Modem ultrapure water (UPW) systems designed for the semiconductor manufacturing industry are very complex nowadays, especially when wastewater recycling is part of the configuration. In this paper, we developed a practical model for system evaluation by correlating the actual UPW consumption rate, eta, with the velocity of water level changes (v) in the storage tank. It was found that the UPW consumption rate is a sole function of the velocity of water level changes in the water storage tank that could be described by the following equation: eta = I - Knu. The model was then applied to a standardized UPW system to examine its accuracy and found good agreement between the estimated and actual results. When process-to-process recycling is included in the model, the equation is still valid; however, the system-related coefficient K decreases with increasing recycling capacity. By properly estimating the engineering parameters of an UPW system, a generic eta-nu graph can also be plotted based on this equation. This eta-nu graph provides accurate UPW flowrate information as the variation of water levels in the DI storage tank is monitored online, which can be easily used for capacity evaluation of an UPW system operating with or without wastewater recycling at the outset of any UPW expansion project.