A buckle achieves improved strap tensile strength by providing a bridge portion, about which a free end of a buckle strap is wrapped in tension, with an inwardly projecting concave surface to create a greater deformation of the strap under tension. The buckle also includes at least one strengthening cross rib on the bridge portion and a strap receiving inset with a concave shaped pocket to improve the strap tensile strength. Furthermore, the buckle includes a bendable retainer element with an inwardly directed clamping tab near its free moving end to retain a portion of the strap within the buckle.