A new algorithm for the calculation of monomer partitioning in emulsion copolymer systems is presented. The algorithm can be applied both to monodisperse and polydisperse systems and the outputs of the algorithm include the distribution of the volume fractions of the monomers according to the particle size distribution, the volume fractions of the monomers in the monomer droplets, and the aqueous phase and the total volumes of all the phases in the system. The algorithm can handle both Interval II and Interval III conditions, using the same set of equations. This results in substantial computer time savings during the simulation of emulsion copolymerization systems. Examples of applications are given.