By means of physical optics method the formulas are obtained for extinction and scattering cross sections and efficiency factors for the case of normal incidence of a plane wave onto the base of a 2a×2b rectangular plate. An estimation of the relative error is obtained in the form of inequality, the right-hand side of which is a linear combination of two integrals being the functions of two diffraction parameters p = ka and q = kb. The error is shown to be less than 3% for the rectangular plate with the minimum size of the base ten times exceeding the wavelength.