Plumbing Fitting Converter

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. The two standards share a nominal name and almost nothing else. Both the outside diameter and the threads per inch differ at most sizes, so the columns below are deliberately kept apart:

NPT and BSP are NOT interchangeable. The thread angles differ (60 vs 55 degrees). Adapters are needed to connect NPT to BSP fittings.

Read the 1/2 and 3/4 inch rows again, because they explain most of the ruined fittings in this subject. At those two sizes both standards happen to use 14 threads per inch and sit within about fifteen thousandths of an inch on diameter. That is close enough to spin together by hand and feel completely normal, and the 5-degree difference in flank angle only bites once you torque it. Everywhere else the thread counts disagree and the mismatch announces itself on the second turn.

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.

One last trap. 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. Don’t try to identify a fitting by measuring it against its name. So when two fittings won’t quite seat smoothly, resist the bigger wrench. A standard mismatch is usually why.