How to calculate Allen key size?

To calculate the size of an Allen key (hex key), measure the distance across the flat sides of the key’s hexagonal head, typically using calipers. Sizes are commonly specified in millimeters or inches, depending on the standard (metric or imperial) used. If you don't have calipers, you can compare the key to a known size or use a sizing chart for reference. Always ensure the key fits snugly into the socket to avoid stripping the screw.