Measure the threaded part on the outside of the threads using a Vernier caliper if possible, and clamping the calipers against the outside of the threads. Do not measure the head of the bolt or the screw.

If you are using a ruler or a tape measure, do your best to measure the outside of the threads by placing the ruler or tape measure across the threaded part and align the zero with the outside thread on the left and checking which number best lines up with the outside thread on the right, for example 16. Assuming this has been done fairly accurately, you now know that the diameter of the bolt is 16mm. When stating the diameter of a metric bolt, use the letter “M” followed by the size. In this instance your bolt should be referred to as a M16 bolt. If your bolt is imperial then it will be measured in inches and in this example it would have measured 5/8” of an inch in which case your bolt diameter is simply 5/8”.

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