Thread Percentage Calculator

The inverse of the tap drill formula

The Tap Drill Size Calculator solves drill diameter = major diameter − (1.299 × pitch × target engagement%) for the drill size. This calculator solves the same equation the other way: engagement% = ((major diameter − drill diameter) ÷ (1.299 × pitch)) × 100 — useful when you already grabbed a drill (maybe the closest one in the index, not the theoretical exact size) and want to know what engagement it's actually going to give you.

Why this matters when substituting drill sizes

The exact theoretical tap drill size rarely lands on a real, physical drill — you round to the nearest one you have. Rounding up (a slightly larger drill) gives you somewhat less engagement than the target; rounding down gives you somewhat more. This calculator tells you exactly how much that substitution actually shifted your engagement percentage, rather than just assuming "close enough."

Reading the result

Most general-purpose tapping targets 75% engagement as a strength-versus-ease tradeoff. If your actual number comes back well above that, expect a harder-tapping hole with higher risk of tap breakage, especially in a blind hole; well below it, expect a weaker thread that may not meet a print's requirement if one was specified.