The drill is the bolt size minus one thread pitch
For a metric thread the pitch is already in millimeters, so an M6 x 1.0 wants a 6 − 1 = 5.0 mm drill. For an inch thread the pitch is 1÷TPI, and the exact rule is major − 1.0825 × pitch, which lands at about 75% thread engagement. In practice you round to the nearest real drill in the fractional, number or letter series.
Tap drill by thread size
| Thread | Coarse (UNC) drill | Fine (UNF) drill |
|---|---|---|
| #4 | #43 (0.089") | #42 (0.094") |
| #6 | #36 (0.107") | #33 (0.113") |
| #8 | #29 (0.136") | #29 (0.136") |
| #10 | #25 (0.150") | #21 (0.159") |
| 1/4" | #7 (0.201") | #3 (0.213") |
| 5/16" | F (0.257") | G (0.261") |
| 3/8" | 5/16" (0.313") | Q (0.332") |
| 1/2" | 27/64" (0.422") | 29/64" (0.453") |
Coarse (UNC) and fine (UNF) threads of the same nominal size take different drills, because fine threads have a shallower pitch and so remove less metal.
Common metric taps
| Thread | Pitch | Tap drill |
|---|---|---|
| M3 | 0.5 mm | 2.5 mm |
| M4 | 0.7 mm | 3.3 mm |
| M5 | 0.8 mm | 4.2 mm |
| M6 | 1.0 mm | 5.0 mm |
| M8 | 1.25 mm | 6.8 mm |
| M10 | 1.5 mm | 8.5 mm |
| M12 | 1.75 mm | 10.2 mm |
| M16 | 2.0 mm | 14.0 mm |
Getting the drill wrong
- Too small. The tap has to cut too much metal, so it binds and can snap off in the hole. Broken taps are hard to remove.
- Too large. The threads come out shallow and weak, and the bolt strips out under load.
- Match coarse to coarse. A drill sized for 1/4-20 UNC leaves the wrong engagement for a 1/4-28 UNF tap. Check which pitch you are cutting first.
- Tap deeper than the bolt. Give a blind hole extra depth so the last few turns of the bolt still meet full thread.
Common questions
What size drill do I use for a 1/4-20 tap?
A #7 drill (0.201 inch). That leaves roughly 75% thread engagement, which is the standard target for a strong, tappable hole.
What is the tap drill for an M6 thread?
A 5.0 mm drill for standard M6 x 1.0. The rule is drill = 6 minus the 1.0 mm pitch, which gives 5.0 mm exactly.
Why not just drill the hole to the full bolt diameter?
Then there would be no material left for the tap to cut threads into, and the hole would be a clearance hole, not a tapped one. The drill has to be smaller than the major diameter so the peaks of the thread have metal to form in.
