How to Size
Wrap a sewing tape measure around your head 1 inch (25mm) above your eyebrows and at the top of your ears. Make sure to measure over your hair, since you'll be wearing the helmet that way. Take multiple measurements and select the largest one.
A size chart with sizing instructions is available on each helmet's page. Follow the instructions to ensure the best fit, as helmets may vary in shape and size.

Know Your Head Shape
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Round oval: Similar width and length.
Fits helmet style: SOA, Mayan/Germanator, Baseball Pro -
Intermediate Oval (most riders): Slightly longer front-to-back.
Fits helmet style: Reversible Polo, Curve
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Long Oval: Noticeably longer front-to-back.
Fits helmet style: Reversible Polo, Curve

Fit test: With the right size, the helmet should make uniform contact. Side pressure = likely round-oval. Forehead hot spot = try a longer shell. Beanie Helmets product pages include shape guidance to help you choose faster.
For more info, read our Biker Guide: Perfect Helmet Fit by Head Shape