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.
Dimensions
in| Designation | TPI | Major (in) | Pitch Ø (in) | Minor ext (in) | Tap drill (in) |
|---|---|---|---|---|---|
| 1/16-27 NPT taper | 27 | 0.3126 | 0.2886 | 0.2587 | — |
| 1/8-27 NPT taper | 27 | 0.4051 | 0.3811 | 0.3512 | — |
| 1/4-18 NPT taper | 18 | 0.5402 | 0.5039 | 0.4594 | — |
| 3/8-18 NPT taper | 18 | 0.6748 | 0.639 | 0.5945 | — |
| 1/2-14 NPT taper | 14 | 0.8402 | 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 | 1.6598 | 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 | 2.3748 | 2.3185 | 2.2488 | — |
| 2-1/2-8 NPT taper | 8 | 2.8748 | 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 0.3126 in across the thread. Tapered sizes show a dash for tap drill: a taper is cut with a stepped drill, so there is no single drill size to quote.