BenchThread Engagement Calculator
What percentage of full thread a given hole leaves — for any hole diameter, not just the ones a chart lists. Enter a hole to get the percentage, or a percentage to get the hole.
Standard 60°-form approximation
What the percentage means
Formula
engagement % = (major Ø − hole Ø) ÷ (1.29904 × pitch)
hole Ø = major Ø − (engagement % × 1.29904 × pitch)
1.29904 = 2 × 0.649519, twice the 60° thread-height factor
Assumptions
- This is the classic Machinery's Handbook approximation for 60° thread forms, UN and ISO metric alike. It ignores the thread's tolerance class and the real crest and root geometry, which is why published charts round to a stocked drill rather than quoting a percentage.
- 75% is a convention, not an optimum. Going from 75% to 100% adds only a few percent of thread strength while roughly doubling tapping torque and the risk of breaking the tap. 60–65% is ordinary practice in tough material.
- Drilled holes come out slightly oversize, more so in ductile material and with a worn bit, so real engagement usually lands under the number here.
- Percentages above about 88% or below 55% are outside the range published charts cover, and this page says so rather than pretending the approximation still holds.
- Roll-form taps displace material instead of cutting it and need a much larger hole. Not covered.
Related
- Tap drill calculator — the cataloged options
- Thread & tap drill charts
- Nearest drill finder