Place the string or tape around your head just above your ear all around your head across the mid-forehead, without holding  the tape too tightly.
 
O/S = One Size 
KN O/S
Centimetres 55 56 57 58 59 60 61 62
Size  S S/M M M/L L L/XL XL XL
UK Size 634 678 7 718 714 738 712 758
US Size 678 7 718 714 738 712 758 734
Inches 21,7 22 22,4 22,8 23,2 23,6 24 24,4