Tap drill formula
- Metric: tap drill (mm) = major diameter โ pitch. M10 ร 1.5 โ 10 โ 1.5 = 8.5 mm.
- Inch: tap drill (in) โ major diameter โ 1 รท TPI. 1/2-13 โ 0.500 โ 0.077 = 0.423 in โ 27/64 (0.4219).
- Both give about 75% thread. Pick the nearest standard drill. In tough materials or deep holes, go slightly larger.
UNC (coarse) tap drills
| Thread | Tap drill | Drill (in) |
|---|---|---|
| #4-40 | #43 | 0.0890 |
| #6-32 | #36 | 0.1065 |
| #8-32 | #29 | 0.1360 |
| #10-24 | #25 | 0.1495 |
| 1/4-20 | #7 | 0.2010 |
| 5/16-18 | F | 0.2570 |
| 3/8-16 | 5/16 | 0.3125 |
| 7/16-14 | U | 0.3680 |
| 1/2-13 | 27/64 | 0.4219 |
| 9/16-12 | 31/64 | 0.4844 |
| 5/8-11 | 17/32 | 0.5313 |
| 3/4-10 | 21/32 | 0.6563 |
| 7/8-9 | 49/64 | 0.7656 |
| 1-8 | 7/8 | 0.8750 |
UNF (fine) tap drills
| Thread | Tap drill | Drill (in) |
|---|---|---|
| #10-32 | #21 | 0.1590 |
| 1/4-28 | #3 | 0.2130 |
| 5/16-24 | I | 0.2720 |
| 3/8-24 | Q | 0.3320 |
| 7/16-20 | 25/64 | 0.3906 |
| 1/2-20 | 29/64 | 0.4531 |
| 9/16-18 | 33/64 | 0.5156 |
| 5/8-18 | 37/64 | 0.5781 |
| 3/4-16 | 11/16 | 0.6875 |
| 7/8-14 | 13/16 | 0.8125 |
| 1-12 | 59/64 | 0.9219 |
Metric coarse tap drills
| Thread | Major โ pitch | Common drill (mm) |
|---|---|---|
| M3 ร 0.5 | 2.5 | 2.5 |
| M4 ร 0.7 | 3.3 | 3.3 |
| M5 ร 0.8 | 4.2 | 4.2 |
| M6 ร 1.0 | 5.0 | 5.0 |
| M8 ร 1.25 | 6.75 | 6.8 |
| M10 ร 1.5 | 8.5 | 8.5 |
| M12 ร 1.75 | 10.25 | 10.2 |
| M14 ร 2.0 | 12.0 | 12.0 |
| M16 ร 2.0 | 14.0 | 14.0 |
| M20 ร 2.5 | 17.5 | 17.5 |
| M24 ร 3.0 | 21.0 | 21.0 |
โ ๏ธ Reference only. Always verify against the manufacturer's specs, your jurisdiction's code and your site's procedures.