Helmet size is determined by the circumference of your head. Once the correct helmet side has been determined, fitting the helmet to your head is a breeze. Best practice is to follow the two finger rule. When the helmet is in the desired position on your head, there should be two finger's width between your eyebrows and the helmet. The helmet strap is properly adjusted when you are able to fit two fingers between the helmet strap and your chin when the strap is buckled.