BenchTap Drill Size Calculator
Enter a thread and get the chart tap drill, then every cataloged drill that lands in the useful engagement band — the choice the chart makes for you.
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How the engagement number is worked out
engagement % = (major Ø − drill Ø) ÷ (1.29904 × pitch)
1.29904 = 2 × 0.649519, twice the 60° thread-height factor
Published tap-drill charts target roughly 75% engagement, which is a compromise, not an optimum: going to 100% adds only a few percent of thread strength while roughly doubling tapping torque and the chance of snapping the tap in the hole. Dropping to 60–65% in tough material is normal practice.
Assumptions
- The chart tap drill is transcribed data, not something this page computes. It is the same value the thread's own reference page publishes, from the same source, and a test asserts the two agree for every thread in the catalog.
- The engagement percentages are the classic Machinery's Handbook approximation for 60° thread forms. They ignore the thread's tolerance class and the real crest geometry, which is why charts round to a stocked drill.
- Drilled holes run slightly oversize, more so in ductile material and with a worn or poorly ground bit, so actual engagement usually lands under the number here.
- Roll-form taps need a substantially larger hole and are not covered.
Related
- Thread & tap drill charts — the full static tables
- Thread engagement calculator — any drill, not just cataloged ones
- Clearance hole calculator — the other half of the joint
- Nearest drill finder