A hard sphere system, using Kelvin tetrakaidekahedron cells, coupled with the mean spherical approximation (MSA), accurately correlates the mean ionic activity, and the individual activities of ions, in aqueous solutions of single 1:1 electrolytes. For the mean ionic activity coefficients, the diameter of the anion is fixed at the Pauling value and the cation diameter is a function of composition. Hydrated diameters of cations are realistic in comparison with Pauling values, and the like and unlike ionic pair correlation functions at contact value are positive in the whole concentration range. The use of a simpler version of the MSA has little effect on the results and dramatically reduces the computation time. The results obtained from the Kelvin mean spherical approximation, K-MSA model, compare favorably with those of other models. The correlation of individual ionic activity coefficients requires composition-dependent diameters for the cation and anion.