Physical and mathematical modeling of characteristics of water-ring vacuum pumps has a number of specific features. Isothermal efficiency, i.e. the ratio of the isothermal gas compression power to the pump power consumed is used as an efficiency indicator. The article describes the existing methods of hydraulic calculation of pumping units. An analysis of physical models, numerical values of empirical coefficients is carried out. An improved algorithm for calculating water-ring vacuum pumps of different manufacturers in the framework of previously developed models is presented. The calculations of compressors manufactured by ZAO Beskom (VVN-3, VVN1-6, VVN1-12) and ERSTVAK (ELRS-45, ELRS-57) are considered as examples. The results of tests of vacuum pumps published by the manufacturers in the open press are used as the initial data for the calculations. The experimental dependences of the performance, power expended, isothermal efficiency of water-ring vacuum pumps on the suction pressure are presented. The results obtained make it possible to improve the accuracy and reliability of predicting the amount of displaced liquid or steam during the operation of the pumping unit.