Bern Nina Adjustable Helmet with White Fleece 18 7/8-20 1/4 in Meets Safety Certification ASTM F 2040, CPSC and EN 1181 Bern's exclusive Zipmold foam is a liquid foam injection process that provides a better weight to strength ratio so Bern can provide customers a thinner helmet Fit system at back of the head customizes to the shape of the head, keeping it stable and comfortable Compatible Bike/Skate/Warm Weather liner available (sold separately)