Wind-induced natural ventilation through openings with small wind pressure differences was examined using large eddy simulation (LES) modelling. This study focused on cases in which the ventilation rate is predicted using a standard Orifice equation. The purpose of this study was to clarify how the ventilation rate is underestimated in such cases for both single-sided and double-sided openings, and to clarify the difference between the effective ventilation rate (purging flow rate, PFR) and bulk airflow rate (AFR), which have not been sufficiently and systematically understood. A simple cubic-room model with two openings was analysed using LES by varying the opening position after validation, and the ventilation rate, velocity field, and wind pressure coefficient were compared with experimental results. The PFR was determined by tracing particles, and the AFR was obtained based on the instantaneous velocity over the openings. The AFR predicted by the Orifice equation was underestimated when the difference in the mean wind pressure coefficient (Delta(C-p) over bar) was less than 0.1. The main feature of this study was to show the ventilation effectiveness defined by the PFR divided by AFR, which was approximately 70-80%, 60%, and 90% for the double-sided openings, single-sided openings on the lateral side, and windward and leeward sides, respectively. Another feature was to propose a method for estimating the AFR reflecting two key phenomena, namely pulsating flow and eddy penetration. In addition, a simple equation considering the standard deviation of wind pressure coefficient difference (sigma(Delta Cp)), and external local velocity around an opening (V-vic) was obtained.