In a method and magnetic resonance system to determine the strength of a magnetic interference field that is caused by an interference object in a magnetic resonance image data acquisition, the interference object having a magnetic susceptibility that differs from the magnetic susceptibility of the material that surrounds the interference object image data acquired with the imaging magnetic resonance measurement that depict an examination subject that contains the interference object are made available in a form that allows the determination of at least one geometric variable of an image artifact in the image data that was caused by the magnetic interference field. The strength of the magnetic interference field that is caused by the interference object is then determined on the basis of the determined geometric variable of the image artifact.