Abstract: A nuclear magnetic flowmeter with a measuring tube through which a multiphase medium can flow and which can be connected to an inlet tube which is located in the flow direction of the medium upstream of the measuring tube and to an outlet tube which is located in the flow direction downstream of the measuring tube. The nuclear magnetic flowmeter is, first of all, characterized essentially in that a medium bypass is assigned to the measuring tube, that the medium bypass includes a bypass tube, an inlet valve and/or an outlet valve and that, for a calibration operation, the bypass tube, on the one hand, can be connected to the inlet tube, and on the other hand, to the outlet tube, specifically via the inlet valve, via the outlet valve or via the inlet valve and via the outlet valve.