Tap drill size

The drill size for a tapped hole — common metric, UNC, and UNF threads at an adjustable thread engagement.

Drill size = major diameter − 1.08253 × pitch × thread%. Common metric, UNC and UNF taps.

75 % is the workshop default — easier to tap than 100 %, nearly as strong.

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How to use

  1. Pick the tap from the metric/UNC/UNF list.
  2. Set the thread engagement % (75 is the workshop default).
  3. Read the drill size and the nearest standard drills.

What it supports

Metric M3–M24 coarse/fine and UNC/UNF #4 to 1 in; engagement 50–95 % (drill = major − 1.08253 × pitch × pct). Suggests the nearest 0.1 mm and 1/64-inch fractional drills — number/letter drills are intentionally not guessed.

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