Snap In Liners Provide Custom Fit; Rear & Side Ventilation; Comfortable Quick-Release Chin Strap; Moisture wicking removable liner is machine washable; Sizes: Small - 15 (6 5/8 - 6 3/4), Medium - 16 (6 7/8 - 7), Large - 17 (7 1/8 - 7 1/4), X-Lg - 18 (7 3/8)