The crack presence changes the shape or size of sections in reinforced concrete beams, and greatly affects the section inertias and the beam stiffness. To assess the instantaneous stiffness of the cracked beams, a method is proposed based on the exact crack depth. The region affected by cracks is determined firstly, and different nonlinear strain distributions are modeled in the effect region to describe the change of concrete strains caused by cracks. The internal force equilibria are adopted to find a solution to the top strain and neutral axis, and according to the solution, the inertias of key sections are calculated to assess the beam stiffness. The proposed method has been validated using experimental results obtained from two-stage tests on five reinforced concrete beams. The dataset stores valuable results calculated by the proposed method.