Head sizes: (cms)

Take a measuring tape and measure around your head just above your ears. Make sure it is not too tight and is comfortable. An average size for a woman is a 57 and 59 for a man.

Ladies hats:
Small/ medium is a 56/57
Medium/ large is a 57/58

Mens Hats:
Medium is a 57
Large is a 59
X Large is a 61