Unsure on what size countersunk hole to use for your ISO flat head fasteners?
Use the image and chart below to determine what dimensions to use, all dimensions are in millimeters apart from the countersink angle.
For example, an ISO M10 flat head screw with a normal fit countersunk hole will require a pilot hole diameter of 11 mm, a countersunk diameter of 22.73 mm, and a countersunk angle of 90°.
Countersunk Hole Size Chart for Flat Head Fasteners (ISO)
All dimensions are in millimeters apart from the countersink angle.
| Fastener Size (Thread) | Pilot Hole Diameter (Close Fit H12) | Pilot Hole Diameter (Normal Fit H13) | Pilot Hole Diameter (Loose Fit H14) | Countersink Diameter | Countersink Angle (°) |
| M3 | 3.2 | 3.4 | 3.6 | 6.94 | 90 |
| M3.5 | 3.7 | 3.9 | 4.2 | 8.96 | 90 |
| M4 | 4.3 | 4.5 | 4.8 | 9.18 | 90 |
| M5 | 5.3 | 5.5 | 5.8 | 11.47 | 90 |
| M6 | 6.4 | 6.6 | 7 | 13.71 | 90 |
| M8 | 8.4 | 9 | 10 | 18.25 | 90 |
| M10 | 10.5 | 11 | 12 | 22.73 | 90 |
| M12 | 13 | 13.5 | 14.5 | 27.21 | 90 |
| M16 | 17 | 17.5 | 18.5 | 33.99 | 90 |
| M20 | 21 | 22 | 24 | 40.71 | 90 |