NPT vs BSP Pipe Threads
NPT and BSP are the two pipe thread systems in general use. They differ in flank angle, crest shape, pitch and diameter, so an NPT fitting does not seal in a BSP port even when the two appear to screw together. NPT is the standard in the United States and Canada; BSP is used almost everywhere else.
| NPT | BSP parallel (G, BSPP) | BSP taper (R, BSPT) | |
|---|---|---|---|
| Standard | ASME B1.20.1 | ISO 228-1 | ISO 7-1, EN 10226 |
| Flank angle | 60° | 55° | 55° |
| Crests and roots | flat | rounded | rounded |
| Thread height | 0.8 × pitch | 0.640 × pitch | 0.640 × pitch |
| Taper | 1 in 16 on diameter | none | 1 in 16 on diameter |
| Seals on | the thread, with sealant | a washer or O-ring at the face | the thread, with sealant |
| Designation | 1/2-14 NPT | G1/2 | R1/2 (male), Rc1/2, Rp1/2 (female) |
Thread form
The 5° difference in flank angle is the reason the two cannot share a gauge or a tap. An NPT male in a BSP female touches only at the crests, leaving the flanks apart, and a thread that does not bear on its flanks neither seals nor carries load properly.
How each one seals
Taper threads (NPT, and BSP taper R into Rc or Rp) wedge together as they are tightened. The wedging gives a mechanical lock, but the crests and roots still leave a spiral passage, so PTFE tape or pipe compound is needed to make the joint tight.
Parallel threads (BSP G) never wedge. ISO 228-1 defines them for joints where the seal is made outside the thread: a bonded washer, an O-ring in a groove or a flat gasket is clamped between the fitting and the face of the port.
Size by size
The nominal sizes look the same but the threads behind them differ. Only the 1/2 and 3/4 sizes have the same pitch in both systems.
| Size | NPT TPI | NPT OD (mm) | NPT pitch (mm) | BSP TPI | BSP major Ø (mm) | BSP pitch (mm) | BSP − NPT Ø (mm) | Pitch |
|---|---|---|---|---|---|---|---|---|
| 1/16 | 27 | 7.94 | 0.941 | 28 | 7.72 | 0.907 | -0.21 | different |
| 1/8 | 27 | 10.29 | 0.941 | 28 | 9.73 | 0.907 | -0.56 | different |
| 1/4 | 18 | 13.72 | 1.411 | 19 | 13.16 | 1.337 | -0.56 | different |
| 3/8 | 18 | 17.14 | 1.411 | 19 | 16.66 | 1.337 | -0.48 | different |
| 1/2 | 14 | 21.34 | 1.814 | 14 | 20.95 | 1.814 | -0.38 | same |
| 3/4 | 14 | 26.67 | 1.814 | 14 | 26.44 | 1.814 | -0.23 | same |
| 1 | 11.5 | 33.40 | 2.209 | 11 | 33.25 | 2.309 | -0.15 | different |
| 1-1/4 | 11.5 | 42.16 | 2.209 | 11 | 41.91 | 2.309 | -0.25 | different |
| 1-1/2 | 11.5 | 48.26 | 2.209 | 11 | 47.80 | 2.309 | -0.46 | different |
| 2 | 11.5 | 60.32 | 2.209 | 11 | 59.61 | 2.309 | -0.71 | different |
| 2-1/2 | 8 | 73.02 | 3.175 | 11 | 75.18 | 2.309 | +2.16 | different |
| 3 | 8 | 88.90 | 3.175 | 11 | 87.88 | 2.309 | -1.02 | different |
| 3-1/2 | 8 | 101.60 | 3.175 | 11 | 100.33 | 2.309 | -1.27 | different |
| 4 | 8 | 114.30 | 3.175 | 11 | 113.03 | 2.309 | -1.27 | different |
Where the pitch is the same, the male will enter the female for a couple of turns and feel as if it fits. This is the dangerous case: the joint can be tightened, but it holds on a few crests and leaks or blows out under pressure. Where the pitch differs by one thread per inch, the parts bind within two or three turns.
Telling them apart
- Count the threads per inch with a thread gauge. 27, 18, 11.5 or 8 means NPT. 28, 19 or 11 means BSP. 14 can be either.
- Look at the crests. Flat crests with sharp corners are NPT; smoothly rounded crests are BSP.
- Check for taper by measuring the diameter at the first and last full thread. NPT and BSP taper (R) narrow toward the end; BSP parallel (G) does not.
- Look for a sealing face. A machined flat or a groove for a washer or O-ring beside the thread means a parallel thread.
- Measure the outside diameter and compare it with the table above.
Connecting NPT to BSP
Use an adapter with one thread of each kind. Adapters are stocked for every common size. Do not rely on extra tape to make mismatched threads hold pressure.
Common questions
Can I screw an NPT fitting into a BSP port?
Not for a pressure-tight joint. In the 1/2 and 3/4 sizes the parts will engage because both have 14 threads per inch, but the flank angles differ and the joint leaks. Use an adapter.
Is BSP tapered or parallel?
Both exist. G (BSPP) is parallel and seals on a washer. R, Rc and Rp (BSPT) are the taper system that seals on the thread.
Which countries use NPT?
The United States and Canada, and equipment built to American standards anywhere, particularly in oil and gas. Most other countries use BSP.
Other charts and tools
Sources: ASME B1.20.1, ISO 228-1 and ISO 7-1. How this site checks its data. Last reviewed 29 September 2026.