The article presents the results of theoretical and experimental studies of the effect of hydraulic friction on the maximum increase in pressure during water hammer. To solve the problem of determining hydraulic friction along a pressure pipe, the telegraph equation was adopted as a mathematical model. As a result of solving the adopted equation, dependence is obtained for calculating the energy loss in a non-stationary process. The calculations of the proposed formula are in good agreement with the experimental data of the author. This proves the legitimacy of the obtained dependence of the author and provides an accurate calculation of pipes for strength during impact and resource saving of pipe material