This article presents a model, based on dimensionless groups, to predict the viscosity of nanoparticle suspensions, nanofluids. This empirical model expresses the viscosity of a nanofluid as a function of the following: viscosity of the base liquid, particle volume fraction, particle size, properties of the surfactant layer, and temperature. According to this model, viscosity changes nonlinearly with nanoparticle loading. Compared to other models, the new model is in good agreement with experimentally determined viscosity data for alumina–water nanofluids.