A simplified method to calculate the effectiveness factors in irregular geometries of washcoats is presented. The method is based on sectioning the washcoat into particles and treating each particle in a 1D approach. This simplified method was compared with a rigorous 2D model for different kinds of geometrical shapes and kinetic expressions of the washcoat. For a first- and 0.75th-order reaction, the simplified method showed a good agreement with the 2D model. For a Langmuir-Hinshelwood rate expression, giving effectiveness factors higher than one, the simplified method could give rise to errors up to 14% due to inhibition effects. However, this was only significant at intermediate Values of temperatures and high surface concentrations on the washcoat. In general, the simplified method gives a good a priori estimation of the effectiveness factor that can replace tedious and time-consuming finite element calculations in irregular shapes of washcoats. A comparison of the CPU-time required to solve a 2D-reactor model has been conducted for the different approaches to evaluate the effectiveness factor.