This is a bit tricky as some people like their hats to come down over their ears and some like them to perch more on top. Use the measurements given togive you an idea of how it wll sit on you.
The circumference is measured where the lining meets the outer, so you need to measure the part around your head where you would like that to sit, ie if you want your ears covered then measure over your ears. The crown to brim distance will tell you how far the hat will go down your head. Measure from the crown of your head, to the front and the back to see where it will sit.
Knitted hats have more stretch, typically 4-5cm. Felted hats have 1-2cm and sewn fabric don't usually have any stretch.
