Use a tape measure (or a length of string that you will have to hold up to a tape measure) to determine the length.
Place the string or tape around your head about 1/8″ above your ear, across the mid-forehead, completely circling your head. Hold the tape firmly, but not too tightly. Basically you need to measure your head exactly where the hat will sit.
Using the chart below, match your head measurement to the corresponding hat size. If your measurement falls between sizes, choose the next largest size.