Molecular weight averages and copolymer composition are commonly used as property indices for copolymers. In gel permeation chromatography (GPC), copolymer molecules are separated based on their hydrodynamic volumes in dilute solutions. For a copolymer with heterogeneity in the composition distribution, GPC chromatogram represents the hydrodynamic volume distribution which is not identical with its true molecular weight distribution that can be calculated from a copolymerization kinetic model. In this paper, a new method is presented for modeling the hydrodynamic volume distribution of a copolymer in a batch free radical copolymerization process. The hydrodynamic volume distribution is calculated by computing the weight fraction of polymers over a number of hydrodynamic volume intervals. Chain length and copolymer composition are related to hydrodynamic volume using the bivariate distribution function and the universal calibration method with composition dependent Mark-Houwink constants.