1/2" NPT has 14 threads per inch and is cut on pipe 0.840" (21.3 mm) across. Measured across the threads, a male 1/2" NPT end is about 0.832" (21.14 mm).
1/2 inch NPT dimensions
| 1/2" NPT | |
|---|---|
| Thread | 1/2-14 NPT |
| Threads per inch | 14 (one thread every 0.0714") |
| Outside of the pipe it’s cut on | 0.840" (21.3 mm) |
| Male thread, measured across | about 0.832" (21.14 mm) |
| Female thread, measured across | about 0.705" (17.90 mm) |
| Taper | 1 in 16 on the diameter (3/4" per foot) |
| Thread angle | 60° |
| Screwed in by hand | 0.320" (4.48 threads) |
| Effective thread length, male end | 0.5337" |
| Overall thread length, male end | 0.7815" |
Pipe outside, taper, angle and lengths: the National Bureau of Standards pipe thread tables (Handbook H28, Table VII.2). Measured diameters: Swagelok’s thread identification guide, which gives them to help identify a thread, not as exact sizes, since every thread has tolerances.
The thread tapers, so a male end is narrowest at its tip and a female fitting is widest at its opening. Swagelok says to measure a tapered male thread with a caliper at the fourth or fifth full thread. Because of the taper, a 1/2" fitting goes on only about 0.320" by hand before it’s snug, well short of the 0.78" of thread on a male end.
For the outside of every kind of 1/2" pipe, threaded or not, see 1/2" pipe OD. To work out an unknown thread, see pipe thread size.
What screws onto 1/2 inch NPT
- Shower arms: Kohler’s K-7397 shower arm has 1/2"-14 NPT at both ends. See shower head thread size.
- Outdoor faucets: Watts makes hose bibbs with a 1/2" male NPT inlet (LFSC8-1 and LFSC8-5), which screw into a 1/2" female fitting. See outdoor faucet replacement.
- Sprinkler spray heads: Rain Bird’s 1800 Series heads have a 1/2" NPT female inlet. See sprinkler head thread size.
- A garden hose: only through an adapter, because hose thread has a different count. See garden hose to 1/2" pipe.
Threads that look like 1/2 inch NPT
Two other 1/2" threads also have 14 threads per inch and are within 0.02" of the same width:
- 1/2" BSP, the British pipe thread, about 0.825" (20.96 mm) across with a 55° thread. See BSP vs NPT.
- 1/2" NPSM, a straight (untapered) 60° thread about 0.818" (20.78 mm) across. A male NPT thread can go into a female NPSM fitting, but a male NPSM thread doesn’t go into a female NPT one. Delta’s U4980 water supply elbow screws onto 1/2-14 NPT and gives the hose a 1/2-14 NPSM outlet. See shower hose thread size.
Sealing 1/2 inch NPT
NPT seals on the thread itself, so the joint needs PTFE tape or pipe thread sealant on the male thread. Swagelok calls for 1/2" wide tape on 1/2" and larger tapered threads. No standard sets a torque or number of turns past hand-tight. For wrap direction and how many wraps each maker gives, see threaded fitting leaking.
Shut off the water before you take a threaded joint apart. What Adapter For covers water fittings, not gas lines.