Electrical impedance tomography is one of the most popular methods to investigate subsurface resistivity and structure; however, it does not take into account the effect of the electrode area because it is based on methods for measuring the resistance of large-scale objects like the Earth. The purpose of the present study is to propose a new theoretical electrical impedance equation to obtain reliable electrical resistance measurements in tests of human-scale objects. The finite element method is applied to investigate how the impedance of a model to which electrodes are to be attached changes depending on the area of the electrodes. Both the electrodes and the model are constructed and calculated through COMSOL. The electrical impedance is simulated according to the area of the electrode, the length of the model, the conductivity, and the frequency of the electric field. On the basis of the simulation results according to the geometric structure of electrodes and the model, a new electric impedance equation for the electrode area is formulated. Unless the shape of the model is a circular plate or a long column, the results fit well with the theoretical calculations. Also, if the model height is similar to the model length, the theoretical impedance can only be defined as an arctangent function. This equation is important for analyzing the effect of the electrode area and is effective for estimating the resistance of human-scale models.