How is tap drill size calculated?

For metric threads the tap drill diameter equals the major diameter minus the pitch, which gives roughly 75 percent thread engagement. For Unified inch threads it equals the major diameter minus one divided by the threads per inch. The tool shows both the standard recommended bit and this calculated value.

Why 75 percent thread engagement and not 100 percent?

A full 100 percent thread adds only about 5 percent more strength but dramatically increases tapping torque and tap breakage risk. The 75 percent target gives nearly all the holding strength while letting the tap cut cleanly, so it is the standard industry recommendation.

What is the drill size for an M6 tap?

A standard M6 by 1.0 mm coarse tap uses a 5.0 mm drill. That follows the rule of major diameter minus pitch, which is 6 minus 1, equalling 5 mm. Fine-pitch M6 threads would need a slightly larger drill.

What happens if the drill is too small or too large?

Too small a hole leaves too much material, raising tapping torque and often snapping the tap inside the hole. Too large a hole produces shallow, weak threads that strip under load. Use the recommended size for reliable, strong threads.

Do these sizes work for blind and through holes?

The drill diameters are the same for both. For blind holes drill deeper than the thread depth to leave room for chips and the tap point, and prefer a bottoming tap to thread close to the bottom of the hole.

Drill & Tap Size Chart

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Tapping a clean, strong thread starts with the right hole. Drill it too small and the tap snaps; too large and the threads strip. This chart pairs every common metric and imperial tap with the correct drill bit and shows the formula behind the recommendation.

How it works

A tapped hole should keep about 75% of the full thread depth. That gives nearly all the holding strength of a perfect thread while leaving enough clearance for the tap to cut without binding. The drill diameter to hit that target is:

metric:  drill Ø(mm) = major Ø − pitch
inch:    drill Ø(in) = major Ø − (1 / TPI)

So an M8 x 1.25 tap needs a 8 − 1.25 = 6.75 mm hole (rounded to the standard 6.8 mm bit), and a 1/4-20 UNC tap needs about 0.25 − 1/20 = 0.20 in, served by a #7 drill at 0.201 in. The table lists the standard recommended bit in green and the exact calculated value beside it.

Example and tips

To tap an M6 x 1.0 hole: drill 5.0 mm, then run the M6 tap, backing it off a quarter-turn periodically to break the chips. Use cutting fluid on steel and aluminium to extend tap life and improve thread finish.

For brittle materials or hand-tapping small sizes, going one drill size larger (reducing thread percentage toward 60–65%) eases tapping torque and reduces breakage at the cost of a little strength. For maximum strength in a critical fastener, stay at the listed 75% size.

Notes

Drill diameters here are nominal. Real holes drill slightly oversize because of bit runout and material spring-back, so measure on a test piece for precision work. Number, letter, and fractional drill designations follow the standard US drill-size series.