Mechanism specificity of ion separations in drinking water is necessary for the study of modeling transfer of calcium salt through nanofiltration membrane. In the present work, we have investigated a mathematical model which can specify the transfer phenomena of solute through a nanofiltration membrane and to predict its reject. The proposed model is based on the extended equation of Nemst-Planck. It is characterized by constants called transfer parameters, which have been determined according to the composition of the initial solution as a binary or a ternary mixture. The established model permitted us to predict the retention of calcium salt. The result of the application of the model has shown that there is a good agreement between the theoretical and experimental curves.