A method of calculating a transient flow rate of a flowed process gas comprises flowing process gas through a mass flow controller into a chamber of known volume and measuring successive data sample points which include pressure data, temperature data, and a time value for each successive data sample point. Groups of successive data sample points are identified wherein each group shares one or more successive data sample points with another group, and ratio values are calculated for each of the successive data sample points wherein each ratio value is a ratio between the pressure data and a product of temperature and gas compressibility data for each respective time value. A line of best fit of the ratio values is determined within at least one group, and then the transient flow rate of the flowed process gas is calculated using a pressure rate of rise technique wherein the pressure rate of rise technique utilizes a ratio value determined from the line of best fit for at least one time value within the at least one group.