The volumetric method is the initial method used in calculating hydrocarbon reserves obtained from the exploration stage obtained geological data from a field in the oil and gas industry. The hydrocarbon reserve calculation obtained from a volumetric method is the result of interpretation data, such as seismic and logging data that have a small range of trust. In this study estimated hydrocarbon reserves in the XY formation gas reservoir using a material balance method or P/Z method in assessing optimism or pessimism of the reserve value generated from a volumetric method. The material balance method or P/Z method was chosen as a reliable method. Because it has better accuracy data such as pressure measurement data, reservoir rock, and fluid analysis data. Cumulative production measurement data compared to interpretation data on a volumetric method. From this research can be concluded that the reserve value produced by the volumetric method is more pessimistic than the reserve value using the material balance method and P/Z method. By knowing pessimism results from the volumetric backup method for the other two methods. it will affect the amount of oil and gas industry reserves so that geological model needs to be done again after more accurate data is obtained.