From the element balances of a complete combustion reaction, the water balance at partial condensation of the steam in an ideal waste gas/steam mixture, as well as the energy balance (1. law) for the combustion reaction and partial condensation, an exact equation for the measuring of the combustion-technical efficiency and of the relative waste gas Loss is derived. For a defined fuel gas, the important initial values of this equation, the waste gas measuring values for the temperature, the relative humidity and the amount of substance of oxygen are sufficient. The practical application is explained with the help of an example.