Smart solutions for gas grids are essential given increasing variations in the quality of natural gas. SmartSim provides an innovative software solution that uses simulation to track gas quality in the grid. This way, calorific value (CV) and other gas properties can be precisely determined and correctly assigned to end customers for gas billing. With SmartSim, grid operators can avoid alternative methods, such as the installation of measurement devices or CV adjustment through conditioning, and thus achieve considerable cost savings. The method can be used for both transport and distribution networks and is already used for billing in several grids in Germany and Denmark. Implementations in other countries are in preparation.