A method for calculating the remaining usage time of a gas cylinder equipped with a pressure reducer, the method comprising the following steps: (a) measuring the pressure of the gas in the cylin-der; (b) calculating the variation of pressure of the gas in the cylinder over time while gas is out-putted; (c) calculating a remaining usage time Tr based on the measured pressure in the cylinder and the calculated variation of pressure. Step (c) takes into account characteristics of the pressure reducer relative to variations of its nominal flow rate along the decrease of its inlet pressure while emptying the cylinder.