In this article, a calculation method is presented that allows estimating the failure probability and the reliability index β for a generical existing bridge. The purpose of the estimation is to create a unified classification for a set of bridges, considering both deficiencies from a static and seismic perspective. This kind of method is based on the structural verifications by considering the Safety Factors and the uncertainties of the adopted analyses for the SF estimation. The parameter able to consider these uncertainties can be determined for traffic loads respecting the threshold of acceptable structures required by the Italian standards; moreover it can be determined for seismic loads by accounting the real variability of peak ground acceleration of bridge’s site. Finally, once the failure probability is estimated it is possible to decide if the current condition of the generic existent bridge is acceptable or if the bridge’s retrofitting must be performed. Moreover, a unique classification of the bridges can be carried out determining the type of intervention best suited to the case or also a complete risk analysis could be performed.