The area of the water prism in the pipe or channel divided by the wetted perimeter. Thus, for a round conduit flowing full or half full, the hydraulic radius is d/4, Another term sometimes used for this quantity is hydraulic mean depth.

The ratio between a volume and the area of the surface that surrounds it. In a plane, it is the ratio between the area of a domain and the length of its circumference. In a porous medium, it is the ratio between the volume of the void space and total area of the solid matrix surface, within a given porous medium domain.

The ratio of the cross-sectional area of a stream and the wetted perimeter.