The development and growth of tourism depend on its sustainability over time and on its benefits for destinations as a whole. However, calculating sustainability is not an easy task. This article focuses on the economic sustainability of tourism growth and, after an exhaustive review of the literature, proposes a quantitative mathematical model to measure it by analysing and calculating leakage in the hotel sector. Leakage analyses the amount of revenue generated by tourists that does not remain in the destination economy. Through a sample of 204 interviews with managers, this study validates the model created and calculates leakage in a mass tourism destination (the Valencian Region in Spain). The paper opens new areas of research in sustainability literature and will be of value to tourism planners and governments in their efforts to implement appropriate tourism development policies.