Thread Percentage Calculator
Already drilled the hole? Work out exactly what thread engagement percentage that drill size actually gives you.
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.