The objective of this research is to determine the real deflection of a concrete joist and correlate the result with theoretical deflection, which is based on a stress vs. deformation model which was proposed by Mander et al. (1988) for monotonic loads of reinforced and non-reinforced concrete. The construction of a concrete joist does not result in a 100% homogenous, isotropic, and linearly elastic element, since its production depends on many conditions, such as aggregate selection, water, cement manufacturing, tests performed for mixture design, the operator in charge of the mixture, and the construction of the joist. Therefore, research was carried out on the variation of real reflection with respect to theoretical calculations. To this effect, 30 simple-concrete and 30 reinforced-concrete joists were elaborated. They were tested by measuring their maximum deflection and comparing it to its theoretical counterpart. To calculate the theoretical deflection, a curvature moment diagram was elaborated with the Rect_Mom software by Restrepo and Rodríguez (2012), which uses the model by Mander et al. (1988). Experimental results showed a greater deflection than the one reported by theoretical calculations.