Smart gels are soft polymeric systems that exhibit a reversible phase transition when exposed to environmental stimuli. A modified Boltzmann sigmoidal equation is proposed to model the phase transition behavior of smart gels. The equation can be solved through nonlinear conventional mathematical techniques by determining the hydrogel volume as a function of a particular stimulus. The theoretical results are in good agreement with the experimental data and are similar to those obtained from multiphysical mathematical models. Moreover, for all of the adjusted parameters, error probabilities approaching zero were obtained.