We developed a design method for an inhomogeneous membrane cascade by adopting the McCabe-Thiele method, which is long established for designing distillation columns. The stage cut value is an independent design parameter in the design procedure and thus has to be set. Within each section, the operating conditions were uniform, but both sections could be operated differently using various combinations of membranes, trans-membrane pressure, temperature and stage cut. The procedure was applied to cascaded nanofiltration for the fractionation of a mixture of fructooligosaccharides of varying molecular weight. The stage and area requirements were strongly dependent on the initial design parameter, the overall stage cut. The total area was related to the overall system cut. However, the overall system cut was dependent on the stage cuts for both sections (top and bottom). The top stage cut could be chosen, whereas the bottom stage cut needed to be calculated iteratively to match the top design at the intersection.