if seven pitch intervals occur over 1/2 inch:

TPI = 7 ÷ 0.5 = 14 TPI

When counting, use the spaces between corresponding thread crests rather than the total number of visible crests. The distance from the first crest to the sixth crest contains five pitch intervals, not six.

For greater accuracy:

Measure across as many complete intervals as possible.
Use a clean, undamaged section of the thread.
Keep the caliper or ruler parallel to the thread axis.
Repeat the measurement at least twice.
Compare the result with a standard metric or inch thread chart.

This method provides an approximate identification. Critical or precision threads should still be verified with a calibrated thread gauge.

Check Whether the Thread Is Parallel or Tapered

Measure the external thread diameter at two locations separated along the thread axis.

If the readings are approximately the same, the thread is likely parallel.
If the diameter changes consistently, the thread is likely tapered.

NPT, NPTF, and BSPT pipe threads have a nominal taper of 1:16 on diameter. This means the diameter changes by approximately 1 unit for every 16 units of axial length.

A short or worn thread may not provide enough length for a reliable taper measurement. In that case, use an appropriate taper gauge or compare the thread against a verified reference component.

Identify the Thread Angle and Profile

Common thread angles include:

Thread system	Thread angle	Typical profile
ISO metric	60°	Symmetrical V-thread
UNC, UNF and UNEF	60°	Symmetrical V-thread
NPT and NPTF	60°	Tapered American pipe thread
BSPP and BSPT	55°	Rounded Whitworth form

A profile projector, optical comparator, thread gauge, or verified reference sample may be required to distinguish similar threads accurately.

Compare the Measurements with a Thread Chart

Once the diameter, pitch, and taper have been measured, compare them with a thread identification chart. Select the thread whose dimensions and characteristics match all observed measurements.

For example, an external thread measuring approximately 20.9 to 21.3 mm may potentially be:

1/2-inch BSP at approximately 20.955 mm
1/2-inch NPT at approximately 21.336 mm
An M21 metric thread, if applicable

Pitch and thread profile are needed to distinguish among them. A 1/2-inch BSP thread has 14 TPI and a 55-degree profile, while a 1/2-inch NPT thread also has 14 TPI but uses a 60-degree profile and different diameter and taper characteristics.

Verify with a Thread Gauge

For maintenance identification, a pitch gauge and caliper may be sufficient. For manufacturing inspection or safety-critical service, verification should be performed with the correct calibrated gauge.

Common inspection tools include:

GO and NO-GO thread plug gauges for internal threads
GO and NO-GO thread ring gauges for external threads
L1 plug and ring gauges for NPT threads
Thread micrometers
Three-wire measuring systems
Optical comparators
Coordinate measuring machines

Do not force an unknown male and female thread together as a method of identification. A partially matching thread can engage for several turns before binding, damaging the flanks, or creating an unsafe connection.

9. How to Select the Correct Thread Pitch

Selecting the correct thread pitch requires balancing strength, assembly speed, adjustment accuracy, material properties, vibration resistance, and product availability. The smallest pitch is not automatically the best choice, and threads with similar dimensions should never be assumed to be compatible.

Match the Existing Thread Standard

When replacing a bolt, fitting, adapter, or instrument connection, first identify the complete thread designation. Confirm the following characteristics:

Nominal diameter or nominal pipe size
Thread pitch or TPI
Parallel or tapered construction
Thread angle and profile
Internal or external thread
Right-hand or left-hand direction
Tolerance or fit class
Required sealing method

For example, identifying a connection only as “1/2-inch thread” is insufficient. It could refer to 1/2-13 UNC, 1/2-20 UNF, G 1/2 BSPP, R 1/2 BSPT, or 1/2-14 NPT. These threads differ in diameter, pitch, profile, taper, and sealing method.

Choose Coarse Threads for General-Purpose Assembly

Coarse threads have a larger pitch and fewer threads per unit length. Common examples include M12 × 1.75 and 1/2-13 UNC.

Coarse threads are generally preferred when:

Fast assembly and disassembly are important
The component is exposed to dirt, corrosion, or minor damage
The female thread is cut into aluminum, cast iron, plastic, or another relatively soft material
Thread stripping is a concern
Frequent field maintenance is expected
Standard hardware availability is important

The deeper thread form and wider spacing make coarse threads more tolerant of contamination and damage. They also require fewer turns to achieve a given engagement length.

Choose Fine Threads for Precise Adjustment

Fine threads have a smaller pitch and more threads per unit length. Examples include M12 × 1.25 and 1/2-20 UNF.

Fine threads may be appropriate when:

Precise axial adjustment is required
Limited radial wall thickness is available
A larger tensile-stress area is desirable for the same nominal diameter
The engagement length is relatively short
Higher clamping control is required
The application uses established fine-thread components

Because fine threads advance a shorter distance per revolution, they provide more accurate positioning. However, their shallow thread depth makes them more vulnerable to cross-threading, contamination, galling, and damage.

Consider the Parent Material

The material containing the internal thread strongly affects pitch selection. A fine pitch creates more engaged threads within a given length, but its shallower thread form may not be ideal for weak or soft materials.

Coarse threads are commonly preferred in:

Aluminum
Magnesium
Cast iron
Plastics
Soft alloys
Components with damaged or contaminated threaded holes

Fine threads are more often used in steel and other materials capable of supporting their smaller thread profile. Thread inserts may be used when a strong, wear-resistant internal thread is required in a soft parent material.

Evaluate Thread Engagement Length

Thread engagement is the axial length over which the external and internal threads contact each other.

An adequate engagement length is necessary to distribute the load and reduce the risk of stripping. The required length depends on:

Fastener diameter
Thread pitch
External and internal thread materials
Tensile load
Shear strength
Tolerance class
Required safety factor

Using more engaged threads does not increase strength indefinitely. The first few engaged threads typically carry a larger portion of the load because loading is not distributed perfectly evenly.

Thread engagement should be verified by engineering calculation or the applicable design standard for critical joints.

Consider Vibration and Dynamic Loading

Fine threads are sometimes selected for applications involving vibration because their smaller lead angle and finer axial movement may offer advantages in maintaining preload. However, thread pitch alone does not guarantee resistance to loosening.

A reliable vibration-resistant joint also depends on:

Correct preload
Tightening method
Friction conditions
Joint stiffness
Fastener elasticity
Locking feature
External loading
Temperature variation

Locknuts, prevailing-torque nuts, mechanical locking devices, safety wire, locking adhesives, or other approved retention methods may be required.

Check Strength and Load Requirements

For the same nominal diameter, fine threads generally have a larger tensile-stress area because their thread roots are not as deep as those of coarse threads. This can provide greater tensile capacity when the fastener material and joint design are otherwise identical.

However, the complete joint must be evaluated for:

Bolt tensile failure
Internal or external thread stripping
Fatigue
Bearing stress
Joint separation
Shear loading
Temperature effects
Corrosion
Required design factor

Do not select a pitch based only on the bolt’s tensile capacity. A high-strength fine-thread bolt can still strip a weak internal thread.

Select the Correct Pipe Thread

For pipe and instrument connections, pitch is only one selection factor. The required thread must also match the connection standard and sealing method.

Thread type	Construction	Typical sealing method
BSPP	Parallel	Gasket, bonded washer, O-ring or sealing face
BSPT	Tapered	Thread interference with suitable sealant
NPT	Tapered	Thread interference with sealant or PTFE tape
NPTF	Tapered Dryseal	Controlled crest-root interference
Metric parallel	Parallel	O-ring, gasket or sealing face, depending on design
SAE straight thread	Parallel	Elastomeric O-ring

Never substitute BSPT for NPT merely because the components appear to have similar diameters or pitches. Use a properly rated adapter when connecting different thread systems.

Consider the Required Adjustment per Revolution

For a single-start thread, axial movement during one complete revolution equals the thread pitch:

Axial movement per revolution = Thread pitch

For a thread with a pitch of 1.5 mm:

Axial movement = 1.5 mm per revolution

For an inch thread, axial movement can be calculated from TPI:

Axial movement per revolution (mm) = 25.4 ÷ TPI

For a 20-TPI thread:

Axial movement = 25.4 ÷ 20 = 1.27 mm per revolution

A smaller pitch produces less axial movement per revolution, providing finer adjustment. A larger pitch produces more axial movement per revolution, allowing faster assembly and disassembly.

Check Commercial Availability

Standard coarse pitches are usually easier to source and less expensive than fine, extra-fine, or special pitches. Before specifying an uncommon pitch, confirm the availability of:

Bolts and screws
Nuts and threaded inserts
Taps and dies
Thread gauges
Replacement parts
Repair equipment
Required material grades and coatings

Using a standard preferred pitch can reduce cost, lead time, and maintenance problems.

Confirm the Tolerance Class

Threads with the same nominal diameter and pitch may have different tolerance classes. The tolerance class controls the allowable variation in thread dimensions and the clearance between mating parts.

Examples include:

Metric external thread: M12 × 1.75–6g
Metric internal thread: M12 × 1.75–6H
Unified external thread: 1/2-13 UNC-2A
Unified internal thread: 1/2-13 UNC-2B

For unified threads:

Class 1 provides a loose fit.
Class 2 provides a general-purpose fit.
Class 3 provides a closer, more controlled fit.
“A” identifies an external thread.
“B” identifies an internal thread.

Surface coatings can change the effective thread dimensions. Galvanizing, plating, painting, or other coatings must therefore be considered when defining thread allowances and tolerances.

Verify Pressure and Temperature Ratings

For fluid systems, a matching thread does not automatically mean that a connection is suitable for service. The complete fitting or component must be rated for the required:

Working pressure
Test pressure
Temperature
Fluid compatibility
Vibration
Thermal cycling
Corrosion exposure
Applicable code or standard

The pressure rating is determined by the complete component design and material, not by pitch alone.

Final Thread Selection Checklist

Before specifying or purchasing a threaded component, verify:

The nominal diameter or pipe size is correct.
The pitch or TPI matches.
The thread profile and flank angle match.
The thread is parallel or tapered as required.
The male and female tolerance classes are compatible.
The engagement length is sufficient.
The selected pitch suits the parent material.
The sealing method is appropriate.
Pressure, temperature, and material ratings are adequate.
The thread can be inspected with the proper gauge.

Conclusion

A thread pitch chart is a practical reference for identifying and comparing metric, UNC, UNF, UNEF, BSP, NPT, and NPTF threads. Metric threads specify the distance between adjacent threads in millimeters, while inch-based systems commonly describe thread spacing in threads per inch.

Thread pitch and TPI can be converted using the following formulas:

Pitch (mm) = 25.4 ÷ TPI

TPI = 25.4 ÷ Pitch (mm)

However, matching pitch values alone does not guarantee thread compatibility. The nominal diameter, thread angle, profile, taper, direction, and tolerance class must also match. This is particularly important when distinguishing between BSP and NPT threads, which may have similar diameters and pitch values but use different thread forms.

For preliminary identification, use a caliper to measure the thread diameter and a thread pitch gauge to determine the pitch or TPI. For manufacturing inspection, pressure-containing connections, or safety-critical applications, confirm the thread using an appropriate calibrated plug, ring, or taper gauge.

Correct thread identification helps prevent cross-threading, leakage, poor engagement, component damage, and premature joint failure. Always verify the complete thread specification before selecting or assembling threaded components.

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