Bolt Sizes

As any handyman, repair person or mechanic can tell you, if you work with small engines long enough you will eventually have to replace a broken or missing bolt. But which size do you need? How do you convert Metric and Standard? This bolt size chart can help!

Bolt Size Chart

If you need to know which socket to use with your bolts, or convert a socket between metric and standard, you can do so with this handy socket size chart.

Bolt Head Size Chart

Bolt Size Standard Diameter Metric Diameter
1/4 0.25 in. 6.35 mm
5/16 0.3125 in. 7.9375 mm
3/8 0.375 in. 9.525 mm
7/16 0.4375 in. 11.1125 mm
1/2 0.5 in. 12.7 mm
9/16 0.5625 in. 14.2875 mm
5/8 0.625 in. 15.875 mm
3/4 0.75 in. 19.05
7/8 0.875 in. 22.225 mm
1 1 in. 25.4 mm
1 1/8 1.125 in. 28.575 mm
1 1/4 1.25 in. 31.75 mm
1 3/8 1.375 in. 34.925 mm
1 1/2 1.5 in. 38.1 mm
1 3/4 1.75 in. 44.45 mm
2 2 in. 50.8 mm
2 1/4 2.25 in. 57.15 mm
2 1/2 2.5 in. 63.5 mm
2 3/4 2.75 in. 69.85 mm
3 3 in. 76.2 mm

All bolt sizes are taken across the head of the bolt so that bolt sizes and socket or wrench sizes are the same. This chart is especially useful if you do not have the right Standard size spanner and need to use the nearest Metric spanner size.

Metric Bolt Sizes

Metric bolts have an M stamped on the bolt head followed by a value. Although Metric bolts are measured in millimeters(mm) the M value is a measurement size, not the size of the bolt.

Use this chart to determine the mm size of the M value stamped on a bolt. Bolt size is measured across the head of hex and square bolts, from flat side to flat side, which is often referred to as head size.

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