Kinetic and quantitative predictions of the internal sulfate attack (ISA) of concrete containing aggregates with iron sulfides have become a challenge, especially in the diagnosis of dams subject to expansive phenomenon. The present study proposes equations to quantify the expansion due to the iron sulfide oxidation and due to the secondary ettringite formation in the concrete. A simplified model to assess the evolution of the concrete expansion with time is outlined and validated with experimental results. Unlike others in the literature, this model allows a direct prediction of the expansions generated by the ISA taking into account the physical and chemical characteristics of the concrete and of the aggregate. The concrete volumetric expansion calculated with this model is then used to estimate the movements of a real dam affected by the ISA. The result obtained fits the measurements and serves to the prediction of future movements of the structure.