Measuring the circumference is an important step for choosing your helmet a incorrect size would result in a helmet or too wide or too tight and this would result in a reduction of driving comfort.

The measurement should be done using a centimetre and positioning it as in the photo around its head.

Head circumference in CM50-5152-5354-5556-5758-5960-6162-6364-65
Men's helmet size xxx x s m i xl xxx xxxl
Head circumference in CM49-5051-5253-5455-5657-58
Size female helmet xxx x s m i