Using a cloth measuring tape, measure all the way around your head at the largest point. This will give the cirumference of your head in inches that can be matched to the proper sized helmet in the list below.

 

xx-Small: 6 1/2 Hat Size (20 1/2 Inches)

x-Small:   6 5/8 - 6 3/4 Hat Size (20 7/8 - 21 1/4 Inches)

Small:      6 7/8 - 7 Hat Size (21 5/8 - 22 Inches)

Medium:  7 1/8 - 7 1/4 Hat Size (22 3.8 - 22 3/4 Inches)

Large:      7 3/8 - 7 1/2 Hat Size (23 1/8 - 23 1/2 Inched)

x-Large:   7 5/8 - 7 3/4 Hat Size (24 - 24 3/8 Inches)

xx-Large: 7 7/8 - 8 Hat Size (24 3/4 - 25 1/8 Inches)