A method for ascertaining volume- or mass-flow of a magnetically conductive medium flowing through a magneto-inductive, flow-measuring device having a predetermined nominal-diameter, wherein a periodically alternating, magnetic field is caused to pass through the flow-measuring device, the actual rise time, until a constant magnitude of the magnetic field is reached, is ascertained, the actual rise time is compared with a desired rise time ascertained in the case of flow of a reference medium through the flow-measuring device, and, on the basis of a difference between actual rise time and desired rise time, a measurement error is ascertained and the actually measured volume- or mass-flow of the magnetically conductive medium is so corrected, that the measurement error is canceled.