Unplanned dilution can pose huge burden on the profitability of underground mining operations, in particular when mining narrow veins. This paper aims at estimating quantitatively unplanned dilution due to the open stope walls overbreak and slough in longhole blasting mining environment. Open stope mining data are used to assess quantitatively the stope dilution level. The parameters considered in the study included the stope depth, the stope average dip, the hydraulic radius, surface character, RMR, powder factor and the ELOS (equivalent linear overbreak/slough) parameter. A dilution index (DI) is defined using the Rock System Engineering method to relate the dilution to its influencing factors. Multinomial logistic regression is used to establish a probabilistic map of the DI. The results indicated that the RMR, hydraulic radius, undercut area and blasting parameter (powder factor) are the most important factors influencing the dilution index. The computed DI was compared to the ELOS and high correlation was observed. This suggests that the proposed DI-chart could be used to quantify the dilution in longhole mining environment and could complement the well-known stability graph method.