Verified by Rachel Mayfield, Supply Chain Analyst - June 2026
Photo by Daniel Smyth on Pexels
Metric tapping drill and clearance hole sizes for coarse-pitch threads M1.6 to M24, from ISO 261 thread pitches and ISO 273 clearance holes. Metric Tap and Clearance Drill Chart The tap drill is the hole you drill before cutting an internal thread; the clearance drill is the through hole a bolt passes freely. For an ISO coarse thread the tap drill is close to the major diameter minus the pitch - an M6 coarse (1.0 mm pitch) taps with a 5.0 mm drill for about 75 percent thread engagement.
| Size |
Coarse pitch (mm) |
Tap drill (mm) |
Close clearance (mm) |
Medium clearance (mm) |
| M1.6 |
0.35 |
1.25 |
1.7 |
1.8 |
| M2 |
0.40 |
1.60 |
2.2 |
2.4 |
| M2.5 |
0.45 |
2.05 |
2.7 |
2.9 |
| M3 |
0.50 |
2.50 |
3.2 |
3.4 |
| M4 |
0.70 |
3.30 |
4.3 |
4.5 |
| M5 |
0.80 |
4.20 |
5.3 |
5.5 |
| M6 |
1.00 |
5.00 |
6.4 |
6.6 |
| M8 |
1.25 |
6.80 |
8.4 |
9.0 |
| M10 |
1.50 |
8.50 |
10.5 |
11.0 |
| M12 |
1.75 |
10.20 |
13.0 |
13.5 |
| M14 |
2.00 |
12.00 |
15.0 |
15.5 |
| M16 |
2.00 |
14.00 |
17.0 |
17.5 |
| M20 |
2.50 |
17.50 |
21.0 |
22.0 |
| M24 |
3.00 |
21.00 |
25.0 |
26.0 |
How to use the metric tap drill chart
For a coarse-pitch thread the quick rule is tap drill equals major diameter minus pitch: M8 coarse is 8 minus 1.25, so a 6.8 mm drill. The chart already lists the standard drill for each size at roughly 75 percent engagement, which is the normal target - going to a larger drill (lower engagement) reduces tapping torque and tap breakage at a small cost in thread strength, which is often worth it in steel.
The clearance columns are for the bolt's through hole, not the thread. Close clearance (ISO 273 fine) keeps parts located precisely; medium clearance (ISO 273 medium) is the general-purpose fit and gives room for slight misalignment. For an M10 bolt, drill 10.5 mm for a close fit or 11.0 mm for a medium fit.
These tap drills are for coarse pitch. Fine-pitch threads (for example M8 fine at 1.0 mm) use a larger tap drill - 7.0 mm rather than 6.8 mm - because the smaller pitch removes less material. Always check the pitch stamped on the tap before drilling.
Related standards
- ISO 261 - ISO general purpose metric screw threads, general plan (pitches)
- ISO 965 - ISO general purpose metric screw threads, tolerances
- ISO 273 - Fasteners, clearance holes for bolts and screws