Real-time water management is a major ongoing challenge for PEM fuel cell technologies. Given the inherently distributed nature of the system, local conditions can change significantly from cell-to-cell. To compensate for this variance, a control-oriented PEM fuel cell model that is capable of capturing localized differences in operating conditions in real time is needed. To fulfill this need, the authors previously investigated the use of multiple, equally sized control volumes (CVs) to represent the cathode channel. This modification to the modeling architecture greatly improved the accuracy over a one CV model, which was incapable of capturing the stack vapor dynamics. However, because the relative humidity distribution in the stack is non-linear, equally sized CVs do not optimize the additional information gained from the multiple cathode CV approach. In this paper, an algorithm to optimally size CVs based on an analytical solution of the relative humidity profile in the cathode channel is presented. The analytical solution was found based on the vapor mass conservation equation in the cathode. This conservation equation includes consideration of electro-osmotic drag, concentration gradient based diffusion, vapor generation, and bulk fluid flow. The solution was validated by correcting the one CV model to match experimental data using the result of the analysis. After applying the analytical adjustment strategy to the one CV model, the dewpoint temperature predicted by the augmented one CV model was in good agreement with experimental results. The solution also revealed a coefficient that relates the current, flow rate, and membrane diffusion in a single term. This coefficient could be used for control decisions to avoid flooding issues in the stack. Furthermore, using the analytical RH profile equation, CVs can be optimally, unevenly sized to improve the modeling of local operating conditions. Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.