The paper presents a method for the computation of the radar cross section of microstrip patches. The approach is based on a combination of ideas derived from the mixed potential integral equation formulation for microstrip antennas, fast convolution algorithms and the biconjugate gradient iterative method. All operations are kept at a vector level and this renders the method capable of handling a large number of unknowns. Numerical results for a rectangular and a circular patch have been compared with measured results available in literature and a good agreement has been observed. Advantages and limitations are discussed.