Men's hat sizes: Men's hat sizes run from 6 3/4 to 8. If you are not sure of your size, you can also go by inches.
Measure your head around the forehead, to the back of the head and back to the middle of the forehead at the largest part. The chart below is a guide. I think you can vary by a 1/4 inch without much problem.
Small: 6-3/4 to 6-7/8 or 21-1/8" to 21.5"
Medium: 7 to 7-1/8 or 21-7/8" to 22-1/4"
Large: 7-1/4 to 7-3/8 or 22-5/8" to 23"
Extra Large 7-1/2 to 7-5/8 or 23.5" to 23-7/8"

Men's hats