Abstract: Apparatus for gauging high pressure gas in particular for the supply of oxygen gas on board on aircraft, the apparatus being associated with a gas tank or with a plurality of gas tanks having their outlets interconnected, the apparatus being wherein each individual tank is provided both with a pressure regulator connected to a common outlet duct, and with a pressure sensor producing a signal proportional to the pressure in the, or each, tank, with the or each, pressure sensor being connected to a calculation and storage unit suitable for determining the content of each individual tank on the basis of the signal associated therewith and taking account of the coefficient of compressibility of the gas contained in each individual tank on the basis of data stored in the storage unit for different values of pressure, and consequently deducing the overall available content, the calculation unit further including a readout for displaying the overall available content.