When to use: Manning's equation estimates the uniform (steady, normal-depth) flow velocity and discharge in an open channel or partially-full pipe. V = (1.49/n)·R^⅔·S^½ (US units), where n is the Manning roughness coefficient, R = A/P is the hydraulic radius, and S is the longitudinal channel slope. Discharge is Q = V·A. Choose the channel cross-section, enter the flow depth and geometry, and the calculator solves the wetted geometry for you.