Using a tape measure, measure your head about one-half inch above your eyebrows where your hat will rest. Be certain that the tape measure is straight, not twisted.
Alternatively, you can use piece of string to measure your head, and then measure the length of the string with a ruler.

Size XS Small Medium Large XL XXL
Fedora Size 6 5/8 6 7/8 7 1/8 7 3/8 7 5/8 7 3/4
INCHES 20 3/4 in 21 1/2 in 22 1/4 in 23 in 23 7/8 in 24 1/4 in
Centimeters 53cm 55cm 57cm 59cm 61cm 62cm