The present invention relates to systems and methods for measuring the amount of fluid. A fluid source is in fluid communication with a measuring system. The fluid source includes a non-collapsible walled container for dispensing or introducing fluid. The measuring system includes a sensor in fluid communication with the container that senses the volume of the gas, and a system that calculates the amount of fluid flowing through an opening of the container based on the amount of gas flowing through an opening of the container. The volume of gas entering or exiting the container is measured rather than directly measuring the amount of fluid leaving or entering the container to determine in a more accurate, less complicated and less expensive manner the volume of fluid.