NPT vs BSP Threads: Differences, Sealing & How to Identify
NPT and BSP are the two pipe thread standards you meet most often, and they are just similar enough to cause real trouble. Both carry fluid and gas, both come in the same nominal sizes, and a fitting from one will happily start to thread into the other. Then it leaks. The mismatch is small on paper (five degrees of thread angle), but it is enough to ruin a seal and, over-tightened, both parts.
This guide covers what actually separates NPT from BSP, why they will not seal together, how each standard makes its seal, and the checks that tell you which one is in your hand.
The core difference: thread angle and taper
Two things define these standards, and both matter for whether a joint seals.
NPT (National Pipe Taper) is an American standard with a 60° thread angle and a tapered thread. The diameter shrinks along the length at a fixed 1-in-16 taper. It follows the Sellers thread form, with flat crests and roots.
BSP (British Standard Pipe) uses a 55° thread angle and the Whitworth form, with rounded crests and roots. BSP comes in two flavours: BSPP (parallel, designated G) and BSPT (tapered, designated R). The parallel BSPP is the one most people mean by "BSP".
Why NPT and BSP will not seal together
When you thread an NPT part into a BSP part, the 60° and 55° flanks contact at only a couple of points instead of along the whole flank. That leaves a spiral gap running the length of the joint. Under pressure, fluid or gas walks straight down that gap. No amount of PTFE tape reliably closes it, and the more you crank to try, the more you deform the softer of the two parts.
Field rule: if a pipe fitting threads in smoothly at first and then feels gritty or unevenly tight, suspect an NPT/BSP mismatch. Back it out and gauge both parts before you seal and pressurise the line.
How each standard seals
This is where the two systems differ most in practice, and it is the part people get wrong.
NPT seals on the threads
Because NPT is tapered, the flanks wedge tighter as you turn. The threads themselves form the seal, and PTFE tape or pipe dope fills the small residual clearance and lubricates assembly. A properly made NPT joint needs no washer.
Parallel BSP (BSPP) seals on a face
BSPP threads are parallel, so they never wedge and never seal on the threads. The seal is made at the fitting shoulder with a bonded washer or an O-ring. Wrapping PTFE tape on a parallel BSPP thread and expecting it to seal is a common and frustrating mistake. The tape does almost nothing there.
Tapered BSP (BSPT) is the middle ground
BSPT is tapered like NPT and does seal on the threads, again with tape or sealant. It still uses the 55° Whitworth angle, so BSPT and NPT remain incompatible with each other despite both being tapered.
How to identify NPT vs BSP
Three quick checks will settle it. You do not need all three, but two agreeing is a safe call.
- Taper or parallel? Lay a straightedge along the crests. If the thread visibly narrows toward the end, it is tapered (NPT or BSPT). If the crests run parallel, it is BSPP.
- Count the TPI with a gauge. At the same nominal size the two standards often differ by a thread or two per inch. A pitch gauge that seats cleanly on one standard's leaf but rocks on the other's is your answer.
- Feel the crest. NPT crests feel sharp and flat (60° Sellers); BSP crests feel rounded (55° Whitworth). It is subtle, but with both types in hand the difference is clear.
For the exact dimensions behind each size, see our NPT thread chart and BSP thread chart, or use the thread cross-reference tool to compare a measured size against both standards at once. If you are working across metric and inch hardware too, the thread measurement guide walks through the caliper-and-gauge method in detail.
Quick comparison
| Property | NPT | BSP |
|---|---|---|
| Thread angle | 60° (Sellers) | 55° (Whitworth) |
| Crest / root | Flat | Rounded |
| Taper | Always tapered | Parallel (BSPP) or tapered (BSPT) |
| Seals on | Threads (tape / dope) | Face washer or O-ring (BSPP); threads (BSPT) |
| Designation | NPT, NPTF | G (parallel), R (tapered) |
| Common region | North America | Europe, Asia, Australia |
The bottom line
NPT and BSP solve the same job with different geometry: 60° flat-crest tapered threads that seal on the threads, versus 55° rounded-crest threads that are usually parallel and seal on a face. The five-degree angle difference means they never mate cleanly, so always confirm the standard before you assemble a pressurised line. A gauge check and a taper check take a minute and save a leak you would otherwise chase for an afternoon.
Frequently asked questions
Are NPT and BSP interchangeable?
No. NPT uses a 60° thread angle and BSP uses 55°, so the flanks do not seat against each other. They may start threading together, but the mismatch leaves a spiral gap that leaks and can damage both parts.
How do I tell if a fitting is NPT or BSP?
Check whether the thread is tapered or parallel, then count the threads per inch with a gauge and feel the crest shape. NPT is always tapered with sharp flat crests; BSPP is parallel with rounded crests. Two of these checks agreeing is a safe call.
Does NPT seal on the threads or need a washer?
NPT seals on the threads. The taper wedges the flanks together and tape or dope fills the gap. Parallel BSPP does not seal on the threads and needs a bonded washer or O-ring at the face.