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Tap Drill Sizing.

Calculate the exact drill hole diameter required for a specific thread engagement percentage. Lower percentages (60-70%) reduce tap breakage in difficult materials; higher percentages (75-80%) provide maximum holding power in soft materials.

e.g., 0.250 for 1/4"

e.g., 20 for 1/4-20

50% (Hard Materials) 90% (Soft Materials)

Required Drill Diameter

Warning: Engagement > 75% significantly increases torque required to tap and the risk of tap breakage, with minimal increase in fastener strength.

The Mathematics

Thread percentage determines how much of the tap's V-profile cuts into the wall of the drilled hole. A 100% thread would mean a hole drilled exactly to the minor diameter.

Imperial Formula (UNC/UNF)

Hole Dia = Major Dia - ((0.01299 × % Thread) / TPI)

Metric Formula

Hole Dia (mm) = Major Dia - ((% Thread × Pitch) / 76.98)

Practical Application

Most standard drill charts target a 75% thread engagement. This is a compromise intended for mild steels.

  • Titanium / Inconel: 60-65% engagement. 75% will almost guarantee tap breakage.
  • Aluminum / Plastics: 75-80% engagement prevents thread pull-out.