A method for determining an actual gas flow rate in a reaction chamber of a plasma processing system is provided. The method includes delivering gas fay a gas flow delivery system controlled by a mass flow controller (MFC) to an orifice, which is located upstream from the reaction chamber. The method also includes pressurizing the gas to create a choked flow condition within the orifice. The method further includes measuring a set of upstream pressure values of the gas via a set of pressure sensors. The method yet also includes applying a calibration factor of a set of calibration factors to determine the actual flow rate. The calibration factor is a ratio of an average of the set of upstream pressure values to an average of a set of golden upstream pressure values, which is associated with an indicated flow rate for an MFC.