As with all personal safety devices, helmet fit is vitally important and can increase the risk of injury if the helmet doesn't fit properly. Carefully measure the head approximately one inch above the eyebrows across a point at the back of the head that gives the largest measurement. Please remember that helmets will "break in" with use. It is important that the helmet fits as tight as one can comfortably withstand to wear.
Please see the charts below for helmet fitting. If you do not know your hat size, please take a cloth tape measure and measure around the circumference of your head, just above your brow line.
The top chart is for adult helmet sizing, the bottom one is for Impact's youth helmet line, which includes inserts that are interchangeable to "resize" the helmet from a small to a large.