A computer based system for taking the physical inventory of liquids dispensed in full and partially full containers. A database stores images of containers and their associated volumes. The containers to be inventoried are identified by a user and input to a computer at a graphical user interface (GUI) input/output. A computer causes the display of an image of the container and a sliding level indicator at the GUI. The level indicator is slidable along the image. The computer calculates a volume of liquid remaining in the container as a function of the position of the level indicator along the image of the container.