Tap Drill and Clearance Hole Chart
Tap drill sizes and clearance holes for metric coarse threads M2 to M24 (ISO 273 medium fit) and common UNC threads, for machinists and designers.
Metric coarse threads
| Thread | Tap drill (mm) | Clearance hole, medium (mm) |
|---|---|---|
| M2 × 0.4 | 1.6 | 2.4 |
| M2.5 × 0.45 | 2.05 | 2.9 |
| M3 × 0.5 | 2.5 | 3.4 |
| M4 × 0.7 | 3.3 | 4.5 |
| M5 × 0.8 | 4.2 | 5.5 |
| M6 × 1.0 | 5.0 | 6.6 |
| M8 × 1.25 | 6.8 | 9.0 |
| M10 × 1.5 | 8.5 | 11.0 |
| M12 × 1.75 | 10.2 | 13.5 |
| M14 × 2.0 | 12.0 | 15.5 |
| M16 × 2.0 | 14.0 | 17.5 |
| M20 × 2.5 | 17.5 | 22.0 |
| M24 × 3.0 | 21.0 | 26.0 |
UNC threads
| Thread | Tap drill |
|---|---|
| #4-40 | #43 (2.26 mm) |
| #6-32 | #36 (2.71 mm) |
| #8-32 | #29 (3.45 mm) |
| #10-24 | #25 (3.80 mm) |
| 1/4-20 | #7 (5.11 mm) |
| 5/16-18 | F (6.53 mm) |
| 3/8-16 | 5/16 in (7.94 mm) |
| 1/2-13 | 27/64 in (10.72 mm) |
Metric tap drill ≈ nominal diameter − pitch (about 75% thread engagement). Clearance holes follow ISO 273 medium series.
Choosing the tap drill
For metric coarse threads the tap drill is the nominal diameter minus the pitch, which gives roughly 75% thread engagement — a good balance between strength and tapping torque. In hard materials or for deep threads, a slightly larger drill reduces tap breakage; in soft aluminum, thread-forming taps need a larger hole than cutting taps.
FAQ
What size drill for an M6 tap?
5.0 mm for M6 × 1.0 coarse thread.
What size drill for a 1/4-20 tap?
A #7 drill (0.201 in, 5.11 mm).