Verified by Rachel Mayfield, Supply Chain Analyst - June 2026
Photo by Nic Wood on Pexels
National Pipe Taper (NPT) thread sizes from 1/16 inch to 2 inch, with threads per inch, pipe outside diameter, and tapping drill sizes from ASME B1.20.1. NPT Thread Sizes Chart 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.
| NPT Size |
Threads/inch |
Pipe OD (in) |
Pipe OD (mm) |
Tap Drill |
| 1/16" |
27 |
0.3125 |
7.94 |
D (0.246") |
| 1/8" |
27 |
0.405 |
10.29 |
R (0.339") |
| 1/4" |
18 |
0.540 |
13.72 |
7/16" (0.438") |
| 3/8" |
18 |
0.675 |
17.15 |
37/64" (0.578") |
| 1/2" |
14 |
0.840 |
21.34 |
23/32" (0.719") |
| 3/4" |
14 |
1.050 |
26.67 |
59/64" (0.922") |
| 1" |
11.5 |
1.315 |
33.40 |
1-5/32" (1.156") |
| 1-1/4" |
11.5 |
1.660 |
42.16 |
1-1/2" (1.500") |
| 1-1/2" |
11.5 |
1.900 |
48.26 |
1-47/64" (1.734") |
| 2" |
11.5 |
2.375 |
60.33 |
2-7/32" (2.219") |
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. The tap drill sizes listed assume no reamer; if you ream the hole to the taper first, go up one drill size.
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 TPI matches - a 1/2 inch NPT and a 1/2 inch BSP are both close to 14 TPI - 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