BSPT vs BSPP vs NPT Thread Guide and Chart
BSP (British Standard Pipe) comes in two forms: BSPT, the tapered thread standardised as R, and BSPP, the parallel thread standardised as G. This guide explains how they differ from each other and from American NPT, and gives the full BSPT/BSPP dimension and tap-drill chart in millimetres and inches.
BSPT vs NPT — the real differences
| Feature | BSPT / BSPP (British) | NPT (American) |
|---|---|---|
| Thread angle | 55° (Whitworth form) | 60° (Sellers form) |
| Crest shape | Rounded | Flat / truncated |
| Taper | BSPT: 1:16 · BSPP: parallel | Always 1:16 taper |
| Sealing | BSPT on flanks + sealant; BSPP on face washer/O-ring | On flanks + sealant |
| Standard | ISO 7 (R), ISO 228 (G) | ASME B1.20.1 |
| Interchangeable with NPT? | No — the 55° vs 60° angle alone makes a leak-free joint impossible. | |
Even where a BSPT fitting threads a few turns into an NPT port, the flanks never fully engage — the joint will weep under pressure. Both are 1:16 taper, but taper rate is not what seals; matching angle, pitch and form are.
BSP vs NPT threads per inch
Same nominal size, different pitch — this is the fastest way to tell them apart at the bench:
| Nominal size | BSP threads per inch | NPT threads per inch |
|---|---|---|
| 1/8" | 28 | 27 |
| 1/4" | 19 | 18 |
| 3/8" | 19 | 18 |
| 1/2" | 14 | 14 |
| 3/4" | 14 | 14 |
| 1" | 11 | 11½ |
| 1-1/2" | 11 | 11½ |
| 2" | 11 | 11½ |
Count the threads per inch on an unknown fitting: 28 TPI on a 1/8 pipe thread means BSP, 27 means NPT. At 1/2 in both are 14 TPI — there the 55°/60° angle and the rounded crest are the tell.
BSPT / BSPP size chart
Basic dimensions for the common BSP sizes — the same numbers apply to BSPP (G) and BSPT (R). Tap drills suit both, since thread depth is identical:
| Size | Major Ø mm | Major Ø in | TPI | Pitch mm | Tap drill mm |
|---|---|---|---|---|---|
| 1/8" | 9.728 | 0.3830 | 28 | 0.907 | 8.8 |
| 1/4" | 13.157 | 0.5180 | 19 | 1.337 | 11.8 |
| 3/8" | 16.662 | 0.6559 | 19 | 1.337 | 15.25 |
| 1/2" | 20.955 | 0.8249 | 14 | 1.814 | 19.0 |
| 3/4" | 26.441 | 1.0410 | 14 | 1.814 | 24.5 |
| 1" | 33.249 | 1.3090 | 11 | 2.309 | 30.75 |
| 1-1/4" | 41.910 | 1.6500 | 11 | 2.309 | 39.5 |
| 1-1/2" | 47.803 | 1.8820 | 11 | 2.309 | 45.5 |
| 2" | 59.614 | 2.3470 | 11 | 2.309 | 57.0 |
Note how far the drill sits from the nominal size: a 1/2 in BSP thread cuts in a 19 mm hole. The nominal figure names the bore of the original pipe, not the thread — same convention as NPT.
What BSPT and BSPP mean
BSP (British Standard Pipe) is the pipe thread family used across the UK, Europe, Asia and Australia. Both variants share the same 55° Whitworth thread form with rounded crests and the same basic dimensions — the difference is the flank:
- BSPT — British Standard Pipe Taper. The thread gets tighter along its length at a 1:16 taper, sealing on the flanks with thread sealant. Standardised in ISO 7 and marked R (male taper) on drawings and fittings.
- BSPP — British Standard Pipe Parallel. Constant diameter along the thread. It does not seal on the flanks; it seals with a bonded seal, O-ring or washer at the face. Standardised in ISO 228 and marked G.
A common shorthand: R = tapered male, G = parallel. If a fitting threads on smoothly with a sealing washer underneath it, that is BSPP; if it tightens and jams progressively like an NPT fitting, that is BSPT.
BSPP vs BSPT — which one seals where
- BSPP (G): used where a face seal is available — banjo fittings, gauge adaptors, ports with a bonded seal or O-ring. Tighten to torque, not feel; the thread itself never seals.
- BSPT (R): used where the thread must seal itself, with PTFE tape or paste — older European plumbing, hydraulic adaptors, pneumatic ports.
- Never mix BSPP into a taper port (or the reverse) without an intermediate sealing washer or an adapter.
BSPT to NPT adapters
Because the standards cannot mate, the universal fix is a dedicated BSPT-to-NPT adapter: one end R-thread, the other NPT, each machined to its own standard. They exist for every common size (1/8, 1/4, 3/8, 1/2...) in both straight and 90° forms. There is no adapter that makes the threads themselves compatible — the adapter simply swaps which standard sits on each side of the joint.