We present a novel non-invasive flow metering procedure where the flow velocity of a fluid is determined using polarized hydrogen nuclei of the fluid as magnetic markers. The metering procedure is based on a time-of-flight method in which magnetic information is applied to the liquid using a permanent magnet and a radiofrequency (RF) pulse. Downstream, the magnetic information is read-out by optically pumped magnetometers (OPM) by measuring the magnetic field produced by the hydrogen. We discuss results, application cases and challenges for industrial usability.