This article presents an approach to rapidly predict the seismic vulnerability existing of low-to mid-rise residential reinforced concrete (RC) buildings. In this procedure, Capacity Index is computed considering the cross-sectional orientation and size and material properties of the components of the structural system. This index is modified considering the several coefficients of possible negative impacts of structural irregularities, which are frequently found in RC buildings. Accuracy of the proposed approach has been tested considering 196 RC buildings subjected to earthquakes. The procedure categorizes the buildings either as safe or as unsafe according to whether they meet the life-safety target performance requirement. The analytical results indicate that the consistency of the proposed approach in this paper is about over 90%. Therefore, the proposed approach can be used to asses the seismic performance and vulnerability levels of existing RC buildings.