Much like shoes, knowing how big your head is will help you buy the proper size motorcycle helmet. Helmet size is determined by head circumference and it is best to measure in centimeters (CM). Measure just above the eyebrows, across the temples of your head. If you have long hair, measure with you hair down. A fabric measuring tape will be the best thing to use for taking a measurement and be sure to have a friend help you out. If you find you are between two sizes of a helmet, we suggest rounding up to the larger size. Every helmet brand and model will fit differently so it is important to check each manufacturer's sizing chart when buying a helmet.