The recommended tap drill for every common inch (UNC/UNF) and metric thread — plus the % thread each one gives you. Tapping hard steel or thin aluminum? Use the calculator to get the exact drill for a lighter thread so the tap doesn't snap.
75% thread is the textbook default. Drop to 55–65% in tough or gummy material — you lose only a little holding strength but the tap cuts far easier and is much less likely to break.
| Thread ▲ | Major Ø | Threads | Tap drill | Tap drill size | ≈% thread |
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Recommended tap drills are the standard 75%-thread values (Machinery's Handbook / ANSI). "% thread" is computed from the listed drill: 76.98 × (major − drill) ÷ pitch.
A #7 bit (0.201"). That gives about 75% thread — the standard strong-but-tappable target. In hard material, a 13/64" (0.2031") drops you to roughly 67% thread for easier tapping.
5.0 mm. The quick metric rule is "drill = major diameter − pitch," so M6×1.0 → 6 − 1 = 5 mm. It works for every ISO coarse thread (M8×1.25 → 6.8, M10×1.5 → 8.5, and so on).
The jump from 60% to 75% thread barely adds holding strength (a few percent) but nearly doubles the torque needed to cut it. In tough alloys, stainless, or a blind hole, a lighter thread saves taps from snapping. Use the calculator to size the larger drill.
No. The tap drill makes the hole you cut threads into (smaller than the screw). A clearance drill makes a hole a screw passes through (larger than the screw). This chart is tap drills.