A liquid container hydrostatic level gauge, in particular a fuel tank level indicator, which operates with a differential pressure sensor, a zero point calibration related to the height of the fuel is performed upon the beginning of measurement, in order to dispense with a requirement for temperature independence and long-term stability of the zero point of the differential pressure sensor. A meter tube is disposed in the container interior, extending from above the maximum liquid level to as far as the container bottom with an opening for receiving fuel within the tank. The differential pressure sensor is connected by one meter input to the upper end of the meter tube and by its other meter input is exposed to the air pressure acting upon the surface of the liquid. An air pump is connected to the upper end of the meter tube between the upper level of the fuel and the input to the pressure sensor. At the beginning of measurement, the meter tube is flooded, and an output value (U.sub.o) of the differential pressure sensor is stored in memory. For determining the fill level, the meter fuel is forced from the tube by pumping air in, and the fill level (h(t)) is determined from a measure of the difference between the instantaneous output signal (u(t)) and the memorized value (U.sub.o).