A temperature-determining device for determining a temperature (TMED) of a medium via a temperature of a surface includes: an ambient-temperature sensor, arranged in surroundings of the surface, for measuring an ambient temperature (TU); a surface-temperature sensor, lying on the surface, for measuring a mixed temperature (TM) lying between the temperature (TMED) of a medium and the ambient temperature (TU); and an arithmetic-logic unit having an approximation formula electronically stored thereon for calculating an approximation (TMEDN) of a temperature of a medium. The approximation formula is a sum of the mixed temperature (TM) and a product of two factors. The first factor results from a difference between the mixed temperature (TM) and the ambient temperature (TU) and the second factor results from a ratio of a dividend to a quotient. The dividend results from a difference between a calibration temperature (TMEDKAL) of a medium and a calibration mixed temperature (TMKAL).