National Pipe Taper (NPT) Thread Chart
Industry Standard: ASME B1.20.1
National Pipe Taper (NPT) is the standard taper pipe thread series used in the United States for sealing pipes and fittings. Governed by the ASME B1.20.1 standard, NPT threads feature a 1:16 taper ratio (3/4 inch per foot) which allows the threads to form a mechanical seal when tightened.
| Size | Ø Major (in) | TPI | Pitch (in) | Ø Pitch (in) | Ø Minor (in) | Tap Drill |
|---|---|---|---|---|---|---|
| 1/16 NPT | 0.3695 | 27 | 0.03704 | 0.3454 | 0.3294 | C |
| 1/8 NPT | 0.4985 | 27 | 0.03704 | 0.4744 | 0.4584 | 21/64 |
| 1/4 NPT | 0.6798 | 18 | 0.05556 | 0.6437 | 0.6197 | 7/16 |
| 3/8 NPT | 0.8448 | 18 | 0.05556 | 0.8087 | 0.7847 | 19/32 |
| 1/2 NPT | 1.0469 | 14 | 0.07143 | 1.0005 | 0.9696 | 23/32 |
| 3/4 NPT | 1.3046 | 14 | 0.07143 | 1.2582 | 1.2273 | 59/64 |
| 1" NPT | 1.6316 | 11.5 | 0.08696 | 1.5751 | 1.5375 | 1-5/32 |
| 1-1/4 NPT | 1.9926 | 11.5 | 0.08696 | 1.9361 | 1.8985 | 1-1/2 |
| 1-1/2 NPT | 2.2426 | 11.5 | 0.08696 | 2.1861 | 2.1485 | 1-3/4 |
| 2" NPT | 2.7176 | 11.5 | 0.08696 | 2.6611 | 2.6235 | 2-7/32 |
| 2-1/2 NPT | 3.2798 | 8 | 0.12500 | 3.1986 | 3.1445 | 2-5/8 |
| 3" NPT | 3.9313 | 8 | 0.12500 | 3.8501 | 3.7960 | 3-1/4 |
| 3-1/2 NPT | 4.4313 | 8 | 0.12500 | 4.3501 | 4.2960 | 3-3/4 |
| 4" NPT | 4.9313 | 8 | 0.12500 | 4.8501 | 4.7960 | 4-1/4 |
Standard FAQs
Do NPT threads require thread sealant?
Yes. NPT threads are designed to seal by wedging together, but they still have microscopic gaps along the crests and roots. Teflon tape or pipe dope is required to guarantee a leak-free seal.
Is NPT compatible with BSPT?
No. NPT and BSPT (British Standard Pipe Taper) have different thread angles (60° vs. 55°) and different pitches in most sizes. They will not seal properly and can damage the threads if mated.