[en] Method for calculating thermodynamic functions in two-phase liquid-vapor region according to analytical dependences is given. It enables to reduce the calculation time, as compared to iterative algorithms. It is assumed that required parameters, characterizing substance state at the boundary of two-phase region are known. Polynomial splines-cubic and square ones-are used to present the boundary of two phase region. Operationability of the method is demonstrated, taking the calculation of copper state in two-phase liquid-vapour region as an example. 5 refs.; 2 figs