Production decline prediction is important to understand the performance and life span of oil and gas wells. The most common prediction method is decline curve fitting based on available production rate data. Such data are fit with different equations that extrapolate to future time. However, the parameters are commonly poorly constrained, especially when the production rate data are limited. In this study, we establish a novel gas isotope interpretation tool to better predict the resource quantity and life span of producing gas wells. This tool is based on the evolution of methane carbon isotope ratios (δ13C1) caused by different gas-releasing processes during production. It requires (1) real-time methane carbon isotope ratio data, (2) continuous gas production rate data for a certain period of time, and (3) basic geological and engineering conditions. We successfully applied the production decline prediction tool to a producing shale gas well in the Barnett Shale. We obtained real-time δ13C1 data for approximately 1 yr using our proprietary, field-deployable gas chromatography–infrared isotope ratio analyzer. The prediction in this well from the isotope method showed a total reserve of up to 7.34–7.75 BCF (2.07–2.19 × 108 m3), which was used to constrain the production decline trend of the study well. The measured production rate data were first fit using the Arps equation, which then joined to an exponential decline curve smoothly at approximately 10 yr, such that the cumulative production calculation from integration of the product rate curve equaled to the total reserve predicted by the isotope method. The novel production decline prediction method thus provided important constraint on the future well production and expected ultimate recoverable reserves.