Not sure about your helmet size? Get an “S1 Helmet Co. Fit Kit” and eliminate the guessing. You’ll get 7 different thickness of sizing liners so you can fit your S1 Helmet perfectly today; and if your head grows in the near future! Size: X Small- XXX Large included in fit kit. The Fit Kit also includes a 2mm top pad to lower the helmet toward, and a 6mm top pad to raise the helmet from, the ears. FIT KIT sizes all S1 Lifer Series shells! (S1 Mini Lifer Helmets 19″ – 20.5″) (S1 Lifer Helmets 21″ – 23.5″) (S1 Mega Lifer Helmet 24″ – 25.5″)