To measure your head you will need to use a cloth measuring tape (or a piece of string that will not stretch, which you will then hold up to a tape measure)
Hold the string or tape measure firmly, about 1/8" above your ear, across your mid forehead and around the back of your head. Basically you need to measure exactly where the hat will sit.
Small - 20.5"
Medium - 21.5"
Large - 22.5"
X-large - 23.5"
If your measurement falls between sizes then go the next size up. Hat sizes, like other apparel, will vary from brand to brand and depending on fit preferences and your specific head shape!