NPT Thread Chart (National Pipe Taper)
All 19 NPT sizes from 1/16 to 12 inch, with threads per inch, pipe outside diameter, pitch diameters, engagement lengths and tap drills from ASME B1.20.1. NPT is the taper pipe thread used in the United States and Canada.
| Thread | TPI | Pipe OD (in) | Pipe OD (mm) | Tap drill | ASME B1.20.1 drill | E0 (in) | E1 (in) | L1 (in) | L2 (in) | K0 (in) |
|---|---|---|---|---|---|---|---|---|---|---|
| 1/16-27 NPT | 27 | 0.312 | 7.94 | D | C | 0.27118 | 0.28118 | 0.160 | 0.2611 | 0.2416 |
| 1/8-27 NPT | 27 | 0.405 | 10.29 | R | Q | 0.36351 | 0.37360 | 0.162 | 0.2639 | 0.3339 |
| 1/4-18 NPT | 18 | 0.540 | 13.72 | 7/16" | 7/16" | 0.47739 | 0.49163 | 0.228 | 0.4018 | 0.4329 |
| 3/8-18 NPT | 18 | 0.675 | 17.14 | 37/64" | 9/16" | 0.61201 | 0.62701 | 0.240 | 0.4078 | 0.5676 |
| 1/2-14 NPT | 14 | 0.840 | 21.34 | 23/32" | 45/64" | 0.75843 | 0.77843 | 0.320 | 0.5337 | 0.7013 |
| 3/4-14 NPT | 14 | 1.050 | 26.67 | 59/64" | 29/32" | 0.96768 | 0.98887 | 0.339 | 0.5457 | 0.9105 |
| 1-11.5 NPT | 11.5 | 1.315 | 33.40 | 1-5/32" | 1-9/64" | 1.21363 | 1.23863 | 0.400 | 0.6828 | 1.1441 |
| 1-1/4-11.5 NPT | 11.5 | 1.660 | 42.16 | 1-1/2" | 1-31/64" | 1.55713 | 1.58338 | 0.420 | 0.7068 | 1.4876 |
| 1-1/2-11.5 NPT | 11.5 | 1.900 | 48.26 | 1-47/64" | 1-23/32" | 1.79609 | 1.82234 | 0.420 | 0.7235 | 1.7265 |
| 2-11.5 NPT | 11.5 | 2.375 | 60.32 | 2-7/32" | 2-3/16" | 2.26902 | 2.29627 | 0.436 | 0.7565 | 2.1995 |
| 2-1/2-8 NPT | 8 | 2.875 | 73.02 | 2-5/8" | 2-39/64" | 2.71953 | 2.76216 | 0.682 | 1.1375 | 2.6195 |
| 3-8 NPT | 8 | 3.500 | 88.90 | 3-1/4" | — | 3.34062 | 3.38850 | 0.766 | 1.2000 | 3.2406 |
| 3-1/2-8 NPT | 8 | 4.000 | 101.60 | 3-3/4" | — | 3.83750 | 3.88881 | 0.821 | 1.2500 | 3.7375 |
| 4-8 NPT | 8 | 4.500 | 114.30 | 4-1/4" | — | 4.33438 | 4.38712 | 0.844 | 1.3000 | 4.2344 |
| 5-8 NPT | 8 | 5.563 | 141.30 | 5-5/16" | — | 5.39073 | 5.44929 | 0.937 | 1.4063 | 5.2907 |
| 6-8 NPT | 8 | 6.625 | 168.27 | 6-3/8" | — | 6.44609 | 6.50597 | 0.958 | 1.5125 | 6.3461 |
| 8-8 NPT | 8 | 8.625 | 219.07 | — | — | 8.43359 | 8.50003 | 1.063 | 1.7125 | 8.3336 |
| 10-8 NPT | 8 | 10.750 | 273.05 | — | — | 10.54531 | 10.62094 | 1.210 | 1.9250 | 10.4453 |
| 12-8 NPT | 8 | 12.750 | 323.85 | — | — | 12.53281 | 12.61781 | 1.360 | 2.1250 | 12.4328 |
Where the diameters are measured
Because the thread is tapered it has no single pitch diameter. E0 is the pitch diameter at the end of the pipe, E1 at the plane where a fitting stops when screwed on by hand, and E2 at the end of the effective thread. The three are tied together by the taper: E1 = E0 + L1 ÷ 16.
Two tap drill columns
The chart gives two drills because published sources differ. The first column is the size printed on standard tap drill charts and in Machinery’s Handbook, and is the one most shops use. The second is the drill ASME B1.20.1 suggests in its appendix for tapping without reaming, which the standard lists up to the 2-1/2 size.
Where the two differ, the chart size is the larger by 1/64 in. It taps more easily but leaves shallower threads at the bottom of the hole, so the smaller drill is the better choice for high pressure or soft material.
No drill is published for 8, 10 and 12 inch NPT. Threads of that size are bored or milled to the minor diameter K0.
Nominal size is not a dimension
NPT sizes name the pipe, following the nominal pipe size (NPS) system. A 1/2 NPT thread is cut on pipe with an outside diameter of 0.840 in (21.34 mm), and nothing on it measures half an inch. To identify a pipe thread, measure the outside diameter and count the threads per inch, then match them to the chart.
Common questions
What is the tap drill for 1/4 NPT?
The tap drill for 1/4-18 NPT is 7/16 in (0.4375 in, 11.11 mm). The standard and the usual shop charts agree on this size.
What is the tap drill for 1/2 NPT?
The tap drill for 1/2-14 NPT is 23/32 in (0.7188 in, 18.26 mm). ASME B1.20.1 suggests the slightly smaller 45/64 in (0.7031 in), which is harder to tap but leaves a fuller thread.
Do NPT threads need sealant?
Yes. NPT threads wedge together but leave a spiral leak path at the crests and roots, so ASME B1.20.1 calls for a sealant such as PTFE tape or pipe compound for a pressure-tight joint. NPTF (Dryseal) threads are made to seal without it.
Other charts and tools
Sources: ASME B1.20.1 Table 2 (dimensions) and Appendix (suggested drills), Machinery’s Handbook (shop tap drill chart). How this site checks its data. Last reviewed 29 September 2026.