To figure out your head size go grab a tape measure and measure the circumference of your head just above your ears and about halfway up your forehead (ie where you would wear your hat). Do this 3 times and take an average reading. Now convert this measurement to your head size according to the table below:

Hat Size 6 7/8 = 55cm = 21 1/2"

Hat Size 7 = 56cm = 21 7/8"

Hat Size 7 1/8 = 57cm = 22 1/4"

Hat Size 7 1/4 = 58cm = 22 5/8"

Hat Size 7 3/8 = 59cm = 23"

Hat Size 7 1/2 = 60cm = 23 1/2"

Hat Size 7 5/8 = 61cm = 23 7/8"

Note that hat size S/M will fit head sizes 6 7/8 - 7 1/8 and L/XL fits 7 1/4 - 7 1/2