M18 × 2.5 tap drill size
15.50 mm drill — or 39/64 (0.6094 in) = 78% thread
M18 coarse series, 2.5 mm pitch. Tap drill Ø = nominal − pitch = 18 − 2.5 = 15.50 mm (≈77% thread, the standard chart value).
| Drill | Size | Thread % |
|---|---|---|
| 15.50 mm | 0.6102 in | 77% |
| 39/64 (inch) | 0.6094 in / 15.48 mm | 78% |
Rounding to your nearest stock drill moves the thread percentage only a few points. Cut taps only — roll-form taps need a larger hole. Verify before you cut.
Need a different thread percentage? Open the tap drill calculator · Metric tap chart · All calculators