Existing tools for environmental certification of buildings are failing in their ability to reach the general public and to create social awareness, since they require not only specialized knowledge regarding construction and energy sources, but also environmental knowledge. In this paper, an open-source online tool for the estimation of the carbon footprint of residential buildings by non-specialized users is presented as a product from the OERCO2 Erasmus + project. The internal calculations, data management and operation of this tool are extensively explained. The ten most common building typologies built in the last decade in Spain are analysed by using the OERCO2 tool, and the order of magnitude of the results is analysed by comparing them to the ranges determined by other authors. The OERCO2 tool proves itself to be reliable, with its results falling within the defined logical value ranges. Moreover, the major simplification of the interface allows non-specialized users to evaluate the sustainability of buildings. Further research is oriented towards its inclusion in other environmental certification tools and in Building Information Modeling (BIM) environments.