Two different conversion modes are used for determining the water hardness from the conductivity of raw water in a water softening device dividing an influent raw water volume flow into two partial volume flows and subjecting a partial volume flow to a full softening process and re-uniting the two partial volume flows thereafter again into an admixed water volume flow. A first calibration curve reflects the maximum water hardnesses occurring at different conductivities; it is used for automatically controlling the regeneration of an ion exchange resin. A second calibration curve reflects the average water hardnesses at different conductivities; it is used for controlling the ratios of the two partial volume flows in the admixed water. With the invention, variations in the correlations between conductivity and water hardness may be taken into account in order to optimize regeneration and to minimize the hardness tolerances of admixed water.