A turbo-machine, which can be operated in an optimized driving range is provided. To this end, a method for operating a turbo-machine having at least one turbo-machine stage, which has at least one rotary shaft is disclosed. According to the method, the following method steps are carried out: a) determining a desired efficiency characteristic value ηsoll of the turbomachine stage; b) determining an actual efficiency characteristic value ηist of the turbo-machine-stage; c) determining a comparison efficiency characteristic value of the turbo-machine stage by comparing the actual efficiency characteristic value ηist and the desired efficiency characteristic value ηsoll to one another; and d) changing at least one operating parameter of the turbo-machine stage subject to the comparison efficiency characteristic value ηvgl, wherein in order to determine the actual efficiency characteristic value ηist, a measuring of a torque of the rotary shaft of the turbo-machine-stage is carried out.