An improved material-balance formulation is presented for determining original hydrocarbons-in-place (OHIP) in waterdrive oil and gas reservoirs. The improved formulation reduces the number of unknowns in the regression analysis through the definition of a combined aquifer/reservoir expansion term (CARET). Field examples analyzed with the CARET formulation are presented illustrating the nonunique nature of waterdrive material-balance solutions for CHIP. A method is presented for reality checking the best-fit aquifer parameters obtained from the regression analysis. The CARET material-balance formulation is shown to provide several advantages over existing regression analysis techniques.