The Lifer helmet is available in 3 different shell sizes to accommodate different size heads.
Measure your head ...
If it measures 18" - 20.25" get an S1 Mini Lifer Helmet
If it measures 20.5"- 23.25" get an S1 Lifer Helmet
If it measures 23.5" - 24.5" get an S1 Mega Lifer Helmet
Fun fact: Most kids ages 6 and under will fit into the S1 Mini Lifer Helmet. And 80% of people ages 7 and up fit into an S1 Lifer Helmet.
If you are buying a helmet for someone and you can't measure their head - follow this general rule of thumb. Ages 6 and under order an S1 Mini Lifer Helmet (and buy a Fit All Kit so they can fit it perfectly) . Ages 7 and up order an S1 Lifer Helmet (and buy a Fit All Kit so they can fit the helmet perfectly).