A hole size needs
its conditions.

Formula and source

For UNC/UNF, D and hole diameter are in inches:

Cut tap: hole = D − (% × 0.01299 ÷ TPI)
Form tap: hole = D − (% × 0.0068 ÷ TPI)

Metric cut tap: hole = D − (% × P ÷ 76.98)
Metric form tap: hole = D − (% × P ÷ 147.06)

For metric equations, D, P and the result are in millimetres. UNEF is outside the reviewed equation scope.

Enter the percentage as 55, 65 or 75; The source’s available drill selections are approximate. Drilling runout can change the hole and the actual thread percentage.

For a single-start thread, synchronous feed in mm/min = RPM × pitch in mm (or RPM × 25.4 ÷ TPI). This is arithmetic, not a machine cycle or tapping-speed recommendation.

Original chart and equations · PDF page 2 ↗