3/8 NPT Tap Drill Size

For 3/8-18 NPT, Allied Machine gives 9/16 in (0.5625 in, 14.29 mm) for tapping without a taper reamer, and Chiggo, Want and Engineering Toolbox's Briggs table print the same drill. The charts we read do not agree: their drills run from 9/16 in (0.5625 in) to 37/64 in (0.5781 in).

If you ream the hole with a 1:16 taper reamer before tapping, drill smaller first. Emuge, a tap maker, lists 14.15 mm (0.5571 in) for that, against 14.4 mm without a reamer.

3/8-18 NPT tap drill: what each chart says

Allied Machine, Chiggo, Want and Engineering Toolbox's Briggs table give 9/16 in. The others give 14.4 mm (Emuge) and 37/64 in (Engineering Toolbox's NPT table and Slugger Tool). Leaving out the reamed hole, the biggest and smallest drills differ by 0.0156 in (0.40 mm).

Why the charts disagree

NPT is a tapered thread. You can drill a straight hole and tap it, or drill smaller and open the hole with a taper reamer before tapping. Emuge gives a hole for each method, and its hole for tapping without a reamer is the larger one. Allied Machine says its drills are for tapping without a taper reamer.

Engineering Toolbox prints two different figures on two pages. Its Briggs pipe tap table, labelled "for direct tapping without reaming", gives 9/16 in. Its NPT thread table gives 37/64 in and does not say which method it assumes.

Want lists other reasons charts differ, among them the thread engagement wanted and the tap's geometry. Apart from the reaming method, none of the charts says what its numbers assume.

Which drill to use

If your tap maker publishes a drill for its pipe tap, use that one. Without one, the two tool makers here are close for tapping without a reamer: 9/16 in from Allied (0.5625 in) and 14.4 mm from Emuge (0.5669 in), 0.0044 in apart. With a taper reamer, start from Emuge's 14.15 mm.

Whichever drill you pick, check the tapped hole with a fitting, a thread gauge or a leak test, as Want advises for pipe threads.

What 3/8-18 NPT means

3/8

The nominal pipe size, not the thread diameter. A 3/8 pipe is 0.675 in (17.14 mm) across the outside.

18

Threads per inch, so one thread every 0.0556 in (1.411 mm).

NPT

National Pipe Taper, defined by ASME B1.20.1. The thread narrows by 1 inch of diameter over 16 inches of length, which is 3/4 in per foot.

Thread data from the NPT table on Wikipedia, which cites Engineering Toolbox and piping-designer.com.

How much thread to cut

Hand-tight engagement is how far a fitting goes in by hand before you tighten it with a wrench. For 3/8-18 NPT it is 0.2400 in, about 4.32 turns, and the effective thread is 0.4078 in, about 7.34 turns (Wikipedia).

On the pipe end, Engineering Toolbox gives about 5/8 in of thread, roughly 11 threads.

NPT, NPTF and NPS

Allied Machine gives the same drill for NPTF, the dryseal version, as for NPT. NPT needs a thread sealant such as tape or paste, while NPTF is made to seal without it (Wikipedia).

Straight pipe thread (NPS) is a different thread and takes a bigger drill: 19/32 in (0.5938 in) for 3/8 NPS, against 9/16 in for NPT (Allied). NPT and NPS threads do not interchange (Engineering Toolbox).

How to tap a 3/8-18 NPT hole

1. Drill the hole. Without a reamer, 9/16 in (Allied) or 14.4 mm (Emuge).
2. If you are reaming, drill 14.15 mm (Emuge), then open the hole with a 1:16 taper reamer. Chiggo says this lowers the tapping torque.
3. Put cutting fluid on the tap (Wikipedia).
4. Hold the tap in line with the hole.
5. Turn the tap forward, then back it off now and then to break the chip (Wikipedia).
6. Check the hole with a fitting, a thread gauge or a leak test (Want).

Drills from Allied Machine, Chiggo, Want, Emuge, Engineering Toolbox's NPT table, Engineering Toolbox's Briggs table and Slugger Tool. Thread data from Wikipedia. Tapping tips from Wikipedia: tap and die. Last reviewed October 12, 2026.