This study used wind tunnel experiments to determine the loss factors of building openings for wind-driven ventilation. It is found that the loss factor is a function of internal porosity, but independent of Reynolds number. The loss factor of a partially open door increased as the door angle decreased. In addition, the influence of thickness ratio on the loss factor is much smaller than the internal porosity. The present study also derived the relationship between the loss factor and the discharge coefficient used in the orifice equation. The experimental results demonstrate that the ventilation rate of wind-driven cross ventilation can be predicted by a resistance model, once the loss factors of the openings and external pressure coefficients are known. Besides, it was found that the flow resistance of the internal opening dominated the ventilation rate when the external openings are large. But the resistance of external opening is the governing parameter when the external openings are small.