BSPT Thread Size Chart: Dimensions, TPI, Pitch & Taper

BSPT threads are widely used in piping, hydraulic, pneumatic, instrumentation, and industrial fluid systems around the world. The term BSPT stands for British Standard Pipe Taper, a tapered pipe thread designed to create a secure mechanical connection between pipes, fittings, valves, and other threaded components.

Unlike parallel pipe threads, BSPT threads gradually change in diameter along the threaded length. When a tapered male thread is tightened into a compatible female thread, the thread flanks progressively engage, creating a tight mechanical fit. In most applications, a suitable thread sealant such as PTFE tape or pipe-thread sealing compound is also used to help prevent leakage.

One of the most important points when selecting a BSPT connection is that the nominal thread size does not directly correspond to the actual outside diameter of the thread. For example, a 1/2-inch BSPT thread has an outside diameter significantly larger than 1/2 inch. For this reason, thread identification normally requires checking several characteristics, including outside diameter, thread pitch, threads per inch (TPI), thread angle, and taper.

This BSPT thread size chart provides a practical reference for common BSPT sizes, thread diameters, pitches, TPI values, and thread designations. It also explains how BSPT threads are measured, how they differ from BSPP and NPT threads, and how to select the correct connection for an industrial piping or fluid system.

1. What Is a BSPT Thread?

BSPT, or British Standard Pipe Taper, is a tapered pipe thread based on the British Standard Pipe thread system. It is commonly used to connect pipes, fittings, valves, pressure instruments, pneumatic equipment, hydraulic components, and other fluid-handling devices.

The defining characteristic of a BSPT thread is its tapered thread form. The thread diameter becomes progressively smaller toward the end of the male fitting. BSPT threads use a nominal taper of 1:16 on diameter, meaning that the thread diameter changes by approximately 1 unit for every 16 units of axial length.

BSPT threads also use the traditional Whitworth thread profile, which has a 55-degree included thread angle with rounded roots and crests. This geometry distinguishes BSP threads from thread systems such as NPT, which normally use a 60-degree thread profile.

Under the ISO 7-1 designation system, tapered and mating pipe threads are commonly identified using the following symbols:

• R – External tapered pipe thread
• Rc – Internal tapered pipe thread
• Rp – Internal parallel pipe thread intended to mate with an R external tapered thread

For example, R 1/2 identifies a 1/2-inch nominal external tapered BSP thread, while Rc 1/2 identifies a compatible internal tapered thread.

An important characteristic of BSPT sizing is that the stated thread size is a nominal pipe size rather than the actual measured thread diameter. A nominal 1/2-inch BSPT male thread, for example, has an outside diameter of approximately 20.955 mm (0.825 in). Therefore, measuring only the fitting with a ruler and matching the result directly to the nominal size can easily lead to incorrect identification.

BSPT connections are widely found in industrial equipment manufactured in the United Kingdom, Europe, Asia, Australia, and many other regions. Typical applications include:

• Compressed-air systems
• Hydraulic piping
• Process instrumentation
• Water and utility piping
• Pumps and compressors
• Pressure gauges and transmitters
• Industrial valves and regulators
• Pneumatic cylinders and actuators
• General-purpose pipe fittings

Because BSPT, BSPP, and NPT connections can sometimes appear visually similar, correctly identifying the thread diameter, pitch, taper, and thread profile is essential before connecting components. Using incompatible thread standards can result in poor engagement, damaged threads, or leakage under pressure.

2. BSPT Thread Terminology and Dimensions

Understanding BSPT thread terminology is essential when identifying a fitting or using a BSPT thread size chart. A BSPT thread is defined by several dimensions, including the nominal size, major diameter, pitch, threads per inch, thread angle, and taper.

Because BSPT uses nominal pipe sizes, the size marked on a fitting should not be confused with its actual measured thread diameter.

Nominal BSPT Size

The nominal BSPT size is the trade size used to identify the connection. Typical designations include:

• R 1/8
• R 1/4
• R 3/8
• R 1/2
• R 3/4
• R 1
• R 1 1/2
• R 2

The nominal size historically related to the approximate bore of the pipe rather than the outside diameter of the thread. Therefore, the actual thread diameter is considerably larger than the nominal size.

For example:

Nominal BSPT Size	Major Diameter
1/8″	9.728 mm
1/4″	13.157 mm
1/2″	20.955 mm
3/4″	26.441 mm
1″	33.249 mm

As a result, a measured thread OD of approximately 21 mm normally indicates a 1/2-inch BSP thread, not a 3/4-inch or 1-inch thread.

Major Diameter

The major diameter is the largest theoretical diameter of the thread.

For an external BSPT thread, it is associated with the diameter across the thread crests. However, because BSPT is tapered, the measured outside diameter changes depending on where along the thread the measurement is taken.

The standardized diameter is referenced at a defined gauge position rather than simply at the extreme end of the fitting.

This is an important consideration when identifying BSPT threads with a caliper. A field measurement may differ slightly from the nominal value shown in a BSPT thread chart.

Minor Diameter

The minor diameter is the diameter measured near the roots of the external thread or corresponding crests of the internal thread.

It is smaller than the major diameter and is primarily important in thread design, machining, tapping, and inspection.

For example, the basic major and minor diameters for a 1/2-inch BSP thread are approximately:

• Major diameter: 20.955 mm
• Minor diameter: 18.631 mm

Pitch Diameter

The pitch diameter is an imaginary diameter where the width of the thread ridge and the width of the thread groove are theoretically equal.

Pitch diameter is particularly important for:

• Thread engagement
• Thread gauging
• Manufacturing tolerances
• Inspection
• Determining whether mating threads will assemble correctly

For tapered BSPT threads, the pitch diameter changes along the axial length because the thread itself follows the specified taper.

Threads per Inch (TPI)

BSP threads are commonly identified by the number of thread crests occurring over one inch of axial thread length. This is called threads per inch, or TPI.

Common BSPT pitches include:

TPI	Pitch
28	0.907 mm
19	1.337 mm
14	1.814 mm
11	2.309 mm

For example:

• 1/8″ BSPT = 28 TPI
• 1/4″ BSPT = 19 TPI
• 3/8″ BSPT = 19 TPI
• 1/2″ BSPT = 14 TPI
• 3/4″ BSPT = 14 TPI
• 1″ BSPT = 11 TPI

TPI is therefore one of the most useful measurements for distinguishing BSPT thread sizes.

Thread Pitch

Thread pitch is the axial distance from one thread crest to the next.

For an inch-based thread specified in TPI, the metric pitch can be calculated using:

Pitch (mm) = 25.4 / TPI

For example, a 14-TPI BSP thread has a pitch of:

25.4 / 14 = 1.814 mm

Likewise, an 11-TPI BSP thread has a pitch of approximately:

25.4 / 11 = 2.309 mm

55-Degree Thread Angle

BSPT threads use the British Standard Whitworth thread form, characterized by a 55° included angle between the two thread flanks.

The thread profile also features rounded crests and roots.

This is an important difference between BSPT and NPT threads. NPT uses a 60° thread angle, so BSPT and NPT should not be treated as equivalent even when their nominal sizes appear similar.

BSPT Taper

One of the defining dimensions of a BSPT thread is its taper.

BSPT tapered threads use a taper of:

1:16 on diameter

This means the thread diameter changes by 1 unit for every 16 units of axial distance.

The equivalent half-angle of the taper is approximately:

1° 47′

The gradual increase in thread diameter causes progressively greater interference as the male thread is screwed into the mating female connection.

Gauge Length

The major diameter listed in a BSPT thread size chart should not be interpreted as the diameter at the extreme end of the fitting.

For tapered threads, dimensions are referenced at a defined gauge plane or gauge length.

For example, typical gauge lengths include approximately:

• 1/16″ BSPT: 4.0 mm
• 1/8″ BSPT: 4.0 mm
• 1/4″ BSPT: 6.0 mm
• 1/2″ BSPT: 8.2 mm
• 1″ BSPT: 10.4 mm

This explains why an actual OD measurement taken at the first thread can be slightly smaller than the major diameter shown in a chart.

Summary of BSPT Thread Geometry

The main characteristics used to identify BSPT threads are therefore:

Characteristic	BSPT Specification
Thread form	Whitworth
Thread angle	55°
Thread type	Tapered
Taper	1:16 on diameter
Size system	Nominal pipe size
Pitch specification	TPI
External tapered designation	R
Internal tapered designation	Rc
Compatible internal parallel designation	Rp

These characteristics should be considered together when identifying an unknown pipe thread rather than relying only on its measured outside diameter.

3. BSPT Thread Standards and Designations

BSPT threads are standardized primarily under ISO 7-1, which specifies thread form, dimensions, tolerances, and designations for pipe threads where pressure-tight joints are made on the threads.

ISO 7-1 covers nominal thread sizes from 1/16 inch through 6 inches and includes tapered external threads, tapered internal threads, and parallel internal threads intended for pressure-tight threaded joints.

ISO 7-1

The principal international standard associated with BSPT jointing threads is:

ISO 7-1 – Pipe threads where pressure-tight joints are made on the threads – Part 1: Dimensions, tolerances and designation.

ISO 7-1 defines characteristics including:

• Thread form
• Nominal thread sizes
• Pitch
• Major and minor diameters
• Taper
• Gauge lengths
• Tolerances
• External and internal thread designations

The standard applies to pipework, valves, fittings, and other pipeline components connected using threads intended to form pressure-tight joints.

R – External Tapered Thread

The letter R identifies an external tapered pipe thread.

Examples include:

• R 1/8
• R 1/4
• R 1/2
• R 3/4
• R 1
• R 2

A designation such as:

R 1/2

therefore indicates an external tapered pipe thread having a nominal BSP size of 1/2 inch.

Rc – Internal Tapered Thread

The designation Rc identifies an internal tapered pipe thread.

Examples include:

• Rc 1/4
• Rc 1/2
• Rc 3/4
• Rc 1

An R male thread may be used with an appropriate Rc female tapered thread.

When tightened, the two tapered threads progressively engage and create the mechanical interference required for a pressure-tight threaded connection.

Rp – Internal Parallel Thread

The designation Rp identifies an internal parallel pipe thread intended to mate with an R tapered external thread.

For example:

R 1/2 × Rp 1/2

represents a tapered male BSP thread connected to a compatible parallel female jointing thread.

Although Rp is parallel rather than tapered, it belongs to the ISO 7 pressure-tight jointing thread system and should not be confused with the G thread designation used under ISO 228.

R, Rc, Rp, and G Thread Designations

The following table summarizes the most commonly encountered BSP-related thread symbols:

Designation	Thread Type	Male/Female	Standard	Pressure-Tight on Thread
R	Tapered	Male	ISO 7-1	Yes
Rc	Tapered	Female	ISO 7-1	Yes
Rp	Parallel	Female	ISO 7-1	Yes, with R male
G	Parallel	Male or Female	ISO 228	No, sealing normally occurs separately

This distinction is particularly important when specifying industrial fittings.

For example:

G 1/2 and R 1/2 do not describe the same type of connection, even though they share the same nominal size and basic 55° BSP thread form.

G threads are parallel, whereas R threads are tapered.

ISO 7-1 vs. ISO 228

ISO 7 and ISO 228 cover different types of BSP pipe-thread connections.

ISO 7-1

• Used where pressure-tightness is made on the threads
• Includes R, Rc, and Rp
• R and Rc are tapered
• Typically requires a suitable thread-jointing medium

ISO 228-1

• Covers parallel pipe threads
• Uses the G designation
• Pressure-tightness is not intended to be achieved by the threads themselves
• Sealing generally relies on an O-ring, bonded washer, gasket, or another sealing surface

Correctly distinguishing ISO 7 BSPT connections from ISO 228 BSPP connections is therefore essential when selecting replacement fittings.

4. Complete BSPT Thread Size Chart

The following BSPT thread size chart covers the standard nominal sizes commonly specified under ISO 7-1 from 1/16 inch through 6 inches. ISO 7-1 defines 15 principal thread sizes within this range.

BSPT Thread Size Chart – Inches and Millimeters

BSPT Size	External Designation	TPI	Pitch (mm)	Major Diameter (mm)	Major Diameter (in)	Minor Diameter (mm)
1/16″	R 1/16	28	0.907	7.723	0.3041	6.561
1/8″	R 1/8	28	0.907	9.728	0.3830	8.566
1/4″	R 1/4	19	1.337	13.157	0.5180	11.445
3/8″	R 3/8	19	1.337	16.662	0.6560	14.950
1/2″	R 1/2	14	1.814	20.955	0.8250	18.631
3/4″	R 3/4	14	1.814	26.441	1.0410	24.117
1″	R 1	11	2.309	33.249	1.3090	30.291
1 1/4″	R 1 1/4	11	2.309	41.910	1.6500	38.952
1 1/2″	R 1 1/2	11	2.309	47.803	1.8820	44.845
2″	R 2	11	2.309	59.614	2.3470	56.656
2 1/2″	R 2 1/2	11	2.309	75.184	2.9600	72.226
3″	R 3	11	2.309	87.884	3.4600	84.926
4″	R 4	11	2.309	113.030	4.4500	110.072
5″	R 5	11	2.309	138.430	5.4500	135.472
6″	R 6	11	2.309	163.830	6.4500	160.872

The major and minor diameters above are basic reference dimensions associated with the BSP thread form. Because BSPT threads are tapered, an outside-diameter measurement taken at another position along the thread will not necessarily equal the tabulated major diameter exactly.

BSPT Size vs. Actual Thread Diameter

One of the most common mistakes when identifying BSPT threads is assuming the nominal size represents the actual outside diameter.

It does not.

For example:

1/4-inch BSPT

• Nominal size: 1/4″
• Actual major diameter: approximately 13.157 mm
• TPI: 19

1/2-inch BSPT

• Nominal size: 1/2″
• Actual major diameter: approximately 20.955 mm
• TPI: 14

3/4-inch BSPT

• Nominal size: 3/4″
• Actual major diameter: approximately 26.441 mm
• TPI: 14

1-inch BSPT

• Nominal size: 1″
• Actual major diameter: approximately 33.249 mm
• TPI: 11

Therefore, thread identification should be based on both diameter and TPI, rather than diameter alone.

Quick BSPT Identification Chart

For field identification, the most common sizes can be simplified as follows:

Measured OD Near Reference Diameter	TPI	Likely BSPT Size
~7.7 mm	28	1/16″
~9.7 mm	28	1/8″
~13.2 mm	19	1/4″
~16.7 mm	19	3/8″
~21.0 mm	14	1/2″
~26.4 mm	14	3/4″
~33.2 mm	11	1″
~41.9 mm	11	1 1/4″
~47.8 mm	11	1 1/2″
~59.6 mm	11	2″

For an unknown fitting, first measure the male thread outside diameter with a caliper. Then use a thread pitch gauge to determine the TPI. Finally, compare both values with the BSPT chart.

For instance, if a male thread measures approximately 20.9 mm and has 14 TPI, it is very likely an R 1/2 BSPT thread.

If the diameter is approximately 26.4 mm with the same 14 TPI pitch, it is instead a 3/4-inch BSPT thread.

This combined diameter-and-pitch method is considerably more reliable than trying to identify a BSPT connection from nominal size or visual appearance alone.

5. BSPT Thread Pitch and TPI Chart

Thread pitch and threads per inch are two of the most useful characteristics for identifying a BSPT thread. While the outside diameter helps narrow down the nominal size, measuring the thread pitch helps confirm the identification and distinguish between several BSPT sizes with similar diameters.

BSPT threads are generally specified using threads per inch (TPI). TPI represents the number of complete thread pitches contained within one inch of axial thread length.

The metric pitch can be calculated from TPI using:

Pitch (mm) = 25.4 / TPI

For example, a BSPT thread with 14 TPI has a pitch of:

25.4 / 14 = 1.814 mm

BSPT threads use only a small number of standard pitches across the common size range, making pitch measurement particularly useful for field identification.

BSPT Thread Pitch and TPI Chart

BSPT Size	TPI	Pitch (mm)
1/16″	28	0.907
1/8″	28	0.907
1/4″	19	1.337
3/8″	19	1.337
1/2″	14	1.814
3/4″	14	1.814
1″	11	2.309
1 1/4″	11	2.309
1 1/2″	11	2.309
2″	11	2.309
2 1/2″	11	2.309
3″	11	2.309
4″	11	2.309
5″	11	2.309
6″	11	2.309

The BSPT size range can therefore be grouped into four main pitch categories:

TPI	Pitch	BSPT Sizes
28	0.907 mm	1/16″, 1/8″
19	1.337 mm	1/4″, 3/8″
14	1.814 mm	1/2″, 3/4″
11	2.309 mm	1″ through 6″

This pattern makes thread identification relatively straightforward.

For example, if the measured thread has:

• Approximately 13.2 mm OD
• 19 TPI

it is likely a 1/4-inch BSPT thread.

If the thread has:

• Approximately 16.7 mm OD
• 19 TPI

it is more likely a 3/8-inch BSPT thread.

The pitch is identical in both cases, so the outside diameter must also be considered.

How to Measure BSPT TPI

The easiest way to determine TPI is to use a thread pitch gauge.

Select the gauge leaf that fits cleanly into the thread grooves. The profile should contact the thread without visible gaps.

A correctly matched BSPT thread gauge should align with several consecutive threads rather than only one or two.

If a pitch gauge is not available, TPI can also be estimated manually.

Measure a known axial distance along the thread, count the number of thread pitches within that distance, and convert the result to threads per inch.

However, a thread pitch gauge is generally faster and more reliable.

Why TPI Alone Is Not Enough

Several BSPT sizes share the same TPI.

For example:

• 1/4″ and 3/8″ both use 19 TPI
• 1/2″ and 3/4″ both use 14 TPI
• Most BSPT sizes from 1″ upward use 11 TPI

Therefore, a correct BSPT identification should combine at least:

Outside diameter + TPI + taper

Checking the 55° BSP thread profile provides an additional confirmation when distinguishing BSPT from other pipe-thread standards.

6. BSPT Thread Taper and Profile

The taper and thread profile are two characteristics that distinguish BSPT from many other pipe-thread systems.

BSPT threads use a 1:16 taper on diameter together with the 55° Whitworth thread form.

These features influence how the male and female threads engage, how the connection is sealed, and whether another pipe-thread standard can safely mate with the BSPT connection.

BSPT 1:16 Taper

A BSPT thread has a nominal taper of:

1 in 16 on diameter

This means that for every 16 units of axial distance along the thread, the diameter changes by approximately 1 unit.

Expressed mathematically:

Change in diameter = Axial length / 16

For example, over an axial thread length of 16 mm, the diameter changes by approximately:

16 / 16 = 1 mm

Over an axial length of 8 mm, the theoretical diameter change is approximately:

8 / 16 = 0.5 mm

Because the thread is tapered, the outside diameter of an R male thread becomes larger as the measurement moves away from the small end toward the fitting body.

This is why measuring BSPT outside diameter at an arbitrary point can produce a slightly different value from that shown in a standard size chart.

BSPT Taper Angle

The 1:16 diameter taper corresponds to a small taper half-angle of approximately:

1° 47′

The included angle between opposite tapered surfaces is approximately twice this value.

The taper is gradual enough to allow several threads to engage before increasing interference generates a tight mechanical connection.

55° Whitworth Thread Profile

BSPT uses the British Standard Whitworth thread form.

The included angle between the two thread flanks is:

55°

The basic profile contains:

• 55° included thread angle
• Rounded thread crests
• Rounded thread roots
• Symmetrical thread flanks

The rounded Whitworth profile is one of the easiest ways to distinguish BSP threads conceptually from NPT threads.

NPT uses a 60° thread angle, while BSPT uses 55°.

BSPT vs. NPT Thread Profile

Although BSPT and NPT fittings may sometimes look very similar, their thread geometries are different.

Feature	BSPT	NPT
Thread angle	55°	60°
Profile	Whitworth	American pipe thread form
Crest/root form	Rounded	Different crest/root geometry
Taper	Approximately 1:16 diameter	1:16 diameter
Pitch series	BSP pitch series	NPT pitch series

The fact that both systems are tapered does not make them interchangeable.

For some nominal sizes, the threads may begin to screw together, but differences in pitch and thread angle can prevent correct flank engagement and reliable sealing.

How the BSPT Taper Creates Thread Engagement

When an R external tapered thread is screwed into an Rc tapered internal thread, the diameters progressively converge.

Initially, the threads engage with relatively little interference.

As the fitting is tightened further, the increasing diameter of the male tapered thread creates progressively greater contact between the mating thread surfaces.

This interference helps create a mechanically secure joint.

However, BSPT threads normally require an appropriate thread-sealing material to fill microscopic spiral leakage paths and surface imperfections.

Common sealing methods include:

• PTFE thread-sealing tape
• Pipe-thread sealant
• Anaerobic thread sealant
• Other sealants approved for the fluid and operating conditions

The correct sealant must be compatible with the process fluid, pressure, temperature, and fitting material.

BSPT Taper vs. BSPP Parallel Thread

BSPT and BSPP use the same basic 55° BSP thread profile but differ fundamentally in thread geometry.

BSPT:

• Tapered thread
• Diameter changes along thread length
• R or Rc designation
• Used for pressure-tight threaded joints

BSPP:

• Parallel thread
• Diameter remains essentially constant
• G designation under ISO 228
• Normally requires a separate sealing surface

This difference can often be identified visually or by measuring the thread diameter at two positions.

7. How to Measure and Identify a BSPT Thread

Correctly identifying a BSPT thread requires more than measuring its outside diameter. A reliable identification should evaluate the diameter, pitch, taper, and thread form.

The following procedure can be used to identify an unknown BSPT male thread.

Step 1: Determine Whether the Thread Is Male or Female

First identify whether the connection has an external or internal thread.

An external tapered BSP thread is normally designated:

R

An internal tapered BSP thread is normally designated:

Rc

An internal parallel thread intended to mate with an R thread is designated:

Rp

This distinction is important because the measurement method differs slightly between male and female threads.

Step 2: Measure the Outside Diameter

For an external male thread, use a digital or vernier caliper to measure the outside diameter across the thread crests.

For example, approximate reference diameters include:

Measured OD	Likely BSPT Size
9.7 mm	1/8″
13.2 mm	1/4″
16.7 mm	3/8″
21.0 mm	1/2″
26.4 mm	3/4″
33.2 mm	1″
41.9 mm	1 1/4″
47.8 mm	1 1/2″
59.6 mm	2″

Because BSPT is tapered, the actual measured diameter depends on the measurement position.

Therefore, treat the OD measurement as an identification guide rather than expecting an exact match to three decimal places.

Step 3: Measure the Thread Pitch

Next, use a thread pitch gauge to identify the thread spacing.

Common BSPT values are:

• 28 TPI
• 19 TPI
• 14 TPI
• 11 TPI

The gauge should fit cleanly between the threads across several consecutive pitches.

If the measured fitting has approximately:

20.9–21.0 mm OD + 14 TPI

the likely identification is:

R 1/2 BSPT

If it measures approximately:

26.4 mm OD + 14 TPI

the likely size is:

R 3/4 BSPT

Step 4: Check Whether the Thread Is Tapered

A BSPT male thread should gradually increase in diameter toward the fitting body.

One practical way to check this is to measure the outside diameter at two different locations along the threaded portion.

For example:

1. Measure near the small end of the thread.
2. Measure again several millimeters closer to the fitting body.

If the second measurement is noticeably larger, the thread is tapered.

For BSPT, the expected taper is approximately 1:16 on diameter.

A parallel BSPP thread should have approximately the same outside diameter along its threaded length.

Step 5: Check the Thread Angle

BSPT uses a 55° Whitworth thread profile.

A dedicated thread-profile gauge or optical measurement system can confirm the thread angle when precise identification is required.

This check is particularly useful when distinguishing BSPT from NPT because NPT uses a 60° profile.

For routine field identification, however, checking diameter, pitch, and taper is usually sufficient to narrow down the thread standard.

Step 6: Compare the Measurements with a BSPT Size Chart

Once the OD and pitch have been measured, compare them with a standard BSPT chart.

For example:

Example 1

Measured values:

• OD: approximately 13.1 mm
• Pitch: 19 TPI
• Thread: tapered

Identification:

R 1/4 BSPT

Example 2

Measured values:

• OD: approximately 20.9 mm
• Pitch: 14 TPI
• Thread: tapered

Identification:

R 1/2 BSPT

Example 3

Measured values:

• OD: approximately 33.2 mm
• Pitch: 11 TPI
• Thread: tapered

Identification:

R 1 BSPT

Step 7: Confirm the Connection Standard

Before installing a fitting, confirm that the mating component uses a compatible thread.

Do not assume that a fitting is BSPT simply because another tapered fitting can begin threading into it.

Check the equipment documentation, fitting marking, manufacturer specification, or applicable thread standard whenever possible.

Particular care should be taken when distinguishing:

• BSPT vs. BSPP
• BSPT vs. NPT
• R vs. Rc
• R vs. Rp
• BSP threads vs. metric threads

Quick BSPT Identification Method

For most field inspections, use the following sequence:

1. Measure thread OD → 2. Measure TPI → 3. Check taper → 4. Compare with BSPT chart

For example:

~21 mm + 14 TPI + tapered = likely R 1/2 BSPT

This method is fast and reliable for identifying most common BSPT fittings used in industrial piping, hydraulic, pneumatic, and instrumentation systems.

If the connection is safety-critical or used at high pressure, final identification should be confirmed from manufacturer documentation or with suitable thread gauges before assembly.

8. BSPT vs. BSPP Thread

BSPT and BSPP are both part of the British Standard Pipe thread family, and both use the same basic 55° Whitworth thread form. However, they differ significantly in geometry, sealing method, and designation.

The most important difference is that BSPT threads are tapered, while BSPP threads are parallel.

BSPT vs. BSPP Comparison

Feature	BSPT	BSPP
Full name	British Standard Pipe Taper	British Standard Pipe Parallel
Thread form	Whitworth	Whitworth
Thread angle	55°	55°
Thread geometry	Tapered	Parallel
Typical designation	R, Rc	G
Main standard	ISO 7-1	ISO 228-1
Pressure-tight on threads	Yes	No
Typical sealing method	Thread sealant	O-ring, gasket, bonded washer, sealing face
Diameter along thread	Changes	Constant
Common applications	Pipe joints, valves, fittings	Hydraulic and pneumatic ports, instrumentation, mechanical connections

BSPT Thread Characteristics

BSPT threads use a 1:16 taper on diameter.

As a male BSPT thread is tightened into a compatible female connection, the increasing thread diameter creates progressively greater mechanical interference.

Common BSPT designations include:

• R – external tapered thread
• Rc – internal tapered thread
• Rp – internal parallel thread intended for use with an R male thread

The sealing function is achieved mainly at the threaded joint, typically with the assistance of an appropriate thread sealant.

BSPP Thread Characteristics

BSPP threads are parallel.

The outside diameter remains essentially constant along the entire threaded portion.

Under ISO 228, parallel BSP threads are commonly identified using the G designation.

Examples include:

• G 1/4
• G 1/2
• G 3/4
• G 1

Because the threads are parallel, tightening them does not create the same wedging action as a tapered BSPT thread.

Instead, sealing normally occurs using a separate sealing element, such as:

• O-ring
• Bonded seal
• Copper washer
• Elastomeric gasket
• Metal sealing face

How to Tell BSPT and BSPP Apart

The easiest field method is to check whether the thread diameter changes along the threaded length.

Measure the diameter near the end of the male thread and then measure again closer to the fitting body.

If the second measurement is larger, the thread is likely tapered.

If both measurements are approximately the same, the thread is likely parallel.

A typical identification sequence is:

Changing OD + 55° profile = likely BSPT

Constant OD + 55° profile = likely BSPP

However, final confirmation should also include nominal diameter and thread pitch.

Can BSPT and BSPP Be Connected?

Certain BSP thread combinations are intentionally designed to work together.

For example, an R tapered male thread may be used with an Rp parallel female thread where the applicable standard and component design permit it.

However, a G thread under ISO 228 should not automatically be assumed to be interchangeable with an R or Rc connection.

The correct connection depends on:

• Thread designation
• Sealing design
• Component specification
• Pressure rating
• Manufacturer requirements

For reliable industrial installations, the entire connection standard should be confirmed rather than relying only on thread size.

9. BSPT vs. NPT Thread

BSPT and NPT are two widely used tapered pipe-thread systems. Because both use tapered threads and similar nominal pipe sizes, they are frequently confused.

However, BSPT and NPT have different thread profiles and, for many sizes, different thread pitches.

They should therefore not normally be treated as interchangeable.

BSPT vs. NPT Comparison Chart

Feature	BSPT	NPT
Full name	British Standard Pipe Taper	National Pipe Taper
Main standards	ISO 7-1	ASME B1.20.1
Thread angle	55°	60°
Thread profile	Whitworth	American pipe thread
Taper	1:16 on diameter	1:16 on diameter
Common designation	R	NPT
Crest/root geometry	Rounded Whitworth profile	Different thread profile
Typical regions	Europe, Asia, UK, Australia	North America
Sealing	Thread interference + sealant	Thread interference + sealant
Direct interchangeability	Generally no	Generally no

Thread Angle Difference

One of the most important differences is the included thread angle.

BSPT uses:

55°

NPT uses:

60°

Although a difference of five degrees may appear small, it changes the geometry of the thread flanks.

When incompatible threads are forced together, the thread surfaces may contact incorrectly rather than engaging across the intended flank area.

This can result in:

• Poor thread engagement
• Localized loading
• Thread deformation
• Difficult assembly
• Leakage
• Reduced connection reliability

BSPT and NPT Pitch Differences

The thread pitch is another important difference.

For example:

Nominal Size	BSPT	NPT
1/8″	28 TPI	27 TPI
1/4″	19 TPI	18 TPI
3/8″	19 TPI	18 TPI
1/2″	14 TPI	14 TPI
3/4″	14 TPI	14 TPI
1″	11 TPI	11.5 TPI
1 1/4″	11 TPI	11.5 TPI
1 1/2″	11 TPI	11.5 TPI
2″	11 TPI	11.5 TPI

This table shows why visual inspection alone can be misleading.

For example, both 1/2-inch BSPT and 1/2-inch NPT use 14 TPI, but the thread angles remain different.

Therefore, matching TPI alone does not prove that two threads are compatible.

Taper Comparison

Both BSPT and NPT use approximately the same basic taper:

1:16 on diameter

This similarity is one reason they are often confused.

A user may see two tapered connections of similar nominal diameter and assume they are the same standard.

However, taper is only one part of thread geometry.

Correct identification must also consider:

• Thread angle
• TPI
• Major diameter
• Thread profile
• Standard designation

Can BSPT and NPT Be Connected?

In some sizes, an NPT fitting may begin to screw into a BSPT port, particularly where the pitch is similar.

This does not mean the connection is correctly matched.

Forcing BSPT and NPT threads together can cause incomplete flank contact and thread damage.

In critical piping, hydraulic, pneumatic, or instrumentation service, use components manufactured to the same specified thread standard or use a properly designed adapter.

Do not rely on excessive PTFE tape or thread sealant to compensate for incompatible thread geometry.

Quick BSPT vs. NPT Identification

A practical identification sequence is:

Step 1: Confirm the thread is tapered.

Step 2: Measure the outside diameter.

Step 3: Measure TPI.

Step 4: Check the thread angle or profile if necessary.

For example:

Approximately 13.2 mm + 19 TPI + 55° profile = likely 1/4″ BSPT

By comparison:

Approximately 13.7 mm + 18 TPI + 60° profile = likely 1/4″ NPT

For high-pressure or safety-critical service, use appropriate thread gauges or manufacturer documentation for final confirmation.

10. How to Select the Correct BSPT Thread Size

Selecting the correct BSPT thread requires more than choosing a nominal pipe size. The thread dimensions, male or female configuration, sealing method, pressure rating, temperature, material, and mating connection must all be considered.

The following process can help reduce selection errors.

Identify the Existing Connection

If replacing an existing fitting, begin by identifying the installed connection.

Check:

• Male or female thread
• Tapered or parallel thread
• Outside or inside diameter
• TPI
• Existing component markings
• Manufacturer documentation

Do not select a replacement based only on the apparent pipe size.

For example, a male thread measuring approximately 21 mm is typically associated with a nominal 1/2-inch BSP thread.

Measure the Thread Diameter

For an external thread, use a caliper to measure across the thread crests.

Compare the result with a BSPT size chart.

Common approximate reference diameters include:

BSPT Size	Major Diameter
1/8″	9.728 mm
1/4″	13.157 mm
3/8″	16.662 mm
1/2″	20.955 mm
3/4″	26.441 mm
1″	33.249 mm
1 1/4″	41.910 mm
1 1/2″	47.803 mm
2″	59.614 mm

Remember that BSPT is tapered, so the measured value varies depending on where along the thread the caliper is positioned.

Confirm the TPI

Use a thread pitch gauge to check the thread pitch.

Typical BSPT values are:

• 28 TPI
• 19 TPI
• 14 TPI
• 11 TPI

The combination of diameter and TPI gives a much more reliable identification than either measurement alone.

For example:

~16.7 mm + 19 TPI = likely 3/8″ BSPT

~26.4 mm + 14 TPI = likely 3/4″ BSPT

Confirm the Thread Designation

Determine whether the connection requires:

• R – male tapered
• Rc – female tapered
• Rp – female parallel mating thread

Do not simply specify “1/2 BSP” when purchasing a fitting if the exact connection type matters.

A better specification would be:

R 1/2 male

or:

Rc 1/2 female

This reduces the risk of receiving a BSPP component when BSPT was required.

Verify BSPT vs. BSPP

Check whether the connection seals through the threads or through a separate sealing surface.

If the thread is tapered and intended to seal at the threaded joint, BSPT is likely.

If the thread is parallel and uses an O-ring, bonded washer, or gasket, the connection may instead be BSPP.

This distinction is particularly important in hydraulic and instrumentation systems where both thread types are commonly encountered.

Verify BSPT vs. NPT

Do not assume that any tapered pipe thread is BSPT.

Before selecting a component, check:

• 55° vs. 60° thread angle
• TPI
• Diameter
• Equipment origin
• Manufacturer specification

Where identification remains uncertain, use dedicated thread gauges.

Select the Correct Sealant

BSPT connections commonly require a suitable sealing material.

Depending on the application, this may include:

• PTFE tape
• Liquid pipe-thread sealant
• Anaerobic thread sealant

The selected sealant should be compatible with:

• Process fluid
• Gas or liquid service
• Temperature
• Pressure
• Fitting material
• Oxygen service, if applicable
• Cleanliness requirements

For specialized services, follow the sealant manufacturer’s instructions and the equipment supplier’s requirements.

Consider Pressure and Temperature

Thread size alone does not determine the allowable working pressure of a fitting.

Pressure capability depends on factors such as:

• Fitting design
• Material
• Wall thickness
• Temperature
• Manufacturing standard
• Thread engagement
• Assembly method

Always use the manufacturer’s pressure-temperature rating rather than estimating pressure capacity solely from the BSPT size.

Check Material Compatibility

BSPT fittings are available in materials such as:

• Stainless steel
• Carbon steel
• Brass
• Bronze
• Plastic
• Other engineering alloys

Material selection should consider:

• Corrosion resistance
• Process-fluid compatibility
• Operating temperature
• Environmental conditions
• Pressure requirements
• Galvanic compatibility

Mixing incompatible metals can introduce corrosion problems even when the thread dimensions themselves are correct.

Avoid Common BSPT Selection Mistakes

Common mistakes include:

• Assuming nominal size equals actual thread OD
• Confusing BSPT with BSPP
• Confusing BSPT with NPT
• Measuring diameter but not TPI
• Ignoring thread taper
• Using incompatible male and female thread standards
• Applying excessive thread sealant
• Over-tightening tapered fittings
• Selecting fittings without checking pressure-temperature ratings

A reliable BSPT selection should therefore follow:

Measure → Identify pitch → Confirm taper → Confirm designation → Verify application requirements

Conclusion

BSPT threads are one of the most widely used tapered pipe-thread systems for industrial piping, hydraulic, pneumatic, instrumentation, and general fluid-handling applications.

A BSPT connection uses a 55° Whitworth thread profile and a 1:16 taper on diameter. Under ISO 7-1, the most common thread designations are R for external tapered threads, Rc for internal tapered threads, and Rp for compatible internal parallel threads.

Because BSPT nominal sizes do not represent the actual measured thread diameter, proper identification should always involve both the thread diameter and pitch.

For example:

• 1/4″ BSPT → approximately 13.157 mm, 19 TPI
• 1/2″ BSPT → approximately 20.955 mm, 14 TPI
• 3/4″ BSPT → approximately 26.441 mm, 14 TPI
• 1″ BSPT → approximately 33.249 mm, 11 TPI

The most reliable field-identification method is:

Measure the diameter → Determine TPI → Check the taper → Compare with a BSPT thread size chart

It is also important to distinguish BSPT from BSPP and NPT. BSPP is parallel and normally seals using a separate sealing element, while NPT uses a different 60° thread profile and a different pitch series for many sizes.

Correct thread identification helps prevent damaged fittings, poor thread engagement, leakage, and incorrect component selection. For critical or high-pressure applications, BSPT connections should always be verified against the applicable standard, manufacturer documentation, or suitable thread gauges before installation.

BSPP vs BSPT Threads Size Chart : Differences & Guide

UN Thread Size Chart: UNC, UNF, UNEF Dimensions

UNC Thread Size Chart: Dimensions, TPI & Tap Drill Sizes