Abstract: A method for measuring the net internal volume of test receptacles each containing an unknown but variable volume of a liquid, such as in testing automatic transmissions containing residual amounts of transmission fluid, to determine the exact volume of fluid needed to be added to fill the transmission to the correct level. The method places a pressurized reference vessel in communication with the internal spaces of a sealed transmission upon opening of a valve allowing the pressure to fall by expansion from the reference vessel into the transmission. The resulting reduced pressure is initially sensed shortly after the valve is opened, and subsequently after a more prolonged interval, allowing the pressure to be substantially stabilized. The receptacle net internal volume is then determined from the relationship between the preliminary pressure in the reference vessel and the stabilized final pressure by application of Boyle's law, and the volume of liquid to be added calculated therefrom.