The size is calculated through the head circumference in the larger point , about cm 2 over the eyebrow.
Here below you find the chart with sizes, the size is calculated with the measure of head circumference.
XS - 6 1/2” - 6 5/8”
S - 6 3/4” – 6 7/8”
M - 7” – 7 1/8”
L - 7 1/4” – 7 3/8”
XL - 7 1/2” – 7 5/8”
XXL - 7 3/4” – 7 7/8”