Analytical long-wavelength approximation for parallelepipeds

•A new analytical long-wavelength approximation for rectangular parallelepipeds is suggested.•The approximation is grounded on replacement of the internal field with a uniform one and is extremely simple.•Numerical calculations has shown that the approximation suggested works in a wide region of parameter values.

We suggest a new analytical long-wavelength approximation for rectangular parallelepipeds based on replacement of the internal field with a uniform one. The approximation is not quite accurate (the typical accuracy is of the order of about 10%) but is extremely simple and works in a sufficiently wide region of parameter values.