At present, when carrying out initial scoping of a project, one of the best ways to do preliminary designs for complicated separation systems is to use residue curve maps. However, in order to compare the different possibilities one needs to do some preliminary costing. As one does not require very accurate costing at this stage, only wishing to eliminate expensive alternatives, one would like to be able to do this directly from the sketches on the residue curve maps. This paper presents methods by which this can be done. In this paper we have limited ourselves to systems with constant relative volatilities, constant molar overflow and the differential equation description of the distillation column. It is shown that, for the rectifying section, by using binary approximations based on the least volatile component, one can obtain reasonable estimates (within a factor of two) for the number of stages. This is done by noting that the changes in the least volatile component are the simplest in nature and using the sum of two binary approximations as the final approximation.