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3/8 NPT Tap Drill Size and Pipe Thread Bit

Use a 37/64 drill bit

3/8-18 NPT · 0.5781 in / 14.68 mm · pipe taper

3/8 NPT (3/8-18) is a common tapered pipe thread for plumbing, pneumatic and hydraulic fittings. The drill is 37/64 in, sized to the pipe taper at the gauge plane — not to a percent-engagement rule.

At a glance

Tap drill	37/64
Inches	0.5781 in
Millimetres	14.68 mm
Threads per inch	18
Thread form	NPT taper pipe (1:16 taper)

Why this size

NPT is a taper pipe thread. The drill is sized to the pipe minor diameter at the gauge plane. Tap drill: 37/64 (0.5781 in / 14.68 mm).

Because NPT threads taper, the drill is not set by the percent-engagement rule used for machine screws. 37/64 in is the correct tap drill for 3/8 NPT — the same standard the 1/8 (21/64), 1/4 (7/16) and 1/2 (23/32) sizes follow.