We have introduced a theoretical mean-field model to describe the swelling of weakly charged microgels as a function of pH, salt concentration and intrinsic material parameters. The model integrates the Poisson-Boltzmann theory for electrostatics with the Flory-Rehner theory and a Langevin elastic model for describing the polymer properties to derive expressions for the microgel osmotic pressure, which is used to determine the equilibrium size of the microgel. Our model extends the work presented in Refs. to consistently incorporate the charge regulation of weak acidic and basic sites in the polymer network.