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National Pipe Taper (NPT) thread sizes from 1/16 inch to 24 inch, with threads per inch, pipe outside diameter, pitch diameter, hand-tight engagement and effective thread length from ASME B1.20.1. NPT uses a 60-degree thread angle and a 1:16 taper that seals on the thread flanks, which is why NPT joints need PTFE tape or pipe dope. The nominal size is the approximate bore of the pipe, not the thread diameter: a 1/2 inch NPT thread measures 0.840 inch (21.34 mm) across the outside and has 14 threads per inch.
| NPT size |
Threads/inch |
Pipe OD (in) |
Pipe OD (mm) |
Pitch dia at pipe end E0 (in) |
Hand-tight engagement L1 (in) |
Effective thread L2 (in) |
| 1/16" | 27 | 0.3125 | 7.94 | 0.27118 | 0.160 | 0.2611 |
| 1/8" | 27 | 0.405 | 10.29 | 0.36351 | 0.1615 | 0.2639 |
| 1/4" | 18 | 0.540 | 13.72 | 0.47739 | 0.2278 | 0.4018 |
| 3/8" | 18 | 0.675 | 17.15 | 0.61201 | 0.240 | 0.4078 |
| 1/2" | 14 | 0.840 | 21.34 | 0.75843 | 0.320 | 0.5337 |
| 3/4" | 14 | 1.050 | 26.67 | 0.96768 | 0.339 | 0.5457 |
| 1" | 11.5 | 1.315 | 33.40 | 1.21363 | 0.400 | 0.6828 |
| 1-1/4" | 11.5 | 1.660 | 42.16 | 1.55713 | 0.420 | 0.7068 |
| 1-1/2" | 11.5 | 1.900 | 48.26 | 1.79609 | 0.420 | 0.7235 |
| 2" | 11.5 | 2.375 | 60.33 | 2.26902 | 0.436 | 0.7565 |
| 2-1/2" | 8 | 2.875 | 73.03 | 2.71953 | 0.682 | 1.1375 |
| 3" | 8 | 3.500 | 88.90 | 3.34062 | 0.766 | 1.2000 |
| 3-1/2" | 8 | 4.000 | 101.60 | 3.83750 | 0.821 | 1.2500 |
| 4" | 8 | 4.500 | 114.30 | 4.33438 | 0.844 | 1.3000 |
| 5" | 8 | 5.563 | 141.30 | 5.39073 | 0.937 | 1.4063 |
| 6" | 8 | 6.625 | 168.28 | 6.44609 | 0.958 | 1.5125 |
| 8" | 8 | 8.625 | 219.08 | 8.43359 | 1.063 | 1.7125 |
| 10" | 8 | 10.750 | 273.05 | 10.54531 | 1.210 | 1.9250 |
| 12" | 8 | 12.750 | 323.85 | 12.53281 | 1.360 | 2.1250 |
| 14" | 8 | 14.000 | 355.60 | 13.77500 | 1.562 | 2.2500 |
| 16" | 8 | 16.000 | 406.40 | 15.76250 | 1.812 | 2.4500 |
| 18" | 8 | 18.000 | 457.20 | 17.75000 | 2.000 | 2.6500 |
| 20" | 8 | 20.000 | 508.00 | 19.73750 | 2.125 | 2.8500 |
| 24" | 8 | 24.000 | 609.60 | 23.71250 | 2.375 | 3.2500 |
How to use the NPT thread chart
Identify an NPT thread by counting threads per inch with a pitch gauge and measuring the male thread OD with calipers, then matching both to the table. NPT tapers at 3/4 inch per foot (1:16), so the OD measured at the end of the thread is smaller than further back.
E0 is the pitch diameter at the small end of the external thread. L1 is the length the male thread enters the fitting by hand before a wrench is needed, and L2 is the full length of effective thread. On a 1/2 inch NPT thread, L1 is 0.320 inch and L2 is 0.5337 inch, so a correctly cut joint goes in a little over halfway by hand and is wrenched the rest.
NPT tap drill sizes
Published NPT tap drill charts do not agree with each other. Two sets of figures are in circulation, one slightly larger than the other, and they differ by a 64th of an inch or so on most sizes. The larger drill leaves less material for the tap to remove and is the easier cut; the smaller drill gives a fuller thread and suits a hole that will be taper-reamed before tapping. The table lists every figure found for each size, and says where the sources agree.
| NPT size |
Published tap drill (in) |
Agreement |
| 1/16" | 0.339 or 0.242 | Sources differ |
| 1/8" | 0.339 or 0.332 | Sources differ |
| 1/4" | 0.438 | All sources agree |
| 3/8" | 0.578 or 0.562 | Sources differ |
| 1/2" | 0.719 or 0.703 | Sources differ |
| 3/4" | 0.922 or 0.906 | Sources differ |
| 1" | 1.156 or 1.141 | Sources differ |
| 1-1/4" | 1.500 or 1.484 | Sources differ |
| 1-1/2" | 1.734 or 1.719 | Sources differ |
| 2" | 2.219 or 2.188 | Sources differ |
| 2-1/2" | 2.625 or 2.609 | Sources differ |
| 3" | 3.250 or 3.234 | Sources differ |
| 3-1/2" | 3.750 | One figure published |
| 4" | 4.250 | One figure published |
| 5" | 5.281 | One figure published |
| 6" | 6.344 | One figure published |
Where two figures are shown, check the recommendation from the maker of your tap before drilling. For a 1/2 inch NPT thread the published drills are 0.719 or 0.703 inch.
NPT, BSP and NPTF
NPT and BSP aren't interchangeable. NPT has a 60-degree thread angle while BSP (British Standard Pipe) has a 55-degree Whitworth form. Even where the pitch is close, the thread profiles and diameters differ enough to cross-thread and leak. Use a dedicated NPT-to-BSP adapter to join the two systems.
NPT seals on the threads, so it always needs a sealant: two to three wraps of PTFE tape applied clockwise, or a brushable pipe-thread compound. NPTF (Dryseal) is a variant with a tighter crest-and-root fit that can seal without tape in hydraulic work, but standard NPT should always be sealed.
Related standards
- ASME B1.20.1 - Pipe Threads, General Purpose (Inch) - covers NPT and NPSC
- ASME B1.20.3 - Dryseal Pipe Threads (NPTF) - sealing without tape
- ANSI/ASME B16 series - pipe fittings dimensioned to these threads
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