It is important to estimate the ventilation rate of animal houses accurately for the purpose of reducing emissions and providing comfortable indoor climate. The orifice equation is widely applied for estimating the airflow rate of naturally ventilated dairy barns. However, to regard openings' discharge coefficient (C-d) as a constant maybe not true for large openings. To gain basic knowledge of the opening size effects on C-d values, experiments with a scaled model were carried out in a large boundary layer wind tunnel. Seven cases with different sidewall opening ratios, varying from 18.6% to 81.4%, were investigated under 8 m/s wind speed conditions. The airflow rates, C-d values of windward openings and airflow patterns were analysed. It was found that the C-d value was considerably dependent on the opening size and varied from 0.67 to 0.94 when the opening ratio increased from 18.6% to 62.7%. This is due to the assumptions of pure pressure-driven flow and uniform velocity distributions in the opening were not satisfied for large opening cases. Our study suggests that C-d should be regarded as a variable under different opening size conditions when using the orifice equation to determine the airflow rate of large openings. (C) 2018 Elsevier B.V. All rights reserved.