NPT pipe threads explained: sizes, threads per inch and sealing
Last updated 2026-11-09
NPT (National Pipe Taper) is the standard tapered pipe thread in the US. The threads taper so the joint tightens as it's screwed together, and a thread sealant fills the small gaps to make it leak-tight.
Pipe size isn't the outside diameter
Pipe sizes are nominal: a name, not a measurement. A 1/2 in. NPT pipe measures about 0.840 in. across the outside. Measure the outside diameter of the threads and use the table to find the size.
| Nominal size | Approx. thread OD | Threads per inch |
|---|---|---|
| 1/8 in. | 0.405 in. | 27 |
| 1/4 in. | 0.540 in. | 18 |
| 3/8 in. | 0.675 in. | 18 |
| 1/2 in. | 0.840 in. | 14 |
| 3/4 in. | 1.050 in. | 14 |
| 1 in. | 1.315 in. | 11.5 |
| 1-1/4 in. | 1.660 in. | 11.5 |
| 1-1/2 in. | 1.900 in. | 11.5 |
| 2 in. | 2.375 in. | 11.5 |
NPT, NPTF, NPS and BSP
- NPT: tapered; needs a thread sealant. The common pipe thread for water, air, gas and oil.
- NPTF (Dryseal): tapered, made to seal metal-to-metal without sealant; used in fuel and hydraulic fittings. NPT and NPTF parts will thread together.
- NPS: straight (not tapered); seals with a gasket or O-ring, not on the threads.
- BSP (BSPT, BSPP): British standard pipe threads, used on many imported tools and compressors. The thread angle and pitch differ from NPT, so BSP and NPT are not interchangeable, even when the size name matches.
Sealing a threaded joint
- Make sure the threads are clean and undamaged.
- Wrap PTFE thread seal tape clockwise (the direction the fitting tightens) 2–3 times, starting a thread back from the end, or brush on pipe thread sealant (pipe dope). Use a product rated for the fluid: gas lines need sealant rated for gas.
- Tighten hand-tight, then about 1–3 more turns with a wrench. Don't over-tighten: it can crack fittings.
Shop pipe fittings, valves, air fittings and couplers and sealants.
Common questions
Why doesn't my 1/2 inch pipe measure 1/2 inch?
Pipe sizes are nominal names. A 1/2 in. NPT pipe is about 0.840 in. across the outside of the threads. Measure the outside diameter and use the size table to identify it.
Can I connect BSP to NPT?
Not directly. The thread angle and pitch are different, so they won't seal reliably. Use an adapter fitting made for BSP to NPT.
Do NPT threads need tape or sealant?
Yes. NPT threads seal with the help of PTFE tape or pipe thread sealant. Straight NPS threads seal with a gasket or O-ring instead.
Need help?
Send us a photo or the measurements and we'll identify the thread: call (908) 824-0916 or send us a message. See also how to read bolt thread sizes.