A novel method for the measurement of volumetric flow rate for gas–liquid flow under vertical setting has been developed in this work. The sensing system consisted of two pairs of non-intrusive optical sensors that emit and receive infrared. The light intensity received is proportional to the phase fraction of gas and liquid in the pipe and hence bubble and slug flow regimes. The actual gas velocities were determined from cross-correlation and combined with phase fractions and pipe size to determine the gas flow rate. For the liquid flow rate, a modified drift flux model was developed to determine the actual liquid velocity based on the identified flow regime of bubble or slug flow. These measurements are then combined with the liquid fraction to determine the liquid flow rate. Measurement errors of ±3.2% and ±4.5% were obtained for liquid flow rate and gas volumetric flow rate measurements, respectively.