The invention relates to a system (1, 16, 19, 28) for measuring a quantity of liquid in an elastic bag (3), a liquid opening (10) for the removal of liquid from the elastic bag (3) being formed in the lower region of the elastic bag (3). The system (1, 16, 19, 28) comprises pressure elements, which bear on opposite walls of the elastic bag (3) and are spaced apart from one another, a drive unit (4), which is designed to drive at least one pressure element, sensors (7), and a control unit (5). The sensors (7) are designed to measure a liquid level (13) of the liquid in the elastic bag (3). The control unit (5) is designed to calculate a quantity of liquid in the elastic bag (3) on the basis of the liquid level (13) and of the distance (21) between the pressure elements. The liquid level (13) of the liquid in the plastic bag (3) can be changed by reducing the distance (21) between the pressure elements by means of the drive unit (4).
