Reinforced concrete (RC) beams manifest a localized high strain response under contact explosion. The commonly used single degree of freedom focuses on global damage, localized damage remains to be challenging. Many researchers proposed to measure the damage degree of residual bearing capacity. In the case of contact explosion, the residual bearing capacity is related to the dimensions of the damage zone, and for RC beams is mainly related to damage zone span. We established a semi-empirical equation for calculating the damage zone span based on the dimensional analysis. The equation was verified by full-scale RC beams damage tests. The influence of gravityon the damage zone span was determined. Additionally, a saturation value was obtained for the damage zone, and this value was only related to the structural and material parameters of the RC beam.