The head size is measured around the head, from the forehead to the little bump where the head turns into the neck. A good guidline is to measure where you’d like the hat to fit. If you prefere to have the hat straight on top of your head, then measure straight around the head. If you prefer to keep the hat a little tilted, measure where you’d like the hat to sit. It’s important not to pull the measuring tape too tight or too loose. The hat should fit comfortable on the head. The sizes on this site is in centimeters.