Measure Your Head
You can 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. If your measurement falls between sizes, choose the next largest size.
Another valid method to determine hat size is to try on a hat and check the Hat Circumference