English hat sizes are the diameter of the head in inches. The American size is the same but add 1/8 of an inch.
The European and Australian hat sizes are the same – both are the circumference of the head in centimetres.
Generally, men’s hat sizes are diameter – women’s circumference.
Hint: Measure your head at about the middle of your forehead. The tape should be snug but not tight. Measure your head 3 times, taking the tape away from you head between each measurement. Write each figure down. Have a friend do the same. Then add your 6 measurements together and divide by 6 (average). The result should be your perfect hat size.