A method for determining a position of a mobile device relative to a B0 field magnet along a z-coordinate axis, and a device and a magnetic resonance tomography unit for performing the method are provided. The device includes a magnetic field strength sensor arranged in a fixed relative position. A characteristic magnetic field strength Bref of the B0 field magnet that emerges for a plurality of x-y coordinate pairs with a same reference z-coordinate zref is ascertained. The device is moved along the z-coordinate axis until the magnetic field strength sensor measures the characteristic magnetic field strength Bref.