Wrap a flexible measuring tape around your head, about 2.5 centimetres above your eyebrows, or in the centre of your forehead. Make sure the tape is level from front to back.

Helmets sizes are listed in centimetres, so try to measure your head in centimetres if possible. If your measuring tape uses the imperial system, multiply the measured inches by 2.54 to get your helmet size in centimetres.