  The neutrino mass experiment KATRIN requires a stability of 3 ppm for the
retarding potential at -18.6 kV of the main spectrometer. To monitor the
stability, two custom-made ultra-precise high-voltage dividers were developed
and built in cooperation with the German national metrology institute
Physikalisch-Technische Bundesanstalt (PTB). Until now, regular absolute
calibration of the voltage dividers required bringing the equipment to the
specialised metrology laboratory. Here we present a new method based on
measuring the energy difference of two $^{83\mathrm{m}}$Kr conversion electron
lines with the KATRIN setup, which was demonstrated during KATRIN's
commissioning measurements in July 2017. The measured scale factor
$M=1972.449(10)$ of the high-voltage divider K35 is in agreement with the last
PTB calibration four years ago. This result demonstrates the utility of the
calibration method, as well as the long-term stability of the voltage divider.
