A aerosol-generating system includes a liquid storage portion, a liquid pressure sensor within the liquid storage portion, and a control unit that is in communication with the pressure sensor. The aerosol-generating system further includes an atmospheric pressure sensor in communication with the control unit. The atmospheric pressure sensor is configured to sense the atmospheric pressure of the environment. The control unit is configured to receive the pressure signals from the liquid pressure sensor and the atmospheric pressure sensor. The control unit is further configured to determine the fill level of the liquid aerosol-forming substrate based on a comparison of the pressure signals from the liquid pressure sensor and the atmospheric pressure sensor.