The GLX 501 series are all equiped with a low-profile, sleek presentation and narrow look combined with an lightweight ABS injection-molded thermoplastic shell. The one thing that sets aside these half helmets from everybody else is the soft and comfortable polyester liner which can be removed and or replaced. For proper fit and size measure the circumference of your head 1" above your eyebrows and around the largest portion of the back of your head. 20 7/8" - 21 1/4" =  X-Small; 21 5/8" - 22" = Small; 22 3/8" - 22 3/4" =  Medium; 23 1/4" - 23 5/8" = Large; 24" - 24 3/8" = X-Large; 24 3/4" - 25 1/4" = XX-Large; 25 5/8" - 26" = XXX-Large.