Determination of transport coefficients in 3D topological insulators is one of the highlighted topics in the literature. The main difficulty of the calculation of transport coefficients comes from the contribution of bulk modes. In this work, electrical conductivity in 3D topologic insulators is considered under zero magnetic fields and it is written in a size dependent form. For this purpose, Weyl conjecture is used. The contribution of the bulk modes to electrical conductivity is analytically derived by depending on the material size, hybridization energy and degeneracy of the carriers.