GDM REBEL Retro Full Face Vintage Matte Black Motorcycle Helmet is DOT FMVSS-218 certified. It features double D-ring chin strap,Its shell is constructed using Injection-Molded ABS with Expanded polystyrene inner shell. It also features Contrasting diamond-stitched quilted foam padding liner. Please refer to the following size guide to determine your correct size. To choose your correct helmet size, please measure the circumference of your head 1" above your eyebrows around your head and refer to the GDM Duke Helmets size guide photo. Please note that the size guide is for reference only and cannot guarantee a perfect fit. SMALL: 21 5/8 – 22 IN 55-56 CM | MEDIUM: 22 ½ -- 22 7/8 IN 57-58 CM | LARGE: 23 1/4 -- 23 5/8 IN 59-60 CM | X-LARGE: 24 -- 24 3/8 IN 61-62 CM | 2X-LARGE: 24 3/4 -- 25 ¼ IN 63-64 CM