For a metric thread, the drill bit for tapping is simply the major diameter minus the thread pitch — M6×1.0 needs a 5.0mm hole, no lookup table required.
This targets roughly 75% thread engagement, the standard compromise between thread strength and how much force it takes to actually cut the tap through the material — 100% engagement is dramatically harder to tap for very little extra holding strength.
That gives about 100% thread engagement, which is more than you want in anything but soft, thin material. In steel a 75% thread cuts far more easily and is barely weaker, and in a hard alloy the full-depth hole is how you snap the tap.
At 10 mm the rule says 8.5 mm and the exact answer is 8.782 mm — 3.2% low. It is never within 2% anywhere in this range.
Subtracting the whole pitch is not the standard — it lands on about 92% thread engagement, where the shops aim for 75%. So the rule always drills SMALL, by 3-4%, and the extra 17% of thread buys almost no strength while making the tap markedly harder to turn. It is the commonest reason a tap snaps in a blind hole.
The rule against the exact answer, computed across the range. Inside the shaded band the shortcut is close enough to use; outside it, reach for the calculator.
Estimate with the rule, then check it against the calculator that models it properly.
Open Screw, Stud & Tap Drill Sizing →For a metric thread, the drill bit for tapping is simply the major diameter minus the thread pitch — M6×1.0 needs a 5.0mm hole, no lookup table required. This targets roughly 75% thread engagement, the standard compromise between thread strength and how much force it takes to actually cut the tap through the material — 100% engagement is dramatically harder to tap for very little extra holding strength.