The two significant types of emissions from fixed-roof storage tanks are "storage" and "working" losses. While the former arise from changes in environmental parameters, the latter result from liquid level changes in the tank. Optimizing the storage policy requires the prediction of the influence of the main operating parameters on the overall emissions. The aim of this work was to develop a mathematical model able to represent the main phenomena involved in such emissions and to validate it by comparison with both laboratory-scale and real-size experimental data. Moreover, because semiempirical relations are available to estimate monthly or annual emissions, a comparison with these relations has been carried out both to further validate the model and to discuss the range of reliability of such relations.