Solute separation characteristics of commercial nanofiltration (NF) membrane Desal-HL were investigated. Donnan steric pore model (DSPM) was applied to the experimental data to study the charge density at the surface of the membrane for various solute radii namely Stokes-Einstein, Born, and Pauling. Sodium chloride and magnesium chloride were used as simple solutes. The effect of pH was studied for all the solutes. Correlations for the charge density for each radius were proposed. It was found that the charge density changes considerably with the ion radius, and, consequently causes remarkable variations in prediction of ions rejections.