Pipe threads: NPT, BSPT and BSPP
The wrong thread screws in half a turn and leaks. See how to tell the standards apart, how each one seals and how many threads per inch each size has.
Types of pipe thread
| Thread | Standard | Form | Angle | Where it seals | Typical use |
|---|---|---|---|---|---|
| NPT | ASME B1.20.1 | Tapered 1:16 | 60° | On the thread itself, with tape or paste | Standard in oil and gas, petrochemicals and ASME B16.11 fittings |
| BSPT (R / Rc) | ISO 7-1, EN 10226-1 | Tapered 1:16 | 55° | On the thread itself, with tape or paste | European and Asian equipment, building hydraulics |
| BSPP (G) | ISO 228-1 | Parallel | 55° | Outside the thread: O-ring, washer or gasket | Pressure gauges, instruments, hydraulics |
| NPSM | ASME B1.20.1 | Parallel | 60° | Outside the thread (gasket or seat) | Mechanical joints, hoses |
Threads per inch and diameter, NPT vs. BSP
| Size | NPT: OD (mm) | NPT: threads/in | BSP: major Ø (mm) | BSP: threads/in |
|---|---|---|---|---|
| 1/8" | 10.3 | 27 | 9.73 | 28 |
| 1/4" | 13.7 | 18 | 13.16 | 19 |
| 3/8" | 17.1 | 18 | 16.66 | 19 |
| 1/2" | 21.3 | 14 | 20.96 | 14 |
| 3/4" | 26.7 | 14 | 26.44 | 14 |
| 1" | 33.4 | 11.5 | 33.25 | 11 |
| 1 1/4" | 42.2 | 11.5 | 41.91 | 11 |
| 1 1/2" | 48.3 | 11.5 | 47.80 | 11 |
| 2" | 60.3 | 11.5 | 59.61 | 11 |
| 2 1/2" | 73.0 | 8 | 75.18 | 11 |
| 3" | 88.9 | 8 | 87.88 | 11 |
| 4" | 114.3 | 8 | 113.03 | 11 |
For NPT the thread outside diameter is the pipe OD (ASME B36.10M). For BSP it is the major diameter at the gauge plane (ISO 228-1 / ISO 7-1).
How to identify the thread
- Count the threads per inch with a thread gauge.
- Measure the outside diameter with a caliper and compare with the table.
- Check whether it is tapered or parallel: on a tapered thread the diameter decreases toward the end.
- If in doubt between 1/2" and 3/4", check the thread angle with 55° and 60° gauges.
Flange bolt threads
- Up to 1": UNC thread (coarse series, ASME B1.1).
- Above 1": 8UN thread, with 8 threads per inch in all diameters.
- Usual material: ASTM A193 B7 stud bolt with A194 2H nut; in stainless, B8M with 8M nut; for low temperature, L7 with grade 7 nut.
Bolt diameter and count per flange are in the tables of each class of ASME B16.5 and ASME B16.47. For the class of threaded fittings, see pressure classes.
Frequently asked questions
Are NPT and BSP compatible?
No. The thread angle is different (60° for NPT, 55° for BSP) and, in most sizes, so is the number of threads per inch. In 1/2" and 3/4" both have 14 threads and will screw together, but they do not seal and may leak under pressure.
How do I know whether a thread is NPT or BSP?
Count the threads per inch with a thread gauge and measure the outside diameter. Then check whether it is tapered (the diameter decreases toward the end) or parallel. With 27, 18, 11.5 or 8 threads it is NPT; with 28, 19 or 11 threads it is BSP.
Which thread do ASME flange bolts use?
Per ASME B16.5, stud bolts up to 1" use UNC (coarse) thread and above 1" use 8UN thread, with 8 threads per inch. Nuts follow ASME B18.2.2 and the material is usually ASTM A193 B7 with A194 2H nuts.
Up to what diameter are threaded fittings used?
In process piping, usually up to NPS 2 (sometimes 1 1/2"), and many piping specifications forbid threads in toxic, flammable or cyclic service. Above that, use butt welds or flanges.
