M18 tap drill size
For a standard 75% thread, drill 15.5 mm before tapping M18 — that's 0.61″ / 15.5 mm.
| Nominal size | M18 |
|---|---|
| Pitch | 2.5 mm |
| Tap drill (75%) | 15.5 mm |
| Tap drill (mm) | 15.5 mm |
| Tap drill (inch) | 0.61″ |
A higher thread percentage barely adds strength (75%→100% adds only ~5% pull-out strength) but sharply raises tapping torque and tap-breakage risk. 75% is the shop standard; drop to 60% in tough or gummy materials.
How the M18 tap drill size is calculated
The tap drill sets the minor diameter of the hole — the space left for the tap to cut threads into. Too large and the threads are shallow and strip easily; too small and the tap has to remove so much material that it binds and snaps. For M18 the formula d = D − (%thread/76.98)·P lands on 15.5 mm at the standard 75 %.
Why 75% is the shop default
More thread engagement is not better. Going from 75% to 100% thread adds only about 5% to pull-out strength — while it can double or triple the tapping torque, which is exactly what breaks taps off in the hole. 75% gets nearly all the strength at far less risk; in stainless, titanium or blind holes drop to 60–65% (the calculator shows the larger drill).
Cut tap vs. roll form
The figure above is for a conventional cut tap that removes material. A roll-form (thrust) tap displaces metal instead of cutting it and needs a larger hole — reusing a cut-tap chart for form tapping gives oversize, loose threads.
Assembling after tapping? M18 bolt torque spec
References
- ISO 724 — Metric screw threads: basic dimensions.
- Machinery’s Handbook, “Tapping and Thread Cutting” — tap drill sizes and thread-engagement relation.
Last reviewed: 2026-07 · Calculation method & assumptions