In this article the straight-line relationship between In A and B, where A and B denote the concentrations of the two types of monomers in any reaction period, is examined to exist in the radical copolymerizations of 1.02 ≥ r1r2 ≥ 0.25. Utilizing this observation, we propose an easy-to-use compositions calculation method in which a single empirical parameter is included. The method is derived without involving the constant monomer reactivity ratios assumption. The copolymerizations of styrene–methyl methacrylate vinyl chloride–vinyl acetate, methyl methacrylate–vinyl acetate, acrylic acid–acrylamide, methacrylic acid–methacrylamide, and sodium methacrylate–methacrylamide are investigated. The instantaneous and cumulative copolymer compositions and the residued monomer compositions computed by this proposal are in very good agreement with the experimental data.