Tap Drill Calculator
Find the drill for a tapped hole from the tap and the percentage of full thread you want. The page works out the exact hole size, then shows the stock drills either side of it and the thread percentage each one would actually give, so you can pick between them knowingly.
Choose a tap type, size and percent of thread.
Method & sources
Percent of thread compares the thread a hole leaves with the full thread form. A hole at the tap's major diameter leaves no thread (0%); 100% is a hole 1.299 × pitch under the major diameter, which is twice 0.6495 × pitch, the depth of a full 60° thread form. Percent of thread scales linearly between the two.
Cut taps: hole = major − 0.01299 × % ÷ TPI for inch taps, and hole = major − 0.01299 × % × pitch for metric taps (the same constant, with the pitch in millimetres).
Form taps: hole = major − 0.0068 × % ÷ TPI for inch taps, and hole = major − % × pitch ÷ 147.06 for metric taps. A form tap displaces metal rather than cutting it, so the hole is larger than for a cut tap at the same percentage.
Drills either side: the exact hole is compared with a combined table of fractional, number, letter and metric drills. When a drill matches the hole exactly it is shown alone; otherwise the next drill smaller and the next drill larger are shown, each with the percent of thread it would give, worked back with the same formula.
Example: a 1/4-20 UNC cut tap at 75% gives 0.250 − 0.01299 × 75 ÷ 20 = 0.2013 in (5.1127 mm). The drills either side are #7 (0.2010 in), which gives 75%, and 13/64 (0.2031 in), which gives 72%. Those are the values you'll see by selecting that tap above.
No thread-percentage recommendation is made by this tool: the percentage you select is only an input to the calculation.