The upgrading of bridges located in seismic areas and built according to obsolete codes is becoming a priority task for highway administrations. Among the possible upgrading strategies, the use of fiber-reinforced plastic (FRP) jackets is gaining widespread acceptance. In this paper, a design equation is proposed to determine the optimal thickness of FRP jackets, to enhance the ductility of existing reinforced-concrete (RC) bridge piers with circular cross sections. The design procedure stems from the definition of an upgrading index, given as the ratio of the target to availability ductility at the pier base section, to be attained through FRP jacketing. The available ductility is that identified through the usual assessment procedures on the RC member set for upgrade, whereas the target ductility is evaluated based on the expected actions on the bridge. The upgrading index is initially defined in general terms and is subsequently extended to the case of piers built in seismic regions. It results in a simple expression in terms of easily computable quantities, such as the ultimate strain and the peak strength of concrete, before and after upgrading. A parametric study on old-code–designed bridge pier sections, upgraded with either glass or carbon fiber jackets, is performed based on a fiber-section model equipped with a newly developed FRP-confined concrete model. This study shows that the index, despite its simplicity, yields excellent predictions of the ductility increase obtained through FRP wrapping, and it is therefore used to develop a design equation. The equation allows the design of the optimal thickness of FRP jackets in terms of the desired upgrading index, mechanical characteristics of the selected composite material, and quantities defining the initial state of the pier section. The design procedure has been applied to available experimental tests of a scaled bridge pier wrapped with FRP and tested to failure, and it has been demonstrated to be very effective.