Membrane processes are gaining importance for water treatment applications as a result of the advances in membrane technology and increasing requirements on water quality. Membrane fouling, which results in loss of productivity, is one of the most important operational concerns of membrane processes. A number of theoretical models have been developed to explain membrane fouling mechanisms. These models use the system parameters (i.e., viscosity, pore size, membrane thickness, pressure) and unsteady-state material and flow balance equations with specific boundary conditions. As a result, they are of scientific interest and have limited use for practical applications. The purpose of the study was to develop a simple flux model to estimate the characteristic fouling times for ultrafiltration systems based on operational data. A simple first-order model was developed based on the resistances-in-series approach to correlate flux to a characteristic clean membrane parameter, system parameters. The model was used to estimate the characteristic fouling times of a laboratory ultrafiltration system.