NPT vs BSP: American and British Pipe Threads Explained
6 min read · updated Oct 2026
Why are there two standards?
In the 1800s, Britain standardised screw threads around Joseph Whitworth's design, with a 55° thread angle. The United States went its own way with William Sellers' 60° design. Pipe threads grew out of both, and the world never merged them. Today, if you buy a tap, pump or garden fitting in Europe, the UK or Australia, it almost certainly has BSP threads. In North America, it's NPT.
The four names you'll see
Each system comes in a tapered version and a straight version:
- NPT — American, tapered. Seals by the threads squeezing together, with tape or paste.
- NPS / NPSM — American, straight. Seals with a washer or O-ring, common on hoses.
- BSPT (marked "R" or "Rc") — British, tapered. Seals on the threads like NPT.
- BSPP (marked "G") — British, straight. Seals with a rubber washer or O-ring. Very common on European taps, filters and pumps.
Side by side
| NPT (American) | BSP (British) | |
|---|---|---|
| Where it's used | USA, Canada | UK, Europe, Australia, Asia, most of the world |
| Thread angle | 60° | 55° |
| Thread tips | Flat | Rounded |
| Shape | Tapered (NPT) or straight (NPS / NPSM) | Tapered (R / BSPT) or straight (G / BSPP) |
| How it seals | Threads wedge tight + tape or paste | Tapered: tape/paste · Straight: rubber washer or O-ring |
| Threads per inch on 1/2" | 14 | 14 |
| Threads per inch on 3/4" | 14 | 14 |
| Threads per inch on 1" | 11.5 | 11 |
Why they won't screw together properly
On the popular 1/2" and 3/4" sizes, both systems have 14 threads per inch. That's the trap: an NPT fitting will start into a BSP one and turn a couple of times, so it feels like it fits. But the 60° and 55° thread shapes don't touch evenly, leaving tiny spiral gaps that water finds. Forcing it further can crack the fitting or strip the threads. On 1" the counts differ (11.5 vs 11), so it will bind after a turn or two.
How to tell which one you have
- Look for markings. "G" or "R" means BSP. "NPT" means American.
- Think about where it came from. Bought or made outside North America? Assume BSP.
- Check for a washer. A flat rubber washer or O-ring in the fitting usually means a straight thread (BSPP or NPSM).
- Count threads on 1". 11.5 per inch is NPT, 11 is BSP.
- Look at the thread tips. Rounded tips point to BSP; flat tips point to NPT. A cheap thread gauge settles it for good.
How to connect NPT to BSP
Don't force them together or rely on extra tape to fill the gap. Use an adapter that has a proper NPT thread on one end and a proper BSP thread on the other. These are hard to find in local hardware stores, which is why printing one is so handy: pick the size and type for each end, and the adapter is made to match both exactly.
A few tips: use PTFE tape on tapered threads (NPT and R), but not on straight G threads — those need a washer. Print with plenty of walls and full infill, and test the fit before relying on it under pressure.
Design your own adapter
Choose a thread for each end, set the lengths, and download a ready-to-print model with a full water passage.
Open the designer →