Bike Helmet Size Guide: How to Measure Head Circumference Correctly
Measure head circumference correctly for a bike helmet with fit checks, size chart tips and links to free helmet size conversion tools.
A bike helmet can only protect well if it fits securely. The most important measurement is head circumference, not age, height or hat size. Use a soft tape measure, then compare your number with the Helmet Size Converter.
Measure Above the Eyebrows
Wrap the tape around the widest part of your head, slightly above the eyebrows and ears. Keep it level and snug. Repeat the measurement once to confirm accuracy, especially if the first result falls between two size ranges.
Test the Retention System
After choosing a size, adjust the rear dial or fit system so the helmet feels stable. It should move with your head, not separately from it. If you feel painful pressure points, try a different size or shell shape.
Adjust the Straps
The side straps should form a V shape around the ears, and the chin strap should be snug enough to fit one or two fingers beneath it. Recheck fit before every ride, especially for kids or shared helmets.