NPT (National Pipe Taper) threads seal on the thread taper, usually with tape or pipe dope. The nominal pipe size is not the actual outside diameter.
| Pipe size | Threads per inch | Pipe OD (in) | Tap drill |
|---|---|---|---|
| 1/16 | 27 | 0.313 | D |
| 1/8 | 27 | 0.405 | R |
| 1/4 | 18 | 0.540 | 7/16 |
| 3/8 | 18 | 0.675 | 37/64 |
| 1/2 | 14 | 0.840 | 23/32 |
| 3/4 | 14 | 1.050 | 59/64 |
| 1 | 11-1/2 | 1.315 | 1-5/32 |
| 1-1/4 | 11-1/2 | 1.660 | 1-1/2 |
| 1-1/2 | 11-1/2 | 1.900 | 1-47/64 |
| 2 | 11-1/2 | 2.375 | 2-7/32 |
| 2-1/2 | 8 | 2.875 | 2-5/8 |
| 3 | 8 | 3.500 | 3-1/4 |
| 4 | 8 | 4.500 | 4-1/4 |
NPT and BSPT
British pipe threads (BSPT) look similar but have a different thread angle and pitch, and won't seal with NPT. Check the thread type before ordering a fitting.