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.
Frequently used tap and drill combinations at a glance
| Tap | Pitch / TPI | Recommended drill | Thread % |
|---|---|---|---|
| M3 | 0.5 mm | 2.5 mm | 75% |
| M4 | 0.7 mm | 3.3 mm | 75% |
| M5 | 0.8 mm | 4.2 mm | 75% |
| M6 | 1.0 mm | 5.0 mm | 75% |
| M8 | 1.25 mm | 6.8 mm | 75% |
| M10 | 1.5 mm | 8.5 mm | 75% |
| M12 | 1.75 mm | 10.2 mm | 75% |
| 1/4-20 UNC | 20 TPI | #7 (0.201”) | ~75% |
| 5/16-18 UNC | 18 TPI | F (0.257”) | ~75% |
| 3/8-16 UNC | 16 TPI | 5/16” (0.313”) | ~75% |
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.
Blind holes: extra depth and tap selection
Through holes are simpler — the tap runs in, threads form, and you’re done. Blind holes need care on two fronts. First, drill deeper than the required thread depth to give the tap a chip reservoir and clearance for the tap’s non-cutting tip. A safe rule is to drill at least 3–5 thread pitches deeper than the required thread depth. Second, for threads near the bottom of a blind hole, switch from a taper tap (which starts cutting only after 8–10 threads) to a plug tap (starts after 3–5 threads) or a bottoming tap (starts on the first thread). Using a taper tap in a short blind hole often produces an untapped dead zone at the bottom that causes fastener seating problems.
Material-specific tips
- Aluminium: generous cutting fluid reduces galling. Use a spiral-flute tap for blind holes — the flutes pull chips upward out of the hole.
- Stainless steel: work-hardens instantly if the tap dwells. Maintain forward pressure, use sulphurised cutting oil, and never dwell at the bottom of a stroke.
- Cast iron: chips are brittle and self-lubricating. Dry tapping is common and works well, but blow the hole clear between passes.
- Mild steel: standard tapping oil. Hand-tapping large sizes (M12+) can be tiring; a tap wrench with a long handle controls torque better than a T-handle.
- Plastic and composites: reduce thread percentage to 60–65% (slightly larger drill) to ease tapping and reduce the risk of cracking brittle materials.
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.