What size helmet do I need for a toddler vs. a school-age child?

Helmets are sized by head circumference in centimeters, not by age. Measure around the widest part of the child's head, just above the eyebrows, and match to the manufacturer's range. Toddler-specific models typically start around 44–50 cm, while school-age helmets cluster around 52–58 cm. Reviewers repeatedly note that fit — snug, level, with the chin strap forming a 'V' under the ears — matters more than brand.