Indonesia occupies the Pacific Ring of Fire so that it prones to earthquakes. Conversely, multistory building structures developed faster due to the demand, including having vertical irregular setbacks, and it could affect the seismic performance of the building. This study investigates the impact of the vertical irregularity index on the ratio of structural damage in reinforced concrete building structures with an stepped setback type. The analysis method employed is a numerical approach, which involves earthquake simulation using nonlinear dynamic analysis, taking into account variations in step type building setbacks. A total of 28 models were utilized with multistory setback designs, each of which had eight stories and a variety of setback variations. The quantification of the structural irregularity indices (ϕs and ϕb) reveals that the values of ϕs and ϕb are contingent upon the number of building panels. The research findings indicate that the performance of a building during an earthquake is significantly influenced by the degree of structural irregularity. The relationship between the irregularity index and the damage index is obtained by variations in stepped setback type buildings. The relationship between the damage index (DI) and predicted damage index (DIpred) and the irregularity index parameters ϕs and ϕb was determined. The damage index ratio was observed to increase as the irregularity index increases.