Your helmet fit is not only critical to the comfort and enjoyment of your helmet, it also plays a critical role in the performance of your helmet in the event of a collision. Never purchase or wear a helmet without ensuring a snug fit. There are several methods and tricks to size a helmet.
To measure your head size, wrap a cloth tape measure horizontally around your head about one inch above your eyebrows and ears. This will measure the largest portion of your head.
Select the helmet size that is closest to your head size. Often times your head measurement may fall between two helmet sizes. When this occurs you should first try the smaller size.
When wearing a helmet, it should be snug. If it is not snug, it is too big for you. With the helmet on and chin strap fastened, if you can move the helmet easily from side to side and up and down without moving your head, the helmet is too big. Try a size smaller.
Be aware that different helmets fit different head shapes. A helmet that will fit one person perfectly may not fit another person with the same size head, due to the shapes of their heads being different