threadsizes

1 NPT Tap Drill Size

1-11-1/2 NPT · American National Standard taper pipe thread · ASME B1.20.1

What size drill bit for a 1 NPT tap?

1-5/32"fractional drill
1.1563inch
29.37mm
4.6turns by hand
Drill size for tapping without a taper pipe reamer. A 1 NPT hole is much bigger than 1 in: the size is the nominal pipe size, and the pipe itself is 1.315 in across.

To scale male thread, side section

1 NPT male thread to scale: OD 1.315 in, hand-tight length L1 0.4 in, effective thread L2 0.6828 in. L1 0.4 in, hand tight L2 0.6828 in, effective OD 1.315 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.

1-11-1/2 NPT ASME B1.20.1

Pipe outside diameter1.315 in / 33.40 mm
Threads per inch11-1/2 TPI (pitch 0.0870 in)
Taper1:16 on diameter
Pitch dia at pipe end (E0)1.2136 in
Pitch dia at L1 (E1)1.2386 in
Hand-tight engagement (L1)0.4 in, 4.6 turns
Effective thread (L2)0.6828 in
Wrench makeup0.2609 in, 3 turns
Thread height0.0696 in

Shop notes

From 1 in pipe upward, NPT goes to 11.5 TPI, an odd half-thread count, and the next several sizes up keep that same pitch. A thread gauge that does not have an 11.5 leaf will not read it correctly, so look for that leaf before calling a fitting something else. British BSP at the same nominal size is slightly coarser, close enough to start and wrong enough to leak.

The outside diameter is 1.315 in. The drill for tapping without a reamer is 1-5/32 (1.1563 in, 29.4 mm), which is a drill press or lathe job rather than something to attempt with a hand drill. The thread is 0.0696 in deep, and the taper tap has to cut all of that from a straight hole, so plan on a big tap wrench and frequent backing off to break the chips.

Hand-tight engagement is 0.400 in, about 4.6 turns, and wrench makeup adds 3 turns (0.2609 in). The fewer threads per inch, the more each turn of the tap changes the fit, so try the fitting early and often while tapping. As with every NPT thread, the seal is on the flanks: use PTFE tape or a pipe thread sealant. For the next size down, see 3/4 NPT.