Size One (small) : everything up to a 55 cm head circumference.

Size Two (med-large) : everything over a 56cm head circumference.

For people on the cusp of the sizes, size 1 hats are shallower, sitting about 1 cm higher above the ears. Size 2 is deeper and can be tightened with the ribbon ties at the back, but only down to 56 cm circumference.