An improved structural bond strength (SBS) model has been developed to quantify the correlation between glass compositions and their chemical durabilities. The SBS model assumes that the strengths of the bonds between cations and oxygens and the structural roles of the individual elements in the glass are the predominant factors controlling the composition dependence of the chemical durability of glasses. The structural roles of oxides in glass are classified as network formers, network breakers, and intermediates. The structural roles of the oxides depend on glass composition and the redox state of oxides. A12O3, ZrO2, Fe2O3, and B2O2 are assigned as network formers only when there are sufficient alkalis to bind with these oxides. CaO can also improve durability by sharing non-bridging oxygen with alkalis, relieving Si0 2 from alkalis. The binding order to alkalis is AI2O3>ZrO2>Fe2O 2>B2O2>CaO>SiO2. The percolation phenomenon in glass is also taken into account. The concentration of network formers has to reach a critical value for a glass to become durable; durable glasses are sufficient in network formers and have a complete network structure; poor durability glasses are deficient in network formers and the network is incomplete and discontinuous. The SBS model is capable of correlating the 7-day product consistency test durability of 42 low-level waste glasses with their composition with an R2 of 0.87, which is better than 0.81 obtained with an eight-coefficient empirical first-order mixture model on the same data set.