Tap Drill & Clearance Hole Calculator
Metric coarse threads M1.6–M30: tapping drill size, clearance holes (close, medium and free per ISO 273), pitch, and the thread engagement you'll actually get. Works both ways — pick a thread, or start from a drill you have and see what it gives you. Every number shows its working. No sign-up.
More thread isn't automatically better — past about 75% you're mostly adding tap breakage and cycle time for very little extra strength.
Or type a target % below and we'll give you the drill.
Tap drill sizes, clearance holes and thread engagement
The tapping drill for an ISO metric coarse thread is simply major diameter minus pitch. An M6×1.0 needs a 5.0 mm drill; an M8×1.25 needs 6.75 mm, rounded to the 6.8 mm drill you can actually buy; an M10×1.5 needs 8.5 mm. That one rule reproduces essentially every published tap drill chart, which is why this calculator computes it and shows you the working rather than making you trust a table.
That rule targets roughly 75% thread engagement — D − P works out at a constant 77% for every metric size — and that is deliberate. Engagement is the proportion of the theoretical full thread form actually cut, and the relationship with strength is heavily diminishing: going from 60% to 100% engagement adds only a few percent of thread strength while roughly doubling the torque needed to drive the tap. In practice a 100% thread is how taps get snapped off in expensive parts. Unless there is a specific reason, 65–75% is the sensible window — and in tough materials like titanium or Inconel, dropping toward 60% is often the right call.
Clearance holes are the other half of the job and follow ISO 273, which defines three fits: close, medium (normal) and free. Medium is the default for general assembly — 6.6 mm for an M6, 9.0 mm for an M8. Close fit is for location-critical joints where you cannot afford the slop; free fit buys you tolerance stack-up forgiveness when several holes must line up.
A practical note for machined parts: blind tapped holes cost more than through holes. A blind hole needs a bottoming tap, more careful depth control, and chip evacuation becomes a real problem — chips have nowhere to go. If your design can use a through hole, or if you can allow a little extra drilled depth below the thread, say so on the drawing; it is one of the cheapest cost savings available.
Tap drill sizes — FAQ
What drill size for an M6 tap?
5.0 mm. The rule for ISO metric coarse threads is major diameter minus pitch: M6 has a 1.0 mm pitch, so 6 − 1 = 5.0 mm. That gives roughly 75% thread engagement, which is the normal target.
What is the tapping drill size for M8, M10 and M12?
M8×1.25 needs 6.75 mm (use a 6.8 mm drill), M10×1.5 needs 8.5 mm, and M12×1.75 needs 10.25 mm (use a 10.2 mm drill). All follow the same major diameter minus pitch rule.
What is thread engagement and what percentage should I use?
Thread engagement is the proportion of the full theoretical thread form actually cut. 65–75% is the normal target. Going to 100% adds only a few percent of thread strength but roughly doubles tapping torque and dramatically increases the risk of snapping the tap — which is far more expensive than the strength is worth.
What size clearance hole for an M6 bolt?
Per ISO 273: 6.4 mm close fit, 6.6 mm medium (normal) fit, or 7.0 mm free fit. Medium is the usual default for general assembly; use close where location matters and free where you need tolerance stack-up forgiveness.
Are blind tapped holes more expensive to machine?
Yes. Blind holes need a bottoming tap, tighter depth control, and chip evacuation is genuinely difficult since chips have nowhere to escape. Through holes are cheaper and more reliable. If your design allows a through hole — or extra drilled depth below the thread — it is one of the easiest cost savings on a machined part.
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