A method and apparatus that solve the problem of accurate measurement of gas flow so that the delivery of gases in semiconductor processing may be performed with greater confidence and accuracy by performing real-time characterization of a lead-line for mass flow controller (MFC) flow verification are provided. In one embodiment a mass flow verifier (MFV) provides rate of rise information to a controller via a digital interface without correcting for lead-line influences. After receiving the rate of rise data from the tool host computer computes a gas mass correction factor in real-time based on at least one of the following: MFC temperature sensor data, lead-line temperature sensor data, lead-line pressure transducer data, and lead-line volume. The rate of rise data and gas mass correction factor are used to compute accurate mass flow. The accurate mass flow information may be used to calibrate the MFC.