To investigate the validity of the material balance or the p/z plot method when applied to single and multi-layered gas reservoirs. For all cases studied, a reservoir is assumed to be a volumetric dry gas reservoir. In this study, applicability of the pseudo-steady state equation (PSSE) in estimating p was also investigated. A reservoir simulator was used in the study to generate p and other parameters needed for generating a p/z plot. All the obtained parameters (including GIIP and reserves) from the simulator were considered as actual values. Both p's obtained from the simulation and the PSSE were used in generating p/z plots. Resulting GIIP's obtained from p/z plots generated from p's from the simulator or actual p's, and from p's from the PSSE were compared to the actual value obtained from volumetric calculation of the simulator. Effects of parameters which cause high deviation of calculated p from the PSSE were also studied. For a single-layered reservoir with two wells, separated-well and combined-well methods were applied. For a two-layered reservoir with one well, flow rate allocation, and separated-layer and combined-layer methods were studied and applied. For a two-layered reservoir with two wells, separated-layer and separated-well, separated-layer and combined-well, combined-layer and separated-well, and combined-layer and combined-well methods were applied. For a three-layered reservoir with one well, both separated-layer and combined-layer methods were applied. Finally, application of the material balance method to real data was carried out.