Building & DIY

Tap Drill Size Calculator (UNC, UNF, Metric)

Pick a thread system and nominal size and it returns the right tap drill along with the major, minor, and pitch diameters and the thread height. The tap drill comes from major diameter minus 1.0825 times the pitch.

How to use
  1. Pick the thread system: UNC, UNF, or Metric.
  2. Choose the nominal thread size.
Estimates only, allow for waste and confirm against local building codes and supplier specs.
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Common questions

What tap drill do I use for a 1/4-20 thread?

A #7 drill (about 0.201 in) is the standard pick for 1/4-20 UNC at roughly 75 percent thread. The calculator gives the exact figure for any size you choose.

How is tap drill size calculated?

Subtract about 1.0825 times the thread pitch from the major diameter. That targets a 75 percent thread, the usual balance of strength and easy tapping.

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