We present the results of durability experiments on commercial and experimental soda-lime-silica glasses. Glasses were subjected to a simple, short-duration alkali leaching experiment after which the K and Na concentrations in the leachate were measured. Our results show that Na is preferentially leached over K from the glass. A multiple variable linear regression was applied to the data to produce a predictive durability model based on glass composition. The model shows that, within the range of the tested compositions, only Al2O3 significantly increases durability while Na2O and K2O decrease durability, with Na2O apparently having a greater effect. Our model, together with a previously published model for chemical strengthening, and composition/viscosity models can be used to formulate a durable glass optimized for rapid Na-K ion exchange strengthening. Specifically, our observations indicate that the replacement of K2O for Na2O in the glass will increase the exchange rate and increase durability.