1/2 NPT Tap Drill Size

For 1/2-14 NPT, Allied Machine gives 45/64 in (0.7031 in, 17.86 mm) for tapping without a taper reamer, and Chiggo and Want print the same drill. The charts we read do not agree: their drills run from 11/16 in (0.6875 in) to 23/32 in (0.7188 in).

If you ream the hole with a 1:16 taper reamer before tapping, drill smaller first. Emuge, a tap maker, lists 17.45 mm (0.6870 in) for that, against 17.8 mm without a reamer.

1/2-14 NPT tap drill: what each chart says

Allied Machine, Chiggo and Want give 45/64 in. The others give 17.8 mm (Emuge), 23/32 in (Engineering Toolbox's NPT table and Slugger Tool) and 11/16 in (Engineering Toolbox's Briggs table). Leaving out the reamed hole, the biggest and smallest drills differ by 0.0313 in (0.79 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 11/16 in. Its NPT thread table gives 23/32 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: 45/64 in from Allied (0.7031 in) and 17.8 mm from Emuge (0.7008 in), 0.0023 in apart. With a taper reamer, start from Emuge's 17.45 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 1/2-14 NPT means

1/2

The nominal pipe size, not the thread diameter. A 1/2 pipe is 0.840 in (21.34 mm) across the outside.

14

Threads per inch, so one thread every 0.0714 in (1.814 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 1/2-14 NPT it is 0.3200 in, about 4.48 turns, and the effective thread is 0.5337 in, about 7.47 turns (Wikipedia).

On the pipe end, Engineering Toolbox gives about 3/4 in of thread, roughly 10 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: 47/64 in (0.7344 in) for 1/2 NPS, against 45/64 in for NPT (Allied). NPT and NPS threads do not interchange (Engineering Toolbox).

How to tap a 1/2-14 NPT hole

1. Drill the hole. Without a reamer, 45/64 in (Allied) or 17.8 mm (Emuge).
2. If you are reaming, drill 17.45 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.