1/4-18 NPT Thread Dimensions and Tap Drill
1/4 NPT, written in full as 1/4-18 NPT, is the American taper pipe thread for nominal 1/4 inch pipe: 18 threads per inch cut on a 1 in 16 taper, on pipe with an outside diameter of 0.540 in (13.72 mm). The nominal size names the pipe and is not a dimension you can measure on the thread.
1/4 NPT is the usual thread on air compressor fittings, regulators and pressure gauges.
- Tap drill7/16"0.4375 in, 11.11 mm
- Threads per inch18pitch 1.411 mm
- Pipe outside diameter0.540 in13.72 mm
- Pitch Ø at hand-tight plane0.49163 in12.487 mm
- Hand-tight engagement0.228 in4.10 threads
- Effective thread length0.4018 in7.23 threads
Where the diameters are measured
A taper thread has no single pitch diameter. ASME B1.20.1 defines it at the end of the pipe (E0), at the plane where a fitting stops when screwed on by hand (E1), and at the end of the effective thread (E2). Each step of 1 in along the axis adds 1/16 in to the diameter.
1/4-18 NPT dimensions
| Dimension | in | mm |
|---|---|---|
| Pipe outside diameter D | 0.5400 | 13.716 |
| Pitch P | 0.05556 | 1.411 |
| Pitch diameter at pipe end E0 | 0.47739 | 12.126 |
| Pitch diameter at hand-tight plane E1 | 0.49163 | 12.487 |
| Pitch diameter at end of effective thread E2 | 0.50250 | 12.763 |
| Hand-tight engagement L1 | 0.2278 | 5.786 |
| Effective thread length L2 | 0.4018 | 10.206 |
| Minor diameter at pipe end K0 | 0.4329 | 10.996 |
| Thread height h (0.8 × P) | 0.04444 | 1.129 |
| Property | Value |
|---|---|
| Threads per inch | 18 |
| Flank angle | 60° (30° each side) |
| Taper | 1 in 16 on diameter (3/4 in per foot) |
| Taper angle to the axis | 1° 47′ (1.7899°) |
| Threads engaged by hand | 4.10 |
| Wrench make-up | 3 threads |
Basic dimensions from ASME B1.20.1 Table 2. Millimetre values are conversions.
1/4 NPT tap drill size
Tap drill for 1/4 NPT: 7/16" (0.4375 in, 11.11 mm)
Standard tap drill charts and ASME B1.20.1 give the same size.
| Method | Drill | in | mm | Against K0 (in) |
|---|---|---|---|---|
| Standard tap drill chartusual choice | 7/16" | 0.4375 | 11.11 | +0.0046 |
| Drill only, no reaming (ASME B1.20.1) | 7/16" | 0.4375 | 11.11 | +0.0046 |
| Drill, then taper ream (ASME B1.20.1) | 27/64" | 0.4219 | 10.72 | -0.0110 |
The hole for a taper tap is a compromise, because a straight drilled hole meets a tapered thread. The minor diameter of the thread is 0.4329 in at the small end (K0) and grows by 1/16 in per inch of depth. For the fullest thread, drill with the reaming size and follow with a 1 in 16 taper pipe reamer.
ASME B1.20.1 notes that its suggested drills do not guarantee fully formed threads over the whole hand-tight length.
How far 1/4 NPT screws in
A 1/4 NPT fitting turns in by hand for 0.228 in (5.8 mm), about 4.1 threads, before the flanks meet. The standard then allows 3 more threads of wrench make-up, for a total engagement of about 0.394 in (10.0 mm). Tolerance on the hand-tight position is one thread either way, so a joint that stops a turn early or late is still within the standard.
NPT threads do not seal by themselves. The crests and roots leave a spiral gap, so ASME B1.20.1 calls for a sealant (PTFE tape or pipe compound) where the joint must be pressure-tight. Dryseal NPTF threads to ASME B1.20.3 have the same pitch and taper but controlled crests and roots that seal without it.
1/4 NPT against G1/4 BSP
The British Standard Pipe thread of the same nominal size has a different pitch (19 against 18 threads per inch), a 55° flank angle instead of 60° and a major diameter of 13.157 mm against 13.716 mm. The two do not screw together properly. Use an adapter.
| Thread | TPI | Major Ø (mm) | Major Ø (in) | Flank angle | Taper |
|---|---|---|---|---|---|
| 1/4-18 NPT (this thread) | 18 | 13.716 | 0.5400 | 60° | 1 in 16 |
| G1/4 (BSPP) | 19 | 13.157 | 0.5180 | 55° | parallel |
Related sizes and tools
Sources: ASME B1.20.1 (dimensions, Table 2; suggested drills, Appendix), Machinery’s Handbook and tap makers’ charts (shop tap drill sizes). How this site checks its data. Last reviewed 29 September 2026.