SIW is considered as an effective rectangular waveguide with periodically loaded vias on its two side walls instead of vertical plates. Effective wave impedances of substrate integrated waveguide (SIW) with varied via-to-via spacings are for the first time numerically de-embedded from three-dimensional (3-D) fullwave algorithm by virtue of a short-open-load (SOL) calibration approach. Full-wave simulation is at first conducted to model a finitecell SIW with two feeding uniform waveguides. Afterwards, three calibration standards are formed and modeled to de-embed the two-port ABCD matrix of the finite-cell SIW section. In addition to propagation constant, effective wave impedance of the SIW can be accurately derived without any prior assumption. After numerical convergence versus unit cells is studied, these parameters are gained to give the complete view on guided-wave characteristics along the SIW.