Hat size is based on the circumference of the head. In order to determine your proper hat size, you must take a measurement of your head.
Using a tape measure, measure your head about one-half inch above your eyebrows where your hat will rest. Be certain that the tape measure is straight, not twisted. Alternatively, you can use piece of string to measure your head, and then measure the length of the string with a ruler. If you are not certain about size, or are between sizes, buy the larger size.