threadsizes

1/2 NPT Tap Drill Size

1/2-14 NPT · American National Standard taper pipe thread · ASME B1.20.1

What size drill bit for a 1/2 NPT tap?

23/32"fractional drill
0.7188inch
18.26mm
4.5turns by hand
Drill size for tapping without a taper pipe reamer. A 1/2 NPT hole is much bigger than 1/2 in: the size is the nominal pipe size, and the pipe itself is 0.840 in across.

To scale male thread, side section

1/2 NPT male thread to scale: OD 0.840 in, hand-tight length L1 0.32 in, effective thread L2 0.5337 in. L1 0.32 in, hand tight L2 0.5337 in, effective OD 0.840 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.

1/2-14 NPT ASME B1.20.1

Pipe outside diameter0.840 in / 21.34 mm
Threads per inch14 TPI (pitch 0.0714 in)
Taper1:16 on diameter
Pitch dia at pipe end (E0)0.7584 in
Pitch dia at L1 (E1)0.7784 in
Hand-tight engagement (L1)0.32 in, 4.48 turns
Effective thread (L2)0.5337 in
Wrench makeup0.2143 in, 3 turns
Thread height0.0571 in

Shop notes

A 1/2 NPT thread is 0.840 in across, more than 3/4 in. That surprises anyone who expects pipe sizes to measure what they say. They never have: the nominal size is a pipe designation, and the outside diameter stays fixed while the wall thickness changes.

For tapping without a reamer, drill 23/32 (0.7188 in, 18.3 mm). At 14 TPI the thread height is 0.0571 in, deep enough that a dull or chipped pipe tap tears the flanks instead of cutting them, and torn flanks are a leak path that tape will not always cover. Back the tap off often and keep the chips moving.

Hand-tight engagement is 0.320 in, about 4.48 turns, then 3 turns with a wrench (0.2143 in). In home plumbing you meet 1/2 NPT constantly; shower arms and most supply fittings use it.

NPT or BSP?

At this size the British pipe thread also runs 14 TPI, which is why the two get mixed up so easily. They are still not the same thread: BSP uses a different thread angle and slightly different diameters. The male will start and feel right for a turn or two, and then you have a joint that weeps no matter how much sealant you use. The same trap exists at 3/4 NPT. Check the markings or the source of the part before blaming the tape.