A method is presented for the determination of the leakage rate for containment vessels of water-cooled reactors. The method is applicable to Type A tests for which the containment vessel is pressurized to some initial overpressure, and subsequent measurements of absolute air pressure and temperature are made to determine the leakage rate. The proposed method incorporates the desirable features of the recommended method for the determination of the leakage rate, namely, that the measured data all have equal statistical weight, the leakage rate is not estimated using finite differences, and the leakage rate is normalized to the initial air content in the containment vessel. The major assumptions of the proposed method are incompressible airflow and a constant absolute air temperature. The proposed method is based on a reasonably accurate description of absolute dry air pressure over time, for which parameters are obtained using a linear regression technique on the transformed pressure measurements. Under the given assumptions the transformed pressure measurements are linear, and therefore, the proposed method avoids the drawback that is encountered in the recommended method of applying a linear model to nonlinear data. The pressure function then is used to determine the leakage rate as a function of time and the integral leakage rate for the duration of the test. Also, the method is readily adaptable to scaling the integral leakage rate to different initial air pressures in the containment vessel. In addition, the assumption of an incompressible airflow is considered to be reasonable for initial Mach numbers less than or equal to 0.4.