Metric tap drill & clearance hole chart
Tapping drill sizes for ISO metric coarse threads, with the three ISO 273 clearance-hole grades alongside. Everything you need at the pillar drill 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 | 0.4 | 1.60 | 2.2 | 2.4 | 2.6 |
| M2.5 | 0.45 | 2.05 | 2.7 | 2.9 | 3.1 |
| M3 | 0.5 | 2.50 | 3.2 | 3.4 | 3.6 |
| M4 | 0.7 | 3.30 | 4.3 | 4.5 | 4.8 |
| M5 | 0.8 | 4.20 | 5.3 | 5.5 | 5.8 |
| M6 | 1 | 5.00 | 6.4 | 6.6 | 7 |
| M8 | 1.25 | 6.75 | 8.4 | 9 | 10 |
| M10 | 1.5 | 8.50 | 10.5 | 11 | 12 |
| M12 | 1.75 | 10.25 | 13 | 13.5 | 14.5 |
| M16 | 2 | 14.00 | 17 | 17.5 | 18.5 |
| M20 | 2.5 | 17.50 | 21 | 22 | 24 |
| M24 | 3 | 21.00 | 25 | 26 | 28 |
| M30 | 3.5 | 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 than the chart buys almost no strength and costs taps.
Blind holes: the drilled depth is not the usable thread depth — deduct the drill point (about 0.3 × diameter for a 118° point) and the tap lead-in, and leave room for chips.
Work out engagement for a specific joint with the thread engagement calculator, or any size with the tap drill calculator.
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