In the ultrafiltration on industrial scale several factors are of large economic importance: yield, capacity energy and investment costs. These factors are interrelated in a rather complex way. In part, these relationships are specific of a given product in a given location: however, next to specific also some general rules can be applied to process design and optimization. In this paper a mathematical model of batch ultrafiltration is proposed. The model includes both the initial feed solution concentration and volume, processing time, variable flux across the membrane as well as membrane area. The developed model enables predictions of the processing time of ultrafiltration and can also be used for design and optimization of industrial processes.