All Little Nutty helmets comply with US CPSC Bicycle Helmet Safety Standards for persons age 5 and older. The Little Nutty helmet is size XS, and fits heads 46cm - 52cm or 18 - 20.5 . See the Little Nutty Size Chart to determine a good fit. If your head measurement is 52cm/20.5 (maximum size of the Little Nutty XS), we recommend ordering a S-M Street helmet. Each Little Nutty helmet has a built-in Spin Dial Fit System to ensure a snug, comfortable fit..a soft, fabric chin strap for comfort..and the revolutionary Nutcase Magnetic Buckle, which means NO PINCHING!