Tap Drill Calculator
75% engagement is optimal, not a compromise.
Work out Tap Drill. 75% engagement is optimal, not a compromise. Names the misconception directly.
Tap drill size
6.99 mm
75% engagement · clearance drill 8.6 mm
Tap drill size is the major diameter minus the engagement fraction of the pitch, times 1.0825 — a constant that falls out of the ISO thread's 60° form. For M8 × 1.25 at 75% that gives 6.99 mm, which is why 6.8 mm is the standard drill. The conventional 75% is not a compromise, it is close to optimal. Going to 100% roughly doubles the tapping torque for about 5% more thread strength, and broken taps almost always follow an undersized hole rather than a worn tap. In soft materials the calculation matters less than the material does. Aluminium and plastics tolerate lower engagement well because the threads are generously oversized for the load anyway; in cast iron and stainless, the torque saving from a slightly larger drill is worth having.
How the Tap Drill Calculator works
Tap and clearance drill sizes for metric threads at any engagement, with the tapping speed. The standard 75% is close to optimal — going higher doubles the torque for almost no extra strength.
Also known as: what drill for m8 tap · tap drill size chart metric · why did my tap break · clearance hole for m10
Frequently asked questions
What drill do I need for an M8 tap?
6.8 mm, which is the standard. The calculation gives 6.99 mm at 75% engagement, and 6.8 is the nearest common drill below it.
Why 75% thread engagement?
Because it is close to optimal. Going to 100% roughly doubles the tapping torque for about 5% more thread strength, and broken taps almost always follow an undersized hole.
How is tap drill size calculated?
Major diameter minus the engagement fraction times pitch times 1.0825 — a constant that falls out of the ISO 60° thread form.
What is a clearance drill?
A hole the bolt passes through freely, nominally the major diameter plus a working gap. Too tight and assembly fights alignment; too loose and the joint relies on the bolt rather than friction.
Does the material change the answer?
The geometry does not, but the practical choice does. Soft materials tolerate lower engagement well; in stainless and cast iron the torque saving from a slightly larger drill is worth having.
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