Troxel Sport Schooling Riding Safety Helmet SEI Certification and Colors XS Up to 20 7/8 inches & Up to 6 5/8 hat size Small 21 1/4 - 21 5/8 inches & 6 3/4 - 6 7/8 hat size Medium 22 - 22 3/8 inches & 7 - 7 1/8 hat size Large 23 1/8 + inches & 7 1/4 + hat size ASTM Approved/SEI Certified, 7 Cooling Vents, Durable, lightweight construction, Removable, washable headliner