A simplified 3D-model for isothermal conditions has been developed to reduce the calculations of the combined rate of pore diffusion and chemical reaction in washcoats of irregular geometries. The method is based on sectioning the washcoat into particles thus treating the solid-phase according to a corresponding boundary value problem. The simplified 3D-model was compared with a rigorous model for five different geometrical configurations of a square monolith with uneven washcoat coating. For the first and power law (0.7th order) kinetics the solutions of the reduced model were in very good agreement in comparison with the more expensive 3D-model. The computational burden could be even further reduced by using a lumped model in spite of peripherally varying concentration profiles. The agreement using the lumped model was satisfactory whenever proper correlations for the mass-transfer coefficient and the effectiveness factor were used.