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BSP Thread Chart

Author: Emma Clarke, Reference Desk3 min readReviewed

BSP threads use a 55 degree Whitworth form with rounded crests, and like NPT the size is nominal: a 1/2 inch BSP thread measures 20.96mm, or 0.825 inch, across the outside. BSPP is parallel and marked G; BSPT is tapered and marked R.

Pipe, fittings & flanges. Based on ISO 228-1, ISO 7-1, BS 21.

BSP thread dimensions

Outside diameter and threads per inch are the two numbers that identify a BSP thread. Both BSPP and BSPT share the same diameters and pitches at a given size, so this table covers either.

BSP male thread dimensions per ISO 228 and ISO 7.
SizeTPIThread OD (mm)Thread OD (in)Pitch (mm)
1/8289.730.3830.907
1/41913.160.5181.337
3/81916.660.6561.337
1/21420.960.8251.814
5/81422.910.9021.814
3/41426.441.0411.814
11133.251.3092.309
1 1/41141.911.6502.309
1 1/21147.801.8822.309
21159.612.3472.309
2 1/21175.182.9602.309
31187.883.4602.309

BSPP against BSPT

BSPP is parallel. The threads do not seal on their own, so the joint needs a bonded seal washer under the head, an O-ring in a machined face, or a soft washer. Marked G in ISO 228.

BSPT is tapered at 1 in 16 and seals on the thread flanks with a sealant, exactly like NPT does. Marked R in ISO 7.

A BSPT male will go into a BSPP female and seal acceptably, which is a common and permitted combination. The reverse, a parallel male into a tapered female, does not seal and should not be used.

Telling BSP from NPT

The threads look similar and the sizes overlap, which is why this catches people out. The two differences that matter are thread angle, 55 degrees for BSP against 60 for NPT, and thread count at some sizes.

At 1/2 inch both are 14 TPI and the diameters are close, so the two will start together and then bind after two or three turns. At 1/4 inch the counts differ, 19 TPI for BSP against 18 for NPT, and a thread gauge separates them immediately.

The reliable test is a thread pitch gauge plus a caliper on the OD. If a fitting starts easily and jams early, stop and measure rather than forcing it, because forcing a mismatched pair damages both.

Where BSP and NPT diverge.
SizeBSP TPINPT TPIDistinguishable by TPI?
1/82827Yes, just
1/41918Yes
3/81918Yes
1/21414No, measure the angle
3/41414No, measure the angle
11111.5Yes
Emma Clarke, Reference Desk editor at Compel GroupEmma ClarkeReference Desk

Emma looks after the reference charts and calculators. Every figure is copied from the named standard on the page rather than generated, and each page shows the date it was last checked. When a value looks wrong, the standard wins and the page gets fixed.

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Reference

BSP Thread Chart questions

What is a BSP thread?
British Standard Pipe, a thread form with a 55 degree Whitworth profile and rounded crests and roots. It is the pipe thread standard across the UK, Europe, Australia and much of Asia, where the US uses NPT.
What is the difference between BSPP and BSPT?
BSPP is parallel and marked G, sealing on a bonded washer or O-ring at the face. BSPT is tapered at 1 in 16 and marked R, sealing on the thread flanks with sealant.
Can BSP and NPT be used together?
No. The thread angles differ, 55 degrees against 60, so the flanks never mate properly. At 1/2 and 3/4 inch both are 14 TPI and will start together before binding, which is exactly how damaged ports happen. Use an adapter.
What size is a 1/2 inch BSP thread?
20.96mm, or 0.825 inch, across the outside of the male thread, at 14 threads per inch. Like NPT, the nominal size refers to the historic bore of the pipe, not to anything measurable on the fitting.
What does G mean on a fitting?
G marks a parallel BSPP thread under ISO 228. R marks a tapered BSPT external thread under ISO 7, and Rp marks a parallel internal thread in that same standard.

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