NPT vs BSP — Pipe Thread Comparison
NPT (National Pipe Taper) and BSP (British Standard Pipe) are the two major pipe thread standards used worldwide. They are not interchangeable despite similar nominal sizes.
| Feature | NPT | BSP |
|---|---|---|
| Standard | ASME B1.20.1 | ISO 228 / ISO 7-1 / BS 21 |
| Thread Angle | 60° | 55° (Whitworth) |
| Thread Form | Flattened crests and roots | Rounded crests and roots |
| Taper | 1:16 (tapered) | BSPT: 1:16 (tapered) BSPP: parallel |
| Seal Method | Thread deformation + sealant (tape/dope) | BSPT: thread deformation BSPP: O-ring or washer seal |
| Primary Use | North America | Europe, Asia, Australia, Africa |
| Compatible? | No — different thread angles make them non-interchangeable. Adapters required. | |
Key Differences
Thread angle: NPT uses a 60° thread angle (same as metric/unified), while BSP uses the Whitworth 55° angle. This means the thread profiles are fundamentally different and cannot mate properly, even when the TPI happens to match.
Sealing: NPT relies on the tapered threads themselves deforming to create a seal, typically assisted by PTFE tape or pipe dope. BSPP (parallel) threads don't seal on the threads at all — they use a bonded washer or O-ring on a machined seat.
Measurement: Nominal pipe size (NPS) refers to a historical bore size, not the actual OD. A "1/2 inch NPT" fitting has an actual OD of 0.840" — the same as "1/2 inch BSP" which has an OD of 0.825". Close enough to thread together a turn or two, but not to seal.
Side-by-Side Dimensions
| Size | NPT TPI | NPT OD (in) | BSP TPI | BSP OD (in) |
|---|---|---|---|---|
| 1/8" | 27 | 0.405 | 28 | 0.383 |
| 1/4" | 18 | 0.54 | 19 | 0.518 |
| 3/8" | 18 | 0.675 | 19 | 0.656 |
| 1/2" | 14 | 0.84 | 14 | 0.825 |
| 3/4" | 14 | 1.05 | 14 | 1.041 |
| 1" | 11.5 | 1.315 | 11 | 1.309 |
| 1-1/4" | 11.5 | 1.66 | 11 | 1.65 |
| 1-1/2" | 11.5 | 1.9 | 11 | 1.882 |
| 2" | 11.5 | 2.375 | 11 | 2.347 |