A set of mathematical expressions is presented to allow the determination of the water content in a PEMFC membrane, and facilitate the calculation of the water balance under variable conditions of hydration on both sides of it. The study is based on a cooperative model of water diffusion through the membrane, which avoids discontinuities in the calculations, gives a natural path to avoid the Schroeder's paradox, correctly predicting the value limit of ponding of water content, and presents a good behaviour to temperature variation. The model has been tested with Nafion 117 literature data. Its features are coherent with physical models. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.