In membrane filtration process, the size of a molecule significantly affects a solute on its transport and retention characteristics. In order to correlate the retention with the molecule size of a solute, molecular geometry has been taken into account to characterize the molecular size in last several years. However, no result has been published to compare the correlation between solute retention and different size descriptors with the consideration of molecular geometry. In this study, the suitability of a molecular size descriptor, "calculated mean size", is evaluated based on large amount of reference data and statistically compared with those previously reported geometric size descriptors. Analysis results show that "calculated mean size" gives a better description of the solute retention by NF membranes in all cases, which justify its suitability as a universal size parameter used in the retention modeling. Statistic results also show that "calculated mean size" may better describe the behavior of solute retention by the higher salt retention membranes. In addition, the comparative study is also helpful to obtain a more accurate description for molecular geometric size. (C) 2009 Elsevier B.V. All rights reserved.