A reference guide covering what size spanner/wrench should be used with a hex bolt, or a hex key with a socket screw.
Use this guide to help you select the right size spanner/wrench or hex/allen key when tightening or loosening a fastener. It's important to use the correct size to prevent damage to the fastener.
The only real caveat to watch out for is on hexagonal fasteners, at sizes M10, M12, M14, and M22. At these sizes, it is notable that DIN standard fasteners (e.g. DIN 931 / DIN 933 / DIN 934) differ slightly from ISO standard fasteners (e.g. ISO 4014 / ISO 4017 / ISO 4032). The difference is only very slight, but it does technically mean that a different size spanner/wrench should be used.
| Metric Diameter | Spanner/Wrench Size (mm) Hex Bolt and Hex Nut |
Hex Key Size (mm) Socket Cap |
Hex Key Size (mm) Socket Grub |
Hex Key Size (mm) Socket Button and Socket Countersunk |
|---|---|---|---|---|
| M1 | 2.5 | |||
| M1.2 | 3 | |||
| M1.4 | 3 | 1.3 | 0.7 | |
| M1.6 | 3.2 | 1.5 | 0.7 | |
| M1.8 | 0.7 | |||
| M2 | 4 | 1.5 | 0.9 | |
| M2.5 | 5 | 2 | 1.3 | |
| M3 | 5.5 | 2.5 | 1.5 | 2 |
| M3.5 | 6 | |||
| M4 | 7 | 3 | 2 | 2.5 |
| M5 | 8 | 4 | 2.5 | 3 |
| M6 | 10 | 5 | 3 | 4 |
| M7 | 11 | |||
| M8 | 13 | 6 | 4 | 5 |
| M10 | ISO: 16 DIN: 17 |
8 | 5 | 6 |
| M12 | ISO: 18 DIN: 19 |
10 | 6 | 8 |
| M14 | ISO: 21 DIN: 22 |
12 | 6 | 10 |
| M16 | 24 | 14 | 8 | 10 |
| M18 | 27 | 14 | 10 | 12 |
| M20 | 30 | 17 | 10 | 12 |
| M22 | ISO: 34 DIN: 32 |
17 | 12 | 14 |
| M24 | 36 | 19 | 12 | 14 |
| M27 | 41 | 19 | ||
| M30 | 46 | 22 | ||
| M33 | 50 | 24 | ||
| M36 | 55 | 27 | ||
| M39 | 60 | |||
| M42 | 65 | 32 | ||
| M45 | 70 | |||
| M48 | 75 | 36 | ||
| M52 | 80 | |||
| M56 | 85 | 41 | ||
| M60 | 90 | |||
| M64 | 95 | 46 | ||
| M68 | 100 | |||
| M72 | 105 | 55 | ||
| M76 | 110 | |||
| M80 | 115 | 65 | ||
| M85 | 120 | |||
| M90 | 130 | 75 | ||
| M100 | 145 | 85 | ||
| M110 | 155 | |||
| M125 | 180 | |||
| M140 | 200 | |||
| M160 | 230 |