This paper presents an analytical method for estimating the flexural strength of reinforced concrete (RC) beams strengthened with externally bonded fibre-reinforced polymer (FRP) laminates. The method is developed from the strain compatibility and equilibrium of forces. Based on the size of external FRP laminates, several flexural failure modes may be identified, namely tensile rupture of FRP laminates and concrete crushing before or after yielding of internal steel reinforcement. Upper and lower limits to the size of FRP laminates used are suggested to maintain ductile behaviour of strengthened RC sections. Comparisons between the flexural strength obtained from the current method and from experiments show good agreement. Design equations for calculating the size of FRP laminates externally bonded to RC sections to enhance their flexural strength are proposed.