A cartridge for an aerosol-generating system includes a liquid storage portion configured to store a liquid aerosol-forming substrate. The liquid storage portion includes one or more flexible walls and is configured to change at least one of the shape and the size of the liquid storage portion upon a change of the volume of the liquid aerosol-forming substrate held in the liquid storage portion. The cartridge includes a sensor configured to detect data of a physical property. The data relates to at least one of a corresponding shape and a corresponding size of the liquid storage portion so that the volume of the liquid aerosol-forming substrate held in the liquid storage portion is determinable from the measured data.