Apparatuses for controlling gas flow are important components for delivering process gases for semiconductor fabrication. In one embodiment, a method of calibrating an apparatus for controlling gas flow is disclosed. Specifically, the apparatus may be calibrated on installation using a two-step process of measuring the volume of gas box downstream from the apparatus by flowing nitrogen gas into the gas box and measuring the resulting temperature and rate of pressure rise. Using the computed volume of the gas box, a sweep of several mass flow rates may be performed using the process gas and the gas map for the process gas. The apparatus is calibrated based on the measured temperature and pressure values, which allow calculation of the actual mass flow rate for the process gas as compared with the commanded mass flow rates.