Silicate systems are of fundamental importance for many metallurgical processes, for the glass industry and also for many aspects of geology. In addition to the phase relations, there are many properties of the liquid phase such as molar volume, surface tension, absorption coefficient, thermal conductivity and viscosity that are important for understanding, simulating and modeling processes involving silicate liquids. Over the past several years, through critical evaluation of all available thermodynamic and phase equilibrium data, we have developed a quantitative thermodynamic description of multicomponent silicate melts using the Modified Quasichemical Model for short‐range ordering. We find that the local structure of the liquid, in terms of the bridging behavior of oxygen, calculated using our thermodynamic description allows us to link the viscosity and the thermodynamics of the silicate liquid. We can thus simultaneously calculate phase relations, thermodynamics and viscosity of the liquid over a wide composition and temperature range. In the present work we outline the viscosity model using selected binary and ternary systems as examples. The model has successfully been applied to melts in the multicomponent Na2O‐K2O‐MgO‐CaO‐MnO‐FeO‐ZnO‐PbO‐Al2O3‐SiO2 system and more elements are currently being added to the database.