© 2024 The Author(s)The Double Focusing Mass Spectrometer (DFMS) onboard the Rosetta spacecraft employs an electrostatic and a magnet sector for energy and mass discrimination, resulting in a high mass resolution. A built-in feedback loop uses the measured magnet temperature to compensate for the temperature dependence of the magnet s field strength. Still, large onboard temperature variations and other effects cause any given mass peak to move over a range of 30 pixels or more on the detector during the mission. The present paper discusses the various factors that contribute to the time variations in the mass calibration relation. A technique is developed to evaluate and correct for these factors. A mass calibration relation that is valid for the DFMS neutral high mass resolution mode measurements throughout the entire mission for the mass range m/z=13u201369 is established and its accuracy is evaluated. The 1σ precision turns out to be less than a single pixel, which is excellent as full peak width at half height is about 12 pixels. The proposed approach provides an a posteriori mass calibration and is useful for all magnet-based mass spectrometers where experimental mass calibration by comparison to reference species, temperature stabilization, and/or electrostatic compensation, are not possible or fail to deliver a mass scale precision that is comparable to the mass resolution of the instrument.