threadsizes

1/4 NPT Tap Drill Size

1/4-18 NPT · American National Standard taper pipe thread · ASME B1.20.1

What size drill bit for a 1/4 NPT tap?

7/16"fractional drill
0.4375inch
11.11mm
4.1turns by hand
Drill size for tapping without a taper pipe reamer. A 1/4 NPT hole is much bigger than 1/4 in: the size is the nominal pipe size, and the pipe itself is 0.540 in across.

To scale male thread, side section

1/4 NPT male thread to scale: OD 0.540 in, hand-tight length L1 0.2278 in, effective thread L2 0.4018 in. L1 0.2278 in, hand tight L2 0.4018 in, effective OD 0.540 in
The thread tapers 1 in 16 on the diameter (3/4 in per foot), drawn to scale. L1 is how far the parts go together by hand; L2 is the effective thread length.

1/4-18 NPT ASME B1.20.1

Pipe outside diameter0.540 in / 13.72 mm
Threads per inch18 TPI (pitch 0.0556 in)
Taper1:16 on diameter
Pitch dia at pipe end (E0)0.4774 in
Pitch dia at L1 (E1)0.4916 in
Hand-tight engagement (L1)0.2278 in, 4.1 turns
Effective thread (L2)0.4018 in
Wrench makeup0.1667 in, 3 turns
Thread height0.0444 in

Shop notes

Drill 7/16 (0.4375 in, 11.1 mm) when tapping 1/4 NPT without a reamer. That one is easy: 7/16 is in every fractional drill set, and the outside diameter of the male thread is 0.540 in, so a quick caliper check tells you what you are holding.

This is probably the most common NPT size in a shop. Air hose quick couplers and compressor fittings in the US are usually 1/4 NPT, and so are the stems of many pressure gauges. Hand-tight engagement is 0.2278 in (about 4.1 turns), followed by 3 turns with a wrench. If a fitting goes in much further than that by hand, the hole was tapped too deep or the fitting is worn, and no amount of tape will give it the taper back.

Same pitch as 3/8 NPT

1/4 NPT and 3/8 NPT both run 18 TPI, so a thread pitch gauge sits perfectly on either and tells you nothing. Measure the diameter instead: 0.540 in for 1/4, 0.675 in for 3/8. The other common confusion is British BSPT, which has a different thread angle and a slightly different pitch at this size. A BSPT fitting will start in an NPT hole and then bind or leak.

Like every NPT joint, it seals on the flanks and needs PTFE tape or thread sealant. Wrap the tape in the direction of the thread so tightening does not peel it back.