An equation is proposed to relate the efficiency of a steam-turbine plant (STP) to the pressure drop Delta P across the plant. The equation considers a STP operating on the Rankine cycle as a system and takes into consideration both thermodynamic and fluid-dynamic aspects of its work. The correlation between this equation and the experimental data was estimated. The physical meaning of the coefficients was established and their numerical values in a wide range of temperature and pressure variations were determined. The equation makes possible functional monitoring of an operating turbine. It can be useful in upgrading and optimizing individual units or designing a new STP.