Cavitation is a heavy threat for spillways with concrete chutes. Besides a proper design and execution, aeration is an effective means to avoid severe damages. A detailed study on bottom aerators of spillway chutes is presented here. Data from laboratory experiments were used to calibrate the coefficients of a physically based equation, which considers the effects of aerator geometry and different roughnesses of the surface of the aerator. After adjustment of scale factors, results computed by this equation showed a good agreement with observed data of different prototypes found in the literature. The main physical concepts of the developed equation are presented. The quantification of the air flow into the water jet was performed by separately considering the gas and liquid phases and using the subpressure under the jet of the aerator as a liaison between the two formulations. Consequently, this subpressure does not appear explicitly in the final formulation and does not need to be known for the quantification of the gas flow. The results show that the approach is suitable for the given problem.