An index was developed to estimate the residual periodontal ligament support for individual teeth during treatment planning for partially edentulous patients. The Residual Periodontal Ligament Index (rPLI) was derived from a formula that calculates the remaining area of periodontal attachment and the Normal Periodontal Ligament Index (nPLI). To illustrate the applicability of the rPLI, the total nPLI scores of the remaining teeth corresponding to Eichner subclasses of partial edentulism were charted by assessing the average occlusal support numerically. The rPLI is proposed to be a possible suitable tool for epidemiologic research on the progression of tooth loss and the survival rate of prostheses.