Not sure about your helmet size? Get an "S1 Helmet Co. Fit All Kit" and eliminate any guessing.
You'll get all 6 different thickness of sizing liners so you can fit your helmet perfectly now or if your head grows .
Size: Small- XXX Large included in fit kit.
FIT ALL KIT SIZES
(S1 Mini Lifer Helmets 19" - 20.5")
(S1 Lifer Helmets 21" - 23.5")
(S1 Mega Lifer Helmet 24" - 25.5")