ASME B1.20.160° flankpipe thread
National Pipe Taper Threads
The US tapered pipe thread. The size is a nominal bore, not the outside diameter — 1/8 NPT measures 0.405 in across the thread. Sealing comes from the taper deforming against the mating thread, so no gasket is used. 14 designations are listed here, all quoted in inches.
Dimensions14 sizes
| Designation | TPI | Major (in) | Pitch Ø (in) | Minor ext (in) | Tap drill (in) |
|---|---|---|---|---|---|
| 1/16-27 NPT taper | 27 | 0.3125 | 0.2886 | 0.2587 | — |
| 1/8-27 NPT taper | 27 | 0.405 | 0.3811 | 0.3512 | — |
| 1/4-18 NPT taper | 18 | 0.54 | 0.5039 | 0.4594 | — |
| 3/8-18 NPT taper | 18 | 0.675 | 0.639 | 0.5945 | — |
| 1/2-14 NPT taper | 14 | 0.84 | 0.7937 | 0.7366 | — |
| 3/4-14 NPT taper | 14 | 1.05 | 1.0035 | 0.9465 | — |
| 1-11.5 NPT taper | 11.5 | 1.315 | 1.2587 | 1.189 | — |
| 1-1/4-11.5 NPT taper | 11.5 | 1.66 | 1.6035 | 1.5339 | — |
| 1-1/2-11.5 NPT taper | 11.5 | 1.9 | 1.8437 | 1.774 | — |
| 2-11.5 NPT taper | 11.5 | 2.375 | 2.3185 | 2.2488 | — |
| 2-1/2-8 NPT taper | 8 | 2.875 | 2.7937 | 2.6937 | — |
| 3-8 NPT taper | 8 | 3.5 | 3.4189 | 3.3189 | — |
| 3-1/2-8 NPT taper | 8 | 4 | 3.9189 | 3.8189 | — |
| 4-8 NPT taper | 8 | 4.5 | 4.4189 | 4.3189 | — |
Sizes are nominal bores, not diameters — 1/8 measures 0.405 in across the thread. Every size in this table is tapered, so the tap drill column is a dash: a taper is cut with a stepped drill, and there is no single drill size to quote.