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Tap drill calculator
For odd sizes, special materials or when you want less than 75% thread. Uses the standard shop formulas for 60° threads.
The formulas
Inch: drill = major diameter − (0.01299 × % thread ÷ TPI). For 1/4-20 at 75%: 0.25 − 0.01299 × 75 ÷ 20 = 0.2013 in, so a #7 drill (0.201 in).
Metric: drill = major diameter − (% thread × pitch ÷ 76.98). At about 77% this becomes the familiar "major minus pitch": M8 × 1.25 → 6.75 mm, drilled 6.8 mm.
Then pick the nearest standard drill from the drill size chart, or look your thread up on the tap drill chart or metric chart.
Frequently asked questions
What percent of thread should I use?
75% is the general standard. 60–65% is common in hard or tough materials such as stainless steel, because it cuts tapping torque sharply while losing little strength.
Where does 0.01299 come from?
It is the full thread depth of a 60° Unified thread per 1 TPI (0.649519 × 2 ÷ 100), so multiplying by % thread and dividing by TPI gives the diameter to remove.