Tap Drill Size Formula
Tap Drill = Major Diameter - (Pitch x Thread % / 76.98)
For 75% thread: Tap Drill approx = Major - Pitch (metric) or Major - 1/TPI (imperial)
Tips
- 75% thread is the industry standard and provides adequate strength for most applications
- Going above 75% adds very little strength but significantly increases tapping torque and tap breakage risk
- For stainless steel and tough materials, consider 60-65% thread engagement
- Always use cutting fluid when tapping, especially stainless steel and titanium