1/4" NPT has 18 threads per inch and is cut on pipe 0.540" (13.7 mm) across. Measured across its threads, a male 1/4" NPT end is about 0.533" (13.54 mm).
1/4 inch NPT dimensions
| 1/4" NPT | |
|---|---|
| Thread | 1/4-18 NPT |
| Threads per inch | 18 (one thread every 0.0556") |
| Outside of the pipe it’s cut on | 0.540" (13.7 mm) |
| Male thread, measured across | about 0.533" (13.54 mm) |
| Female thread, measured across | about 0.436" (11.07 mm) |
| Taper | 1 in 16 on the diameter (3/4" per foot) |
| Thread angle | 60° |
| Screwed in by hand | 0.2278" (4.10 threads) |
| Effective thread length, male end | 0.4018" |
| Overall thread length, male end | 0.5946" |
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 is tapered, narrowing toward the tip of a male end, so measure it with a caliper at the fourth or fifth full thread. A 1/4" fitting turned by hand engages about 0.23" of the thread, out of 0.5946" on the male end.
To identify an unknown thread from 1/4" to 1", see pipe thread size.
What screws onto 1/4 inch NPT
- Air compressor fittings: Milton’s 1/4" M-style couplers (715-4) have a 1/4" female NPT thread, and its M-style plugs (727) a 1/4" male NPT thread. Milton says the couplers interchange with any maker’s that meet the dimensions of military specification MIL-C-4109, and rates them to 300 psi.
- Pressure gauges: WIKA’s model 111 gauges come with a 1/8", 1/4" or 1/2" NPT male thread, and the 1/2" isn’t offered on its 1-1/2" to 2-1/2" dials. The same gauges are also sold with G threads (EN 837-1) and R threads (ISO 7), so check the marking on a replacement.
Threads that look like 1/4 inch NPT
- 3/8" NPT has the same 18 threads per inch, so counting won’t tell them apart. Measure instead: Swagelok lists a male 3/8" NPT thread at about 0.668" (16.98 mm), about 0.135" wider than 1/4".
- 1/4" BSPT (ISO 7/1, the tapered British thread) is about 0.518" (13.16 mm) across with 19 threads per inch and a 55° thread. At this size a thread pitch gauge can tell it from NPT, unlike at 1/2" and 3/4". See BSP vs NPT.
Sealing 1/4 inch NPT
Like every NPT size, it seals on the threads, so wrap PTFE tape or brush pipe thread sealant onto the male thread. Swagelok calls for 1/4" wide tape on 1/8", 1/4" and 3/8" tapered threads, and says there’s no standard for torque or number of turns. Other makers’ wrap counts are on threaded fitting leaking.
Let the air out of a compressor hose or tank, or shut off the water, before you unscrew a fitting. What Adapter For doesn’t cover gas lines.