Metric thread calculator

For coarse metric threads the tap drill is the diameter minus the pitch — an M8×1.25 needs a 6.75 mm hole. That rule gives about 75 percent thread engagement, which is the standard target.

Seventy-five percent is not an arbitrary number: going to 100 percent engagement adds only about five percent more thread strength while roughly doubling the tapping torque, which is how taps get snapped off inside workpieces. Under-engaging is the safer error. The material matters too — in aluminium and plastic, 65 to 70 percent is often better because the threads strip before the fastener fails anyway, and in hard steel a larger hole makes tapping survivable. The minimum tapping depth of 1.5 times the diameter is a rule of thumb for steel into steel; soft materials need more thread engaged to reach the same holding strength.