Many people believe that the wrench that fits a nut or bolt is the actual size of the bolt. This is not true -- bolt heads of various sizes are often found on bolts of a given diameter. To make things more complicated, an even larger selection of nuts require various wrench sizes for the same bolt diameter. Knowing the correct size of a nut or bolt is especially important when those parts become worn and need to be replaced.
Determine the diameter of the bolt shaft to correctly identify the bolt size. The easiest way to do this is to place a vernier caliper over the diameter of the bolt and read the size on the vernier scale. If a verier caliper is not available, go on to the next step.
Remove the nut from the bolt and insert the threaded end of the bolt into a drill bit gauge. A 3/8-inch bolt will fit snugly into a 3/8-inch drill gauge. If a drill gauge is not available, go on to the next step.
Place a steel rule or tape measure across the centre of the bolt's threads. Determine the size of the bolt diameter by the measurement on the steel rule.
Another way to determine the size of a bolt is to learn the wrench sizes that fit the bolt diameter. For example, a bolt that uses a 9/16-inch wrench is a 3/8-inch bolt.