Problems with filling can lead to rebar corrosion and impair the durability and all other functional aspects of reinforced concrete structures. However, the prevention of concrete filling defects is left to the skill, judgment, and experience of on-site operators. In this research, we develop a methodology for quantitatively estimating the rate of occurrence of concrete filling defects. By this, we hope to promote the efficient and effective prevention of such defects at construction sites. Therefore, as a model for defective filling, we propose a mortar loss model in which the mortar component of concrete free falling into the frame adheres to the reinforcing bars in the frame, and based on that, we conducted an experiment. Using actual measurement data, we quantitatively estimated the risk of defective filling.