Tap Drill Calculator

75% engagement is optimal, not a compromise.

75% engagement is optimal, not a compromise. Tap and clearance drill sizes for metric threads at any engagement, with the tapping speed.

Written and maintained by Mohit PatelLast checked August 4, 2026How we build these

Thread

Thread engagement

Material

Tap drill size

6.99 mm

75% engagement · clearance drill 8.6 mm

Tap drill6.985 mm

Clearance drill8.6 mm

Thread pitch1.25 mm

Major diameter8 mm

Minor diameter at full form6.647 mm

Suggested tapping speed394 rpm

Note75% is the usual target and close to optimal — nearly all the strength, at a fraction of the tapping torque.

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

Where to go next

The Tap Drill question rarely arrives on its own. These are the ones that usually come with it:

Thread Pitch Calculator — Pitch and lead differ the moment you add a start.
Bolt Torque & Preload Calculator — Ninety percent of your torque is fighting friction.
Cutting Speed Calculator — Spindle speed is not cutting speed.
Speed Distance Time Calculator — Any one of the three from the other two, in four units.

Sources

ASTM International standards

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.

Put this calculator on your own site

Free to use, on any site, commercial or not. Paste this where you want it to appear. It is a plain iframe, so it works in WordPress, Squarespace, Wix, Webflow, Ghost and anything else that accepts HTML.

The one-line version

<iframe src="https://www.thecalclibrary.com/embed/drill-tap-calculator" width="100%" height="640" style="border:1px solid #e2e8f0;border-radius:12px" loading="lazy" title="Tap Drill Calculator"></iframe>
<p style="font:13px/1.5 system-ui,sans-serif;margin:6px 0 0;color:#64748b">Powered by <a href="https://www.thecalclibrary.com/drill-tap-calculator" style="color:#64748b">Tap Drill Calculator</a> from The Calc Library</p>

The only condition is that the credit line below the frame stays in place. That one line is what pays for the tool being free — it is how anyone else finds it.