Verified by Rachel Mayfield, Supply Chain Analyst - June 2026
Photo by mefodiy on Pexels
Spanner and hex-key sizes for metric hexagon bolts and socket cap screws, M3 to M30. Spanner Size Chart for Metric Bolts The spanner size is the across-flats (A/F) width of the bolt head, set by ISO 4014 and ISO 4017 for hex bolts and ISO 4762 for the hex-key socket in cap screws. An M10 hex bolt takes a 16 mm spanner under the current ISO standard, and an M10 socket cap screw takes an 8 mm hex key.
| Bolt size |
Spanner A/F ISO (mm) |
Older DIN A/F (mm) |
Hex key (mm) |
| M3 |
5.5 |
- |
2.5 |
| M4 |
7 |
- |
3 |
| M5 |
8 |
- |
4 |
| M6 |
10 |
- |
5 |
| M8 |
13 |
- |
6 |
| M10 |
16 |
17 |
8 |
| M12 |
18 |
19 |
10 |
| M14 |
21 |
22 |
12 |
| M16 |
24 |
- |
14 |
| M18 |
27 |
- |
14 |
| M20 |
30 |
- |
17 |
| M22 |
34 |
- |
17 |
| M24 |
36 |
- |
19 |
| M27 |
41 |
- |
19 |
| M30 |
46 |
- |
22 |
How to find the right spanner size
Match the bolt thread size in the first column to the across-flats spanner size. The thread size (M10) is the diameter of the thread, not the spanner size - an M10 bolt doesn't take a 10 mm spanner. If you don't know the thread size, measure the across-flats width of the head with calipers and read the chart backwards.
Two width standards exist. Current ISO 4014/4017 sets M10 at 16 mm and M12 at 18 mm. Older DIN 931/933 bolts, still common in machinery built before the 1990s, used 17 mm for M10 and 19 mm for M12. The DIN column lists the sizes that differ - carry both a 16 and a 17 mm spanner if you work on mixed-age equipment.
Socket cap screws (ISO 4762) use a hex key, not a spanner, and the key size is much smaller than the head: an M10 cap screw takes an 8 mm hex key. See the metric bolt torque chart for the tightening torque once you have the right tool engaged.
Related standards
- ISO 4014 / ISO 4017 - Hexagon head bolts and set screws (across-flats widths)
- ISO 4762 - Hexagon socket head cap screws (hex-key sizes)
- DIN 931 / DIN 933 - Older hex bolt standard (17 mm for M10, 19 mm for M12)