Tap drill calculator

The drill for any thread and percentage, with the nearest real drill sizes.

Uses the standard shop formula: drill = major diameter − 0.01299 × % thread × pitch (pitch = 1 ÷ TPI). About 75% thread is usual for steel; 60–65% is easier to tap in tough materials and loses little strength. Form (roll) taps need a larger hole — use the tap maker's size for those.