Measure your head at the place where the headband will rest. Keep in mind that everyone's head-shape is different. If you happen to fit between two sizing brackets, it is better to go to the lower sizing bracket if you have a narrow head and up to the next bracket if you have a rounder shape to your head. Please note: This item is not certified by DOT and not intended to be used as a safety equipment. This will not provide any protection to the head. If you need DOT certified motorcycle helmets please check our other listings or our store where we sell the smallest and lightest DOT helmets in various styles.