Bell makes helmets in many sizes, including: Large: 59-63 cm / 23.25 - 24.75 in. Medium: 55-59 cm / 21.75 - 23.25 in. Small: 51-55 cm / 20-21.75 in. Universal Fit Adult: 54-61cm / 21.25 - 24 in. Universal Fit Women’s: 50-57cm / 19.75 - 22.5 in. Universal Fit Youth: 50-55cm / 19.75 - 21.75 in. Infant: 47-52cm / 18.5 - 20.5 in. In order to determine the size you need, you can measure the circumference around your head. Simply wrap a measuring tape around your head, being sure to locate the tape just above your ears and keeping the tape level front to back.