As part of an investigation into single-sided natural ventilation, a computational fluid dynamics study was performed to analyse the impact on the airflow rate of the dimensions and position of a large rectangular opening and of the temperature difference between inside and outside air. An empty room with a rectangu-lar opening in the external wall was assumed and the Bernoulli formula used to calculate discharge coefficients Cd. The vertical position of the opening was found to have the greatest impact on the discharge coefficient. Cd is also influenced by the opening width and wall thickness. By contrast, the horizontal distance of the opening from the side-wall and the opening area are of minor importance only. Experimental results served for spot verifica-tion of CFD results.