The paper presents a reformulation of the standard pressure-time method for flow rate measurement in closed conduits. According to the IEC 60041 standard, the method is used with turbines flow rate cut-off. Its current formulation requires complete closure of the turbine shut-off device, generally guide vanes for reaction turbines and valves for impulse turbines, which may cause wear and tear on the machine. Any leakage through the closed shut-off device also needs to be measured. In the present work, a new formulation of the pressure-time method is derived, extending the actual use. The newly formulated method allows the determination of the flow rate at any instant of time from a load variation and, thus, the initial, incremental, and final flow rate. The load variation may be positive or negative. Measurements of any leakage flow are eliminated. Numerical and experimental cases are used to assess the validity and applicability of the proposed formulation. The numerical results demonstrate the correctness of the derivation. The experimental results exemplify the applicability of the method. Limitations of the derived methodology are discussed.