Tap Drill Calculator

Pick the drill bit for cutting an internal thread with a tap. For a metric thread the tap drill is the major diameter minus the pitch (about 75% thread engagement); for a unified inch thread it's the major diameter minus one over the threads-per-inch. The tool computes the theoretical drill and matches it to the nearest standard bit — a fractional, number or letter drill for imperial sizes, a 0.1 mm-step metric drill for metric sizes — so you can grab the right bit from the box. Useful for tapped holes in metal, 3D-printed and machined parts, and repair work. Pure arithmetic — the 75% rule is a practical compromise that gives full-strength threads without a tap that binds or breaks; harder materials sometimes use a slightly larger hole (65%) and softer a smaller one (85%), and form (roll) taps need a larger pre-drill than cut taps. Everything runs locally in your browser.