Metric Hex Bolt Head Dimensions
When working with metric fasteners, the wrench size (width across flats) follows a different pattern than US bolts. Here’s the quick reference:
| Bolt Size | Width Across Flats (mm) | Width Across Corners (mm) | Head Height (mm) |
|---|---|---|---|
| M5 | 8 | 9.24 | 3.5 |
| M6 | 10 | 11.55 | 4.0 |
| M8 | 13 | 15.01 | 5.3 |
| M10 | 16 (or 17) | 18.48 | 6.4 |
| M12 | 18 (or 19) | 20.78 | 7.5 |
| M14 | 21 (or 22) | 24.25 | 8.8 |
| M16 | 24 | 27.71 | 10.0 |
| M18 | 27 | 31.18 | 11.5 |
| M20 | 30 | 34.64 | 12.5 |
| M22 | 32 (or 34) | 36.95 | 14.0 |
| M24 | 36 | 41.57 | 15.0 |
| M27 | 41 | 47.34 | 17.0 |
| M30 | 46 | 53.12 | 18.7 |
| M33 | 50 | 57.74 | 21.0 |
| M36 | 55 | 63.51 | 22.5 |
Notes
- M10, M12, M14: Two common head sizes exist depending on the standard (ISO 4014/4017 vs. DIN 931/933). The smaller size is newer ISO; the larger is older DIN. Check your spec.
- Property class is stamped on the head (8.8, 10.9, 12.9) — unlike US bolts which use grade markings (hash marks).
Related: Hex Bolt Grade ID & Mechanical Properties | Metric Tap Drill & Clearance Chart