In this paper, a formulation based on the moment–curvature diagrams was proposed to calculate the effective flexural stiffness (EFS) of fiber-reinforced polymer (FRP) reinforced concrete (RC) beam members. A parametric study was carried out to encompass the effect of different factors, such as tensile reinforcement ratio, concrete compressive strength, modulus of elasticity, and aspect ratio. The proposed formulation was verified with numerous experimental data reported by various researchers. To extend the validation of the formulation into the beams of RC frames, the EFS (EIe) of cracked GFRP (glass FRP) and CFRP (carbon FRP) RC beam sections for four types of frames were calculated. Based on the analytical study, experimental approach, and frame analysis approach, the EFS of FRP RC beams is less than 0.184 times the gross flexural stiffness (EcIg). According to the results obtained from various approaches, the range between 0.05 and 0.18 for the EFS ratio (EIe/EcIg) of FRP RC beams is proposed.