Discharge from sluice gate flows is commonly calculated using the Torricelli outflow velocity, which is inaccurate and must be corrected by a discharge coefficient. Moreover, this approach commonly only considers the relative gate opening, without including the impact of three-dimensional (3D) effects, scaling effects, different velocity profiles, and friction forces. Aiming for a theoretical approach that can address all flow effects for sluice gate discharge calculations, the authors applied the momentum balance theory to this problem. First, the control volume was introduced, and parameterization equations for the pressure distributions and momentum coefficients at the control volume borders for both the standard and the inclined sluice gates were determined using computational fluid dynamics (CFD) simulations. The results show good agreements with the discharge measurement results of frequently quoted experimental studies from other authors, demonstrating the potential of this approach. Also, one example of the impact of the 3D effect of various channel widths was investigated with the momentum balance theory.