We study numerically the tortuosity--porosity relation in a microscopic model of a porous medium arranged as a collectin of freely overlapping squares. It is demonstrated that the finite-size effects and the discretization errors, which were ignored in previous studies, may cause significant underestimation of tortuosity. The simple tortuosity calculation method proposed here eliminates the need for using complicated, weighted averages. The numerical results presented here are in good agreement with an empirical relation between tortuosity ($T$) and porosity ($ϕ$) given by $T-1\propto \lnϕ$, that was found by others experimentally in granule packings and sediments. This relation can be also written as $T-1\propto R S/ϕ$ with $R$ and $S$ denoting the hydraulic radius of granules and the specific surface area, respectively.