Bontrager's Circuit WSD is a lightweight road helmet that features massive vents for maximum airflow. It features their one-hand-adjustable Headmaster fit system, plus it has an in-mold strap retention system with FlatLock strap adjusters for a superb fit and serious safety. The premium fit pads not only give you a custom feel, they also help keep your head dry on those hot rides. - Small = 51-57cm - Medium = 54-60cm