High-speed steels (HSS) exhibit a microstructure that comprises a martensitic matrix with carbides. Due to the generally high thermal stability of carbides, it is necessary to perform hardening at very high austenitizing temperatures. Nevertheless, there are certain carbides in HSS that are not dissolved. Therefore, the composition of the austenite, which can be transformed into martensite, is significantly different from the alloys’ nominal composition. Consequently, commonly applied formulae for the calculation of martensite start temperature (Formula presented.) cannot be used for HSS. The current study demonstrates how empirical (Formula presented.) formulae, which are basically applied for low-alloyed steels, where no carbides are present at austenitizing temperature, can be modified for high-alloyed HSS by applying thermodynamic calculations. Thermo-Calc software is utilized to calculate the composition of the austenite at two different austenitizing temperatures, and with these compositions (Formula presented.) are calculated subsequently. For experimental verification, (Formula presented.) of four alloys, which are quenched from these austenitizing temperatures, are determined using dilatometry. The experimental results show good agreement with the corresponding thermodynamic equilibrium calculations. Furthermore, the results reveal that Co does not raise (Formula presented.) as predicted by the commonly applied empirical formulae. Therefore, adapted formulae for HSS in a wide composition range are proposed.