Metric thread calculator
For coarse ISO metric threads the tap drill is the nominal diameter minus the pitch: an M8×1.25 takes a 6.75 mm hole. That shortcut lands on 76.98 percent thread engagement every time, whatever the size, which is why the charts print it and nothing else.
How to use the metric thread calculator
The shortcut is exact arithmetic underneath, not a rule of thumb. On the ISO 60-degree profile, thread engagement is 76.98 × (diameter − hole) ÷ pitch, so putting the hole at diameter minus pitch returns 76.98 percent for M3 and M24 alike. One line covers the whole series, which is the only reason it is memorable.
Chasing 100 percent is the classic mistake. Going from 75 to full engagement buys roughly five percent more thread strength and close to doubles the torque needed to cut it, which is how taps end up sheared off inside a workpiece with no good way to get them out. Under-engaging is the cheap error. Material shifts the target: aluminium, brass and plastics do better at 65 to 70 percent, because the threads strip before the fastener yields anyway, and hardened steel wants a slightly larger hole for no reason other than keeping the tap alive.
The clearance figures come from ISO 273, which defines three fits rather than one. An M8 bolt takes 8.4 mm close, 9 mm medium and 10 mm free, and the choice is about how much tolerance stack-up the assembly has to swallow, not about how tight the finished joint is. Depth is a separate question again: 1.5 diameters is the working minimum for steel into steel, and soft materials need more thread engaged to reach the same pull-out load, which is the point at which a thread insert becomes cheaper than a deeper hole.
What people use it for
- Finding the tap drill for a size that is not on the chart in the tap box lid
- Backing off engagement before tapping aluminium or plastic
- Choosing between a close and a free clearance hole for a bracket
- Setting a blind hole deep enough for full thread engagement
- Working out the drill for a fine-pitch thread on an existing part
Questions
6.75 mm for coarse M8×1.25 — the diameter minus the pitch, which puts engagement at 76.98 percent.
76.98 × (diameter − hole size) ÷ pitch. Set the hole at diameter minus pitch and the pitch cancels, which is why the shortcut works at every size.
It adds about five percent thread strength for roughly double the tapping torque. That trade is how taps get snapped off inside the work.
Yes. Aluminium, brass and plastics do better at 65 to 70 percent because the thread strips before the bolt fails; hardened steel benefits from a slightly larger hole.
At least 1.5 times the diameter in steel, and more in soft materials, where a thread insert is often cheaper than the extra depth.
The hole in the part the bolt passes through. ISO 273 gives three fits — close, medium and free — for how much tolerance stack-up the assembly has to absorb.