Pick the drill before the tap
The hole before the thread: the tap drill size for metric and imperial taps.
Tools used in this guide
The job
Drill too small and the tap snaps; too big and the threads strip. The rule of thumb is major diameter minus ~75 % of the pitch — close enough for shop work, kind to taps.
Size and pitch
Pick the tap from the metric/UNC/UNF list and set the engagement (75 % default). The result gives the exact drill size plus the nearest 0.1 mm metric and 1/64-inch fractional sizes.
Worked example
M10×1.5 at 75 % → drill ≈ 8.78 mm → use 8.8 mm (or 11/32 in). 1/4-20 at 75 % → ≈ 5.32 mm → 13/64 in.
Where it stops
75 % engagement is a compromise between strength and tap torque. Hard materials may want more engagement (smaller drill); soft aluminum tolerates less. Number/letter wire-gauge drills are intentionally not suggested — check an index if the fraction lands between sizes. Pipe taps are not covered.