Method for calculating in real time, the autonomy duration of a non-refrigerated tank and defined by a setting pressure of the valves p valve, the shape thereof and the dimensions thereof, as well as the evaporation rate thereof, said tank containing natural gas being distributed in: - a natural gas layer in a liquid state, defined at a given instant t by the temperature thereof T liq (t), the composition x liq (t), and the filling rate of the tank by said natural gas layer; - a natural gas layer in the gaseous state (g), defined at a given instant t by the temperature thereof T gas (t) and the composition thereof x gas (t), and a pressure p(t); said method being characterised in that it consists of an algorithm comprising the following steps: A. at an instant t 0, the initialisation of the physical parameters of said natural gas layers is proceeded with, by measuring using pressure and temperature sensors, of the pressure of the gas p(t 0), and the temperature of the liquid T liq (t 0); while the respective compositions of the liquid x liq (t 0) and gaseous x gas (t 0) phases are known input data corresponding, either to the respective compositions of the liquid and gaseous phases at the time of loading the tank, or to average compositions for the LNG type used; B. for each instant t greater than t 0, a predetermined volume of natural gas in the gaseous or liquid state is subtracted, said volume corresponding to the functioning state of the tank at this instant t; and based on the volume of natural gas remaining after subtraction, the physical parameters p(t), T gas (t), and T liq (t) are calculated, using equations based on conserving the mass and the energy of the liquid and gaseous natural gas contained in the tank; C. while the pressure p(t) is less than p valve, the calculation of step B is reiterated for the instant following t+δt, with a constant physical time step δt; D. as soon as during N iterations of the process for calculating p(t), p(t+δt),..., p(t+N ∗ δt), the pressure p(t+N ∗ δt) becomes equal to or greater than p valve the calculation is stopped; E. the autonomy duration sought is equal to the total duration N ∗ δt covered by the algorithm at the time of stopping the calculation. - a natural gas layer in a liquid state, defined at a given instant t by the temperature thereof T liq (t), the composition x liq (t), and the filling rate of the tank by said natural gas layer; - a natural gas layer in the gaseous state (g), defined at a given instant t by the temperature thereof T gas (t) and the composition thereof x gas (t), and a pressure p(t); A. at an instant t 0, the initialisation of the physical parameters of said natural gas layers is proceeded with, by measuring using pressure and temperature sensors, of the pressure of the gas p(t 0), and the temperature of the liquid T liq (t 0); while the respective compositions of the liquid x liq (t 0) and gaseous x gas (t 0) phases are known input data corresponding, either to the respective compositions of the liquid and gaseous phases at the time of loading the tank, or to average compositions for the LNG type used; B. for each instant t greater than t 0, a predetermined volume of natural gas in the gaseous or liquid state is subtracted, said volume corresponding to the functioning state of the tank at this instant t; and based on the volume of natural gas remaining after subtraction, the physical parameters p(t), T gas (t), and T liq (t) are calculated, using equations based on conserving the mass and the energy of the liquid and gaseous natural gas contained in the tank; C. while the pressure p(t) is less than p valve, the calculation of step B is reiterated for the instant following t+δt, with a constant physical time step δt; D. as soon as during N iterations of the process for calculating p(t), p(t+δt),..., p(t+N ∗ δt), the pressure p(t+N ∗ δt) becomes equal to or greater than p valve the calculation is stopped; E. the autonomy duration sought is equal to the total duration N ∗ δt covered by the algorithm at the time of stopping the calculation.