To predict the overall average porosity of a packing of unequal spheres, Ouchiyama and Tanaka (Ind. Eng. Chem. Fundam. 1981, 20, 66.) established a simple packing model and derived an analytical formula. In this model two uniformity assumptions were introduced, which lead to systematic errors in the model, making the prediction of the average porosity higher or the prediction of the packing density lower than experimental data. This paper proposes a theoretical correction for this model. For binary packings of spheres, the correction parameter is fixed. The prediction by our correction is in excellent agreement with experimental data for a large range of ε̄0, the porosity of a packing of equal spheres, and k, the ratio of the smaller to the larger diameter of spheres.