Method and apparatus for determining a mass rate of flow of gas by a rate of change of pressure includes an inlet for communicating a flow of gas from a source of gas whose mass flow rate is to be measured. A volumetric container is connected to the gas inlet to receive the gas. The volumetric container precisely defines a standard volume in its interior. Positioned within the volumetric container is a heat conductive assembly for maintaining the interior of the system substantially isothermal during pressure changes of the gas as gas flows into or leaves the container. Attached to the container is a pressure measuring transducer which measures the gas pressure within the container. The rate of change of the gas pressure under isothermal conditions within the container is indicative of the rate of mass flow of the gas into or out of the volumetric container.