Things You'll Need
Bolt width gauge (metric and standard)
Nut and bolt gauge (metric and standard)
Thread pitch gauge (metric and standard)
Bolts and nuts will never interchange both standard and metric measurements on the same nut or bolt. Only metric measurement or standard measurement will apply for each individual nut and bolt. Once you've taken one measurement and ascertained a metric or standard measurement (excluding the shank length), only apply the correct metric or standard measuring devices to continue with that same nut or bolt.
Bolts commonly use four types of measurement, while nuts use only two. Bolts are measured in length of shank, bolt head size, width of shank (the diameter of the threaded bolt body) and the thread pitch (the size of the thread). Nuts are measured by the width of the hex shape and by their thread pitch. Bolts and nuts come in many different measurements and cannot be interchanged. Most nut and bolt measurements come in either metric sizing or standard sizing. Strength of nuts and bolts is also considered a measurement and determined by their grade of alloy.
Place the head of a bolt or a nut into a nut and bolt gauge. Nut and bolt gauges are available in most hardware and automotive parts stores and come in both metric and standard sizing. One hole of the gauge will fit the bolt head or nut snugly (be sure you're not mixing up metric and standard gauges) and will not allow the bolt head or nut to wiggle inside the gauge. The size of the bolt head or nut will be stamped on the gauge pertaining to the hole you choose.
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Measure the shank length of the bolt from the bottom of the head (where it meets the shank) to the bottom of the bolt, using a measuring tape to determine its overall length. Since the difference between metric and standard length bolts is very slight, most often, this measurement is done in inches.
Place the shank of the bolt into the bolt width gauge. This measurement is the diameter of the shank and can be in either metric or standard sizing. Use each to determine the difference. Three-eighths inch will be slightly smaller than a 10-mm measurement, but a ½ inch will be slightly bigger than a 12-mm measurement. Because millimeter and inches fluctuate, you'll have to try both metric and standard gauges to see which one is more snug to the bolt, which will determine its actual diameter width. The hole in the gauge will be stamped with the size width.
Unfold and attach the thread pitch gauge to the threads of the bolt. Again, because millimeter and inch measurements are slightly different, there will only be one gauge key on the thread gauge that will fit the threads perfectly. Each key will have a measurement stamped on it, revealing the pitch measurement.
Thread a nut onto the shank of a bolt once you have determined the thread pitch size of the bolt. A nut that threads onto a bolt and does not allow a wiggling motion on the shank of the bolt is the correct nut for the bolt, and the pitch is identical.