ESDU 16005 provides a method of predicting the mass flow rate of a leak discharging into an external stream. The leak mass flow rate is dependent on the pressure difference across the leak orifice@ the geometry@ size@ and orientation of the leak orifice and the external flow conditions. The method requires knowledge of the leak orifice area together with values of the maximum orifice dimensions in both the streamwise direction and perpendicular to it. A discharge coefficient is used@ which is an efficiency parameter enabling the prediction of the change in mass flow rate from its isentropic value@ due to friction@ viscous and real gas effects. The method is derived using flat-plate test data for flows discharging into external streams through various rectangular@ square@ elliptical and circular orifice geometries in thin plates. In addition to these geometries the method should also be applicable to other similar geometries. The method is applicable for any boundary-layer thickness@ external flow Mach numbers up to 1.1 and unit Reynolds numbers greater than 4.7 ?? 106 m?C1. The lower limit of applicability for unit Reynolds number reduces to 1 ?? 106 m?C1 when the leakage flow streamtube is partially immersed in the external flow boundary layer. Leak mass flow ratio is a useful parameter and is the ratio of the mass flow rate through the leak orifice to the mass flow rate in the external stream. Sketches comparing predicted data with test data show that in most cases predictions lie within ??0.05 for leak mass flow ratio.