This is a great chart to help explain helmet sizing.

If your head circumference is 18.5" - 20.5" you should get S1 Mini Lifer Helmet. (this is a small helmet shell) 

If your head circumference is 20.5" - 23.5" you can get the S1 Lifer Helmet, S1 Lifer w Brim, S1 Lifer w Visor, S1 Retro Lifer , S1 Retro Lifer Helmet (NTA 8876) or the S1 Lifer Fullface (Avail Holiday 2026),

If your head is 23.5" - 25.5" you should get the S1 Mega Lifer Helmet. (it is a bigger helmet shell) 

All S1 Lifer Helmets are CPSC Certified for Skate, Bike, Roller Skate, E-Bike (Class 1 & 2) .

The S1 Retro Lifer (NTA 8876) Helmet is certified for Class 3 E-Bikes are go up to 28 mph.

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Helmet Sizing Chart