Tapping Drill Size Calculator
This tool uses formulas only—no lookup tables—to compute the correct tap drill size for metric (M) and Unified (UNC/UNF) threads at your chosen % thread. Enter the major diameter and pitch/TPI. Results show in your preferred units with a dual-unit hint.
Formulas used
Metric (M): Tap Drill ≈ Major − k × Pitch, where k ≈ %Thread / 75. For 75% thread, this reduces to the shop rule Major − Pitch.
Unified (UNC/UNF): Tap Drill ≈ Major − (0.0130 × %Thread) / TPI (inches). For 75% thread and 1/4‑20, this gives ≈ 0.201″ (≈ #7).
Practical approximations for general tapping. Confirm with tap manufacturer guidance for critical fits or difficult materials.