Tap drill calculator
The tap drill for an M8 × 1.25 thread at 75% engagement is 6.8 mm, from drill Ø = major Ø − (pitch × %) ÷ 76.98. This page computes the tap drill diameter for any metric or inch thread and thread-engagement percentage, then shows the nearest standard drill and the actual percent of thread it produces.
65–75% engagement suits most work. Drop lower (50–60%) for tough materials, small taps, and deep holes, where full-depth threads multiply torque and break taps for almost no strength gain.
- Tap drill
- 6.782mm
- Tap drill (converted)
- 0.2670in
Nearest standard drills
| Drill | Size (in) | Size (mm) | Actual % thread |
|---|---|---|---|
| 6.8 mmbest | 0.2677 | 6.800 | 73.9 |
| H | 0.2660 | 6.756 | 76.6 |
| 17/64 | 0.2656 | 6.747 | 77.2 |
Metric drill Ø = major Ø − pitch·% ÷ 76.98. Inch drill Ø = major Ø − 0.01299·% ÷ TPI.
Sizing a fit instead of a hole to tap? The fits & limits calculator works out hole and shaft deviations from ISO 286.
How is tap drill size calculated?
A tap does not cut a full-depth thread; the drilled hole is left slightly larger than the thread’s minor diameter so only part of the thread flank is formed. The percent of thread sets how much.
For metric threads: drill Ø = major Ø − (pitch × %thread) ÷ 76.98. For inch threads: drill Ø = major Ø − (0.01299 × %thread) ÷ TPI. The constant 76.98 (and its inch equivalent 1 ÷ 0.01299) comes from the geometry of the standard 60° thread form.
What percent of thread should I use?
Going from 75% to 100% thread engagement adds only a few percent of thread strength but sharply increases tapping torque. Most shops tap at 65–75%. Reduce it for hard alloys, thin sections, blind holes, and small-diameter taps to cut torque and reduce breakage.