The strengthening of existing structural systems and members by externally bonded FRP systems is nowadays a well consolidated technique. The design of this reinforcement practice relies upon the definition of the FRP design strength, which is obtained from the characteristic values through appropriate partial factors, which are commonly indicated in building codes or design guidelines. Specific environmental conversion factors are also generally introduced in codes to take into account the degradation of mechanical properties of FRP systems under various exposure conditions. Following the principle of ‘design-by-testing’ pursued by Eurocode 0 (EN 1990), the paper presents the results of a statistical analysis based on a large experimental activity developed for the qualification of several FRP products by different manufacturers. Results of tensile tests under standard ambient conditions were used to calibrate the partial safety factors, while the results obtained after various types of ageing conditions allowed to determine the environmental conversion factors; each type of ageing test was, indeed, associated to the exposure conditions classes foreseen by codes (i.e., “exterior” and “aggressive”). The paper will discuss the obtained results in comparison to the factors currently prescribed by national and international design provisions, especially evidencing the differences among various types of fibres (carbon, E-glass, aramid and steel) and highlighting, thus, some aspects that can be taken into account in future revisions of design guidelines.