Metric Spanner/Wrench and Hex Key Sizes

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