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.
How to use the metric thread calculator
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.
Questions
6.75 mm for coarse M8×1.25: the diameter minus the pitch.
Full engagement adds about five percent strength for roughly double the tapping torque. Seventy-five percent is where the trade stops being worth it.
Yes. Aluminium and plastic do better at 65 to 70 percent; hard steel benefits from a slightly larger hole to keep the tap alive.
At least 1.5 times the diameter in steel, more in soft materials.
The hole in the part the bolt passes through. Close is a snug fit; normal allows for tolerance stack-up.