In this paper the effect of different geometries on the performance of spiral wound modules is examined. The analysis is based on an analytical solution of the simplified transport equations presented in previous papers(1,2). An explicit equation has been proposed, so that the effects of the number of leaves, permeate channel height, brine channel height and length to width ratio on the module specific productivity have been examined. As a result of the analysis an optimum geometry for the FT30SW 2.5 " membranes has been proposed. The method has been applied for sea-water spiral-wound modules and gives very reliable results for recovery ratios up to 40%. Although the method was applied to FT30SW 2.5 " membranes, it can be also used in any type of membrane, as long as the characteristics of the membrane, such as water permeability coefficient, friction parameters etc., are known. It is believed that the analytical procedure presented in this work is a very useful tool for the optimum spiral-wound module design, not only because of its accuracy but also because it is quick and simple.