Tap Drill Size Calculator

Where this formula comes from

For a standard 60° thread form, the theoretical full thread height works out to about 1.299 times the pitch. Thread engagement percentage describes how much of that full triangle actually gets cut into the hole wall, so the pilot hole diameter is the major diameter minus that fraction of the full thread height: drill = major diameter − (1.299 × pitch × engagement% / 100), where pitch is 1÷TPI for imperial threads. Plug in a 1/4-20 thread at 75% engagement and this gives 0.2013 in — the long-standard #7 drill (0.201 in) used for that exact thread.

Why 75% instead of 100%

A 100% thread engagement is stronger on paper but dramatically harder to tap, especially by hand, since it removes less material and leaves the tap fighting more resistance the whole way down — it also increases the odds of breaking a tap in a blind hole. 75% engagement gives roughly 90% of the full thread's strength in most materials while tapping noticeably easier, which is why it's the default most machinists reach for unless a print specifically calls out a different percentage.

Rounding to a real drill

This calculator gives you the exact theoretical diameter; the actual drill you chuck up needs to be a real, physical drill size. Round up to the nearest standard fractional, number, or metric drill using the Drill Bit Size Chart — rounding down instead would push engagement above your target and make the hole harder to tap than intended.