Work out tap drill size instantly with clear inputs, formula shown and shareable results.
Tap drill size trades thread depth against tapping torque: drill diameter equals major diameter minus the engagement fraction times 1.299 divided by threads per inch. Seventy-five percent engagement is the shop standard because it retains about 95 percent of the thread strength while cutting tapping torque roughly in half compared with full depth.
Tap drill
Drill = major diameter - (percent engagement x 1.299 / tpi)
Minor diameter
Basic minor = major - 1.299 x pitch
Drill = major diameter - (percent engagement x 1.299 / tpi). Tap drill size trades thread depth against tapping torque: drill diameter equals major diameter minus the engagement fraction times 1.299 divided by threads per inch.
In tough or work-hardening materials, or for deep holes in blind bores, 60-65 percent reduces tap breakage substantially at little cost in strength.
This calculator takes 3 inputs: Thread major diameter, Threads per inch, Thread engagement target. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.