Pipe threads: BSP (G/R) vs NPT
Caliper the male thread’s outside (major) diameter and count threads per inch — those two numbers identify the family. BSP cuts a 55° Whitworth form; NPT cuts 60°. At ½″ and ¾″ they share 14 TPI and differ by well under half a millimetre in diameter: they will screw together and they will leak. Never mate the families.
| Size | BSP major Ø (mm) | BSP TPI | Pitch (mm) | NPT major Ø (mm) | NPT TPI |
|---|---|---|---|---|---|
| 1/8″ | 9.728 | 28 | 0.907 | 10.29 | 27 |
| 1/4″ | 13.157 | 19 | 1.337 | 13.72 | 18 |
| 3/8″ | 16.662 | 19 | 1.337 | 17.14 | 18 |
| 1/2″ | 20.955 | 14 | 1.814 | 21.34 | 14 |
| 3/4″ | 26.441 | 14 | 1.814 | 26.67 | 14 |
| 1″ | 33.249 | 11 | 2.309 | 33.40 | 11.5 |
| 1-1/4″ | 41.910 | 11 | 2.309 | 42.16 | 11.5 |
| 1-1/2″ | 47.803 | 11 | 2.309 | 48.26 | 11.5 |
| 2″ | 59.614 | 11 | 2.309 | 60.32 | 11.5 |
BSP majors per ISO 228 (G, parallel) and ISO 7 (R/Rp, taper) — identical majors, different sealing: G seals on a washer or O-ring, R and NPT seal on the tapered thread with tape or dope. NPT major ≈ the pipe OD at the end of the taper.