BSP and NPT are the two most commonly confused hydraulic and pneumatic threads. They look almost identical, and at some sizes they’ll even start to thread together – but they are not interchangeable, and forcing them together leaks or damages the threads.
The key differences
- Origin: BSP (British Standard Pipe) is used across Australia, the UK, Europe and Asia. NPT (National Pipe Tapered) is the American standard.
- Thread form angle: BSP uses a rounded 55° Whitworth form; NPT uses a sharper 60° form.
- Taper: NPT is always tapered. BSP comes both parallel (BSPP / “G”) and tapered (BSPT / “R”).
- Sealing: NPT seals on the threads (with sealant or PTFE tape). BSPT also seals on the thread, while BSPP seals on a bonded washer or O-ring.
The thread-pitch trap
At some sizes the two share the same threads-per-inch (TPI), which is exactly why they seem to fit:
| Size | BSP TPI | NPT TPI |
|---|---|---|
| 1/4" | 19 | 18 |
| 3/8" | 19 | 18 |
| 1/2" | 14 | 14 |
| 3/4" | 14 | 14 |
| 1" | 11 | 11.5 |
At 1/2" and 3/4" the TPI matches, so the threads engage – but the differing 55° vs 60° form means they never seal properly and will weep or gall. At 1/4", 3/8" and 1" the pitch differs, so they won’t even run together.
How to tell them apart
Measure the TPI with a thread gauge and check the thread form angle. If you need an exact ID method for these and the other common threads (JIC, ORFS, Metric, UNO), see our full thread identification guide.
Need to convert between threads? BSP-to-NPT and other adaptors are in stock.