Pipe Thread Sizes - BSP Chart
Verified by Rachel Mayfield, Supply Chain Analyst - April 2026
British Standard Pipe (BSP) thread sizes from 1/8 inch to 4 inch, covering threads per inch, major outside diameter, and tapping drill sizes. Pipe Thread Sizes - BSP Chart BSP threads use a 55-degree thread angle (Whitworth form), which distinguishes them from NPT threads (60-degree). Both parallel (BSPP/G) and taper (BSPT/R) versions share the same thread dimensions - the difference is that BSPT has a 1:16 taper for self-sealing joints.
| BSP Size | Threads/inch | Major OD (mm) | Tapping Drill (mm) | BSPP / BSPT |
|---|---|---|---|---|
| 1/8" | 28 | 9.728 | 8.566 | G1/8 / R1/8 |
| 1/4" | 19 | 13.157 | 11.445 | G1/4 / R1/4 |
| 3/8" | 19 | 16.662 | 14.950 | G3/8 / R3/8 |
| 1/2" | 14 | 20.955 | 18.631 | G1/2 / R1/2 |
| 5/8" | 14 | 22.911 | 20.587 | G5/8 / R5/8 |
| 3/4" | 14 | 26.441 | 24.117 | G3/4 / R3/4 |
| 1" | 11 | 33.249 | 30.291 | G1 / R1 |
| 1 1/4" | 11 | 41.910 | 38.952 | G1-1/4 / R1-1/4 |
| 1 1/2" | 11 | 47.803 | 44.845 | G1-1/2 / R1-1/2 |
| 2" | 11 | 59.614 | 56.656 | G2 / R2 |
| 2 1/2" | 11 | 75.184 | 72.226 | G2-1/2 / R2-1/2 |
| 3" | 11 | 87.884 | 84.926 | G3 / R3 |
| 4" | 11 | 113.030 | 110.072 | G4 / R4 |
How to use the BSP thread chart
Measure the outside diameter of the male thread with calipers and count the threads per inch (TPI) with a thread pitch gauge. Match both values to the table. Note that BSP sizes don't match the actual thread diameter - a 1/2 inch BSP thread has a 20.955 mm OD, not 12.7 mm. The nominal size refers to the approximate bore of the pipe, not the thread diameter.
BSPP (G) parallel threads need a bonded seal or O-ring to seal. BSPT (R) taper threads seal on the thread itself and need PTFE tape or thread sealant. Don't mix them - a parallel male thread into a taper female fitting won't seal properly.
The most common confusion is BSP vs NPT. Both are pipe threads but they're not interchangeable. BSP has a 55-degree thread angle, NPT has 60 degrees. A 1/2 inch BSP has 14 TPI while a 1/2 inch NPT has 14 TPI too, but the diameters differ and they'll cross-thread and leak.
Related standards
- BS 21 - Pipe threads for tubes and fittings where pressure-tight joints are made on the threads (BSPT)
- ISO 7-1 - Pipe threads where pressure-tight joints are made on the threads (taper)
- ISO 228-1 - Pipe threads where pressure-tight joints aren't made on the threads (parallel)
- BS EN ISO 228-1 - UK implementation of parallel pipe threads
- BS 2779 - Specification for pipe threads (parallel) for mechanical joints
References
- BS 21:1985 - Pipe threads for pressure-tight joints (taper)
- ISO 7-1:1994 - Pipe threads - tapered
- ISO 228-1:2000 - Pipe threads - parallel
Frequently Asked Questions
What is the OD of a 1/2 inch BSP thread?
A 1/2 inch BSP thread has a major outside diameter of 20.955 mm (0.825 inches). The nominal 1/2 inch size refers to the approximate pipe bore, not the thread diameter. The tapping drill size for a 1/2 inch BSP internal thread is 18.631 mm.
What is the difference between BSPP and BSPT threads?
BSPP (G) is British Standard Pipe Parallel - the thread has a constant diameter along its length. BSPT (R) is British Standard Pipe Taper - the thread tapers at 1:16 (about 1 degree 47 minutes). BSPT seals on the thread form itself. BSPP needs a bonded seal, O-ring, or washer to seal. Both use the same 55-degree Whitworth thread angle.
Can I use BSP and NPT fittings together?
No. BSP and NPT threads are not interchangeable. BSP uses a 55-degree thread angle while NPT uses 60 degrees. Even where the TPI count matches (e.g. 1/2 inch at 14 TPI), the thread diameters and profiles differ. Forcing them together will cross-thread, leak, and likely damage both components. Use an adapter fitting to convert between systems.
What are the thread angles and types of threads specified in the Pipe Thread Sizes - BSP Chart?
The Pipe Thread Sizes - BSP Chart specifies a 55-degree thread angle for both parallel (BSPP/G) and taper (BSPT/R) versions. These threads differ from NPT threads, which have a 60-degree angle.
How do you distinguish between BSPP and BSPT in the chart?
In the Pipe Thread Sizes - BSP Chart, BSPP and BSPT are distinguished by their applications; BSPP is parallel with no taper, while BSPT has a 1:16 taper designed for self-sealing joints. Both types share the same thread dimensions but differ in the presence of a taper.
What tapping drill sizes are listed for 3/4 inch BSP threads according to the chart?
For 3/4 inch BSP threads, the Pipe Thread Sizes - BSP Chart lists a major outside diameter (OD) of 26.441 mm and a tapping drill size of 24.117 mm.
How does one choose between parallel and tapered BSP threads for pipe connections?
When choosing between BSPP and BSPT in the Pipe Thread Sizes - BSP Chart, consider whether you need a seal without additional sealing compound (BSPT) or if a simpler connection without taper is sufficient (BSPP).
What are some UK suppliers mentioned regarding BSP fittings based on this chart?
The Pipe Thread Sizes - BSP Chart doesn't list specific suppliers but mentions that UK engineers can find BSP fittings from manufacturers and distributors such as Ferguson, Wolseley, and Ryco.
How does the availability of different sizes in the chart affect cost for UK projects?
Availability of different sizes in the Pipe Thread Sizes - BSP Chart varies by supplier. Smaller sizes like 1/8 inch may be more readily available than larger ones such as 4 inches, impacting cost and lead time for UK project requirements.
What are the thread counts per inch for a 3/8 inch BSP size listed on this chart?
The Pipe Thread Sizes - BSP Chart shows that a 3/8 inch BSP size has 19 threads per inch with a major outside diameter of 16.662 mm and a tapping drill size of 14.950 mm.