How to find the right size hat
An average head size for men is size 7 3/8 (using U.S. sizing), 58 cm (using European sizing) or 7 1/8 (using UK sizing).
The average head size for women is 7 1/4 (using U.S. sizing). Of course, head sizes and shapes can also differ from country to country.
How to measure your head
Take a measuring tape and place it round the circumference of the head, slightly above the ears.