4XS

6 ⅜  

20 1/16

51

3XS

6 ½ - 6 ⅝  

20 ½ - 20 ⅞

52 - 53

2XS

6 ¾ - 6 ⅞  

21 ¼ - 21⅝  

54 - 55

XS

7

22

56

S

7 ⅛  

22 ⅜

57

M

7 ¼ - 7 ⅜  

22 ¾ - 23 ¼

58 - 59

L

 7 ½

23 ⅝  

60

XL

7 ⅝ - 7 ¾

24 - 24 ⅜

61 - 62

2XL

7 ⅞ - 8

24 ¾ - 25 ¼

63 - 64

3XL

8 ⅛ - 8 ¼

25 ⅝ - 26

65 - 66

4XL

8 ⅜ - 8 ½

26 ⅜ - 26 ¾

67 - 68

Measuring the circumference of the head should be measured  approximately one inch above the eyebrows and at a point in the back of the head that gives you the largest possible measurement.