The objective of this work is to present a simplified mathematical approach for back calculation in the Wagner-Nelson method. The literature reported values of in vitro study, obtained using a discriminative and biorelevant test methodology, were used to generate the calculated plasma concentration of a drug. The volume of distribution, elimination rate constant and the dose of the drug are needed to predict the calculated plasma concentration. For validating the proposed equation, Wagner-Nelson method was adopted to generate the values of elimination rate constant and volume of distribution in a literature reported in vivo data set. The applications of the rearranged form of the Wagner-Nelson equation are shown in judging in vitro in vivo correlation and also in dosage form design. The steps for obtaining the rearranged form of the Wagner-Nelson equation are given in an appendix. The proposed method can be used if the custom made computer programs are not available.