MECHANICAL DESIGN · METRIC REFERENCE
Metric Counterbore Size Chart M1.6–M48: Diameter & Depth
A counterbore is a flat-bottom recess for a socket-head cap screw. Find counterbore diameter A, depth B, under-head countersink diameter C and clearance hole D for normal or close fit. All dimensions are in mm.
18 sizes shown · all dimensions in mm
| Screw | Counterbore Ø A | Depth B | Under-head countersink Ø C | Clearance D · normal | Clearance D · close |
|---|---|---|---|---|---|
| M1.6 | 3.50 | 1.60 | 2.00 | 1.95 | 1.80 |
| M2 | 4.40 | 2.00 | 2.60 | 2.40 | 2.20 |
| M2.5 | 5.40 | 2.50 | 3.10 | 3.00 | 2.70 |
| M3 | 6.50 | 3.00 | 3.60 | 3.70 | 3.40 |
| M4 | 8.25 | 4.00 | 4.70 | 4.80 | 4.40 |
| M5 | 9.75 | 5.00 | 5.70 | 5.80 | 5.40 |
| M6 | 11.20 | 6.00 | 6.80 | 6.80 | 6.40 |
| M8 | 14.50 | 8.00 | 9.20 | 8.80 | 8.40 |
| M10 | 17.50 | 10.00 | 11.20 | 10.80 | 10.50 |
| M12 | 19.50 | 12.00 | 14.20 | 13.00 | 12.50 |
| M14 | 22.50 | 14.00 | 16.20 | 15.00 | 14.50 |
| M16 | 25.50 | 16.00 | 18.20 | 17.00 | 16.50 |
| M20 | 31.50 | 20.00 | 22.40 | 21.00 | 20.50 |
| M24 | 37.50 | 24.00 | 26.40 | 25.00 | 24.50 |
| M30 | 47.50 | 30.00 | 33.40 | 31.50 | 31.00 |
| M36 | 56.50 | 36.00 | 39.40 | 37.50 | 37.00 |
| M42 | 66.00 | 42.00 | 45.60 | 44.00 | 43.00 |
| M48 | 75.00 | 48.00 | 52.60 | 50.00 | 49.00 |
No matching size. Choose “Show all”; for unlisted sizes, use the drawing and fastener data.
Source: Carr Lane — Trigonometry Tables & Handy References, Rev. 9/2021, p. 9. Checked against this reference on 2026-10-26. These are the source’s machining reference dimensions, not a complete ISO 4762 or other standard specification. Confirm the drawing, actual head dimensions and tolerances before machining.
Reading A, B, C and D
A is the cylindrical recess diameter; B is depth from the surface to its flat seat. C refers to the chamfer at the through-hole entrance under the head, not the head diameter of a flat-head screw. D is the through hole.
Common size examples
M6: Counterbore Ø11.20 × 6.00 deep; normal clearance Ø6.80, close clearance Ø6.40.
M8: Counterbore Ø14.50 × 8.00 deep; normal clearance Ø8.80, close clearance Ø8.40.
M10: Counterbore Ø17.50 × 10.00 deep; normal clearance Ø10.80, close clearance Ø10.50.
Reference tables may use different allowances. Check head recess, washers, corner radii and tool access separately.
Frequently asked questions
Why do other counterbore charts give different values?
Clearance and head-recess allowances vary between design references. This table uses one named source consistently instead of mixing fit conventions. It does not override a project drawing.
What about M18 or M22?
The cited reference does not list these sizes. This page does not interpolate machining dimensions; consult the actual fastener data and the project’s hole-design specification.
How do I calculate a conical countersink depth?
For a conical countersink, use h = (D − d)/(2 tan(θ/2)). With a 90° included angle, depth is half the difference between major and pilot diameters. Open the countersink depth calculator →