Selecting the correct metric wrench size can save time, prevent damage to fasteners, and make projects easier. A handy way to choose the right size is to start by understanding the sizes commonly found in metric wrench sets, which usually range from 6mm to 24mm. Check the fastener or bolt you're working with; it often has a stamped size, especially if it's part of automotive or machinery work. If you don't know the size, use a caliper or an adjustable wrench to measure the bolt head's diameter in millimetres. Then, match it with a wrench close in size, testing fit to ensure it doesn’t wiggle or feel loose. Keeping a chart of standard bolt and wrench size pairings can also be a quick reference for common tasks, saving you the hassle of trial and error.

METRIC SIZES
Thread Size
Diameter (mm)
Hexagon Size S (mm)
Hexagon Size J (mm)
M 3 5.5 2.0
M 4 7 3
M 5 8 4
M 6 10 5
M 8 13 6
M 10 17 8
M 12 19 10
M 14 22 12
M 16 24 14
M 18 27 14
M 20 30 17
M 22 32 17
M 24 36 19
M 27 41 19
M 30 46 22
M 33 50 24
M 36 55 27
M 39 60 27 (30)
M 42 65 32
M 45 70 -
M 48 75 36
M 52 80 36
M 56 85 41
M 60 90 46
M 64 95 46
M 68 100 50
M 72 105 55
M 76 110 60
M 80 115 65
M 85 120 70
M 90 130 70 (75)
M 95 135 -
M 100 145 85
M 105 150 -
M 110 155 -
M 115 165 -
M 120 170 -
M 125 180 -
M 130 185 -
M 140 200 -
M 150 210 -