NPT vs BSP: How to Tell Them Apart
They look identical, they thread together, and they will not seal. Here is how to identify which one you are holding.
Why this is genuinely hard
Most thread standards are easy to distinguish because the sizes don't line up. NPT and BSP are the opposite case: they were designed independently around the same nominal pipe sizes, so a 1/2″ fitting in each standard is within a few thousandths of an inch of the other. At 1/2″ and 3/4″ they even share the same threads per inch.
The consequence is that a BSP male will start into an NPT female, take several turns, and feel like it is seating. It then leaks — sometimes immediately, sometimes after a pressure cycle or a thermal cycle, which is worse because the failure happens after the job is signed off.
The comparison, size by size
| Nominal | NPT major | BSPT major | Difference | NPT TPI | BSPT TPI | Pitch |
|---|---|---|---|---|---|---|
| 1/8″ | 0.405″ | 0.383″ | 22 thou | 27 | 28 | different |
| 1/4″ | 0.54″ | 0.518″ | 22 thou | 18 | 19 | different |
| 3/8″ | 0.675″ | 0.656″ | 19 thou | 18 | 19 | different |
| 1/2″ | 0.84″ | 0.825″ | 15 thou | 14 | 14 | same |
| 3/4″ | 1.05″ | 1.041″ | 9 thou | 14 | 14 | same |
| 1″ | 1.315″ | 1.309″ | 6 thou | 11.5 | 11 | different |
| 1-1/4″ | 1.66″ | 1.65″ | 10 thou | 11.5 | 11 | different |
| 1-1/2″ | 1.9″ | 1.882″ | 18 thou | 11.5 | 11 | different |
| 2″ | 2.375″ | 2.347″ | 28 thou | 11.5 | 11 | different |
| 2-1/2″ | 2.875″ | 2.96″ | 85 thou | 8 | 11 | different |
| 3″ | 3.5″ | 3.46″ | 40 thou | 8 | 11 | different |
| 4″ | 4.5″ | 4.46″ | 40 thou | 8 | 11 | different |
Four ways to identify a pipe thread
1. Thread angle gauge. The definitive test. A 55/60° pitch gauge costs very little and settles it in seconds. NPT is 60°, BSP is 55°. If you work on imported equipment at all, own one.
2. Count threads per inch. Works everywhere except 1/2″ and 3/4″, where both standards use 14 TPI. At 1/8″ NPT is 27 and BSP is 28 — a difference you will only catch with a gauge, not by eye.
3. Measure the outside diameter. Compare against the table above. The gap ranges from 6 thou at 1″ to 85 thou at 2-1/2″, so a caliper resolves it everywhere — but at the tighter end a tape measure or a rule will not. The 1″ and 1-1/4″ sizes are the ones to be careful with.
4. Where did the equipment come from? Not proof, but a strong prior. North American plumbing, pneumatics and hydraulics are overwhelmingly NPT. European, British, Australian, Japanese, Chinese and Indian equipment is usually BSP. A machine imported into the US frequently has BSP ports with NPT adapters already fitted — and those adapters get lost.
BSPP and BSPT are also different from each other
BSP splits into two families that are commonly confused with each other as well:
| BSPT (R) | BSPP (G) | |
|---|---|---|
| Form | Tapered | Parallel |
| Seals on | The thread itself, with sealant | A bonded washer or O-ring at the face |
| Marking | R1/2, Rc1/2 | G1/2 |
| Sealant on threads | Yes | No — it will not help and may hide the real problem |
A BSPP male can thread into a BSPT female, which is a common source of joints that never quite seal. If a fitting has a captive washer or a smooth sealing face, it is parallel and the seal happens at that face, not in the threads.
If you have to connect NPT to BSP
Use a proper adapter with one standard on each end. Do not force a mismatched pair and compensate with extra sealant — the joint may hold on the bench and fail under pressure, vibration or thermal cycling. In gas, hydraulic or steam service that is a safety issue rather than an inconvenience.