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NPT vs BSPT and BSPP: Pipe Thread Comparison

Guide · Updated October 28, 2026

American NPT and British BSPT pipe threads share the same nominal sizes, the same 1:16 taper and, at 1/2 and 3/4 in, the same 14 threads per inch, so they get mixed up constantly. They are not interchangeable. NPT (ASME B1.20.1) uses a 60° flank angle with flat crests and roots; BSPT, called R for male and Rc for female in ISO 7-1, uses the 55° Whitworth form with rounded crests and roots. BSPP, called G in ISO 228-1, is the parallel British thread that seals on a washer or O-ring, not in the thread. Values below are basic male major diameters from ASME B1.20.1 and ISO 7-1 as tabulated in published thread charts.

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Step by step

  1. Taper or parallel: Lay a straight edge along the crests; a taper of 1:16 on diameter shows over three or four threads. Parallel means BSPP (G) or NPS.
  2. Count the pitch: Use a pitch gage: 27, 18 or 11-1/2 TPI says NPT; 28, 19 or 11 TPI says British.
  3. Check the angle: At 1/2 and 3/4 in both are 14 TPI. Seat a 60° and a 55° gage blade; the one that sits without rocking is the thread form.
  4. Measure the diameter: Mic the male thread a few threads in and compare with the chart, remembering the size is nominal pipe bore, not thread OD.
  5. Check the seal: A bare taper sealed with tape or dope is NPT or R. A flat face, bonded washer or O-ring under the shoulder means a parallel G thread.
  6. Use the right gage: Verify with an L1 ring or plug for NPT, or an ISO 7-1 gage for R threads.

NPT vs BSPT comparison chart

NPT diameters are the gauge-plane major diameter (E1 plus thread height) from ASME B1.20.1; R diameters are the ISO 7-1 gauge-plane major diameter, which is the same basic diameter as G. Above 2 in, NPT drops to 8 TPI while British stays at 11 TPI.

Thread form differences

Rp is a parallel British female thread made to accept a tapered R male; Rc is the tapered British female. A G male will not seal in an Rc port without a face seal.

Sizes that falsely assemble

At 1/2 and 3/4 in the pitch matches and the diameters are within about 0.3 mm, so an NPT male will thread into a BSPT port and tighten. The 5° flank mismatch leaves a spiral leak path that tape may hide at first but that weeps under pressure or vibration. At 1 in and 1-1/4 in the diameters are within 0.1 mm but the 11 vs 11.5 TPI mismatch binds after one or two turns. 1/8, 1/4 and 3/8 in bind quickly because of the 27/28 and 18/19 pitch mismatch.

Worked example: identifying an unknown fitting

A male fitting measures 0.523 in (13.28 mm) across the crests a few threads from the end, and an 18 TPI blade fits cleanly. 18 TPI only occurs on NPT, and 13.3 mm is close to the 1/4 in row, so it is 1/4-18 NPT. If a 19 TPI blade had fit instead, it would be R 1/4 or G 1/4; check for taper to tell which. For a 1/2 in fitting at 14 TPI, only the 60° vs 55° angle check decides.

Worked example: pitch over ten threads

On a 1 in fitting where diameter does not settle it, measure across ten threads. NPT: 10 ÷ 11.5 = 0.870 in (22.09 mm). BSPT: 10 ÷ 11 = 0.909 in (23.09 mm). A 1 mm difference is easy to see with calipers.

Common problems

Taping a BSP fitting into an NPT port because it threaded in, calling every British thread BSPT when it is really parallel G, measuring the fitting OD and expecting it to equal the nominal size, and using an NPT tap for an Rc hole.

FAQ

Are NPT and BSPT interchangeable?

No. The thread angle differs (60° vs 55°) and most sizes have different TPI.

Which sizes have the same TPI?

1/2 and 3/4 in are 14 TPI in both systems.

What is the taper on NPT and BSPT?

Both are 1:16 on diameter, 3/4 in per foot.

What does G mean on a thread?

A parallel British pipe thread per ISO 228-1 (BSPP).

What does R mean on a thread?

A tapered British male pipe thread per ISO 7-1 (BSPT).

How do I tell 1/2 NPT from 1/2 BSPT?

Use a 60° and a 55° thread gage; only the angle differs.

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