The present paper aims to improve the applicability of the pressure-time method for flow rate evaluation by eliminating the necessity to cut off the machine discharge completely and taking into account the compressibility of the fluid. As a result, the flow rate determination is possible for any load changes, increase or decrease. The method uses the water hammer equations with the differential pressure measured between two cross-sections. Furthermore, the instantaneous flow rate is analytically linked to the pressure variation and the losses. The method is presented and validated against numerical results.