Unplanned dilution can pose huge burden on the profitability of the operations in mines employing the open stoping mining methods, if these mines are not adequately designed. The empirical dilution graph methods are commonly used for these purposes due to their practicability. Nevertheless, these methods as any empirical approach may lead to poor design performance due to additional factors such as site-specificity. Hence, the aim of this study is to propose a dimensionless index, the dilution index (DI), capable of relating well-unplanned dilution with its influencing parameters to describe the susceptibility of the stopes to unplanned dilution. The interactions between these influencing factors are quantified using the Rock Engineering Systems (RES) which is a practical tool for determining complex and highly nonlinear correlation among the input parameters via the interaction matrix. The parameters of the DI system consist of 12 parameters related to the stope geometry, rock mass properties, and the in situ stress. Overall, the validation results showed excellent agreement between the newly developed index and the actual dilution. It was concluded that the DI could be used to quantify the dilution in open stoping operations and could complement the well-known dilution graph method.