Getting the right fit is very personal and important and its well worth the time to get your measurements correct. There are several techniques to measuring your head the most accurate is using a cloth measuring tape also called a “tailors tape”.
(XS) 21 3/4" (S) 22 1/2" (M) 23 1/4" (L) 23 3/4”
Another way is to use a fitted baseball hat that is sized usually in the range of 7 1/8” - 7 5/8”.
Please keep in mind how and where you wear your hat (low on the forehead, in windy environments, loose over the hair) Do NOT use other hats in your collection for sizing.