Tap drill & clearance hole chart
Tapping drill sizes for ISO metric coarse threads, with the three ISO 273 clearance grades alongside. Everything the pillar drill needs, on one sheet.
| Thread | Pitch | Tapping drill | Clearance close | Clearance medium | Clearance free |
|---|---|---|---|---|---|
| M1.6 | 0.35 | 1.25 | 1.7 | 1.8 | 2 |
| M2 | 0.40 | 1.60 | 2.2 | 2.4 | 2.6 |
| M2.5 | 0.45 | 2.05 | 2.7 | 2.9 | 3.1 |
| M3 | 0.50 | 2.50 | 3.2 | 3.4 | 3.6 |
| M4 | 0.70 | 3.30 | 4.3 | 4.5 | 4.8 |
| M5 | 0.80 | 4.20 | 5.3 | 5.5 | 5.8 |
| M6 | 1.00 | 5.00 | 6.4 | 6.6 | 7 |
| M8 | 1.25 | 6.75 | 8.4 | 9 | 10 |
| M10 | 1.50 | 8.50 | 10.5 | 11 | 12 |
| M12 | 1.75 | 10.25 | 13 | 13.5 | 14.5 |
| M16 | 2.00 | 14.00 | 17 | 17.5 | 18.5 |
| M20 | 2.50 | 17.50 | 21 | 22 | 24 |
| M24 | 3.00 | 21.00 | 25 | 26 | 28 |
| M30 | 3.50 | 26.50 | 31 | 33 | 35 |
Why 77% and not 100%?
Thread strength rises very little between about 75% and 100% engagement, while tapping torque and tap breakage rise steeply. Most shops tap at 70–80% deliberately. Going finer buys almost no strength and costs taps.
Blind holes catch people out
Drilled depth is not usable thread depth. Deduct the drill point (about 0.3 × diameter for a 118° point) and the tap lead-in, then leave room for chips. Run out of room and the tap snaps on the bottom.
Which clearance grade?
Close where the joint has to locate something. Medium is the sensible default. Free when parts are welded, painted or stacked up and you need the slop to assemble at all.
Softer material, less engagement
Free-machining aluminium taps happily at the lower end of the 70–80% band. Easing engagement is the cheapest way to stop breaking small taps — the joint rarely notices, because the failure moves to the bolt long before the thread strips.
Stainless is a different job
Austenitic grades work-harden as you tap. Spiral-point through, spiral-flute blind, cut the speed and use proper tapping compound rather than flood coolant. Dwelling in the cut is what glazes the hole.
Check the specific joint
Engagement depends on material and length of engagement, not diameter alone. Work out a real case with the thread engagement calculator, or any size at all with the tap drill calculator, at dalloways.com/tools.
Typical reference values: the governing standard or your material certificate takes precedence.
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