Product details:

Electra Solid Color Bike Helmet Durable ABS hard shell for protection and EPS foam liner for security Detachable visor for when you want sun protection, velcro for when you don't Fit retention system provides secure and snug fit Easy to use magnetic Fidlock buckle Size SMALL fits circumference 48cm - 54cm and hat sizes 6 1/2 - 6 3/4. Size MEDIUM fits circumference 55cm - 58cm and hat sizes 6 7/8 - 7 1/4. Size LARGE fits circumference 59cm - 61cm and hat sizes 7 3/8 - 7 5/8. Small size generally fits children while medium and large sizes fit adults.