Plumbing Fitting Converter
Convert between NPT (National Pipe Thread) and BSP (British Standard Pipe) thread sizes.
Look up thread dimensions for common pipe sizes.
Select a pipe size — see NPT and BSP thread details.
NPT and BSP are the two main pipe thread standards worldwide.
NPT (National Pipe Thread), used in North America:
- Tapered threads (seal by wedging)
- 60-degree thread angle
- Uses Teflon tape or pipe dope for sealing
BSP (British Standard Pipe), used in most other countries:
- BSPT (tapered) and BSPP (parallel) variants
- 55-degree thread angle
- Parallel threads use a washer or O-ring to seal
Common size equivalents:
| NPT Size | BSP Size | OD (in) | TPI |
|---|---|---|---|
| 1/8" | G1/8" | 0.405 | 27 |
| 1/4" | G1/4" | 0.540 | 18 |
| 3/8" | G3/8" | 0.675 | 18 |
| 1/2" | G1/2" | 0.840 | 14 |
| 3/4" | G3/4" | 1.050 | 14 |
| 1" | G1" | 1.315 | 11.5 |
| 1-1/4" | G1-1/4" | 1.660 | 11.5 |
| 1-1/2" | G1-1/2" | 1.900 | 11.5 |
| 2" | G2" | 2.375 | 11.5 |
NPT and BSP are NOT interchangeable. The thread angles differ (60 vs 55 degrees). Adapters are needed to connect NPT to BSP fittings.
The detail that causes the most leaks is how each thread actually seals. NPT and BSPT are tapered, so they wedge tighter as you screw them in and seal on the threads themselves, which is why they need PTFE tape or pipe dope to fill the spiral gap. BSPP is parallel and seals on a flat washer or O-ring at the face instead, so wrapping tape around a parallel thread does almost nothing. Match the sealing method to the fitting or it will weep no matter how tight you crank it.
Two more traps worth flagging. Like pipe sizes generally, the label is nominal: a “1/2 inch” fitting measures about 0.84 inches across the thread, because the number refers to a rough bore, not anything you can put a ruler on. And the genuinely risky case is the near-miss. A 1/2-inch NPT and a 1/2-inch BSP are close enough to start threading together and feel fine, then they cross-thread and leak under pressure or crack the fitting later. When two fittings won’t quite seat smoothly, a standard mismatch is usually why.
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