Abstract-In this paper, an extremely fast technique is introduced to evaluate the shielding effectiveness of a rectangular enclosure of finite wall thickness and numerous square or circular small apertures subject to a normally incident plane wave. The technique is based on the traditional waveguide circuit model where the enclosure is replaced with an equivalent shorted transmission line. In the proposed circuit model, the perforated thick wall is represented by an equivalent impedance which is derived from the reflection coefficient. The computation results are in very good agreement with measurements. Additionally, further results are compared to a generalized modal MoM technique which are in excellent agreement as the number of apertures increase. Besides accuracy, the method is extremely efficient and easy to implement compared to the numerical techniques.