Despite the numerous applications of pressurized graphene membranes in new technologies, there is still a lack of accurate mechanical models. In this work we propose a continuum model for circular graphene membranes subjected to uniform lateral pressure. We adopt a semi-inverse method by defining the kinematics of deformation and we describe the material behavior with a stored energy function that takes into account both nonlinearity and anisotropy of graphene. From the equilibrium we obtain an expression of the applied pressure as a function of the deflection of the membrane. A finite element (FE) model in nonlinear elasticity is presented and the results are used to validate the analytical model. A comparison with other models, numerical simulations and experiments from the literature demonstrates the advantages of the model proposed in this work. Differently from our entirely nonlinear approach, all the continuum models in the literature are based on the assumption of linear elastic material, which is suitable only when deformations are small. The present model gives a comprehensive description of the mechanics of pressurized graphene membranes.