55-59 cm. Features a wind tunnel-aided design, In-Mold polycarbonate shell with EPS liner, Roc Loc 5 Fit System, and 5 Vents with internal Channeling Product details: https://www.triathletesports.com/giro-advantage-2-helmet-2016/ Small cracks at the earflaps. Other than that it is in very good shape.