Method for the real-time calculation of the residual chemical energy E contained in a pressurised tank defined by its shape and its dimensions and containing a layer of liquefied natural gas, said layer of LNG being defined at a given instant t, by its temperature T, its density ρ, and its level h in said tank; said method consisting of an algorithm that comprises, at a given instant t, the following steps: A. Acquisition of the characteristic parameters of the layer of LNG by measurement: - of the level h of the layer of LNG in the tank, using a level sensor; - of the temperature T using a temperature sensor; and - of the density ρ using a density sensor; and B. Determination of the total mass m t of the LNG contained in the tank; said method being characterised in that said algorithm further comprises, for each instant t, the following steps: C. Calculation of the mass gross calorific value GCV mass of the LNG using a function f taking as parameters the temperature and the density of the liquid according to the formula: GCV mass = f T ρ D. Calculation of the residual chemical energy E according to the formula: E = GCV mass ∗ m t wherein the function f that connects the mass gross calorific value GCV mass to the parameters T and ρ is of the form: f T ρ = A T + B ∗ ρ where: - A is a constant value for a given temperature - B is a constant independent of the composition. said method being carried out by a calculator intended to be connected to level, temperature, and density sensors of which said tank is provided with. said method consisting of an algorithm that comprises, at a given instant t, the following steps: A. Acquisition of the characteristic parameters of the layer of LNG by measurement: - of the level h of the layer of LNG in the tank, using a level sensor; - of the temperature T using a temperature sensor; and - of the density ρ using a density sensor; and B. Determination of the total mass m t of the LNG contained in the tank; A. Acquisition of the characteristic parameters of the layer of LNG by measurement: - of the level h of the layer of LNG in the tank, using a level sensor; - of the temperature T using a temperature sensor; and - of the density ρ using a density sensor; and - of the level h of the layer of LNG in the tank, using a level sensor; - of the temperature T using a temperature sensor; and - of the density ρ using a density sensor; and B. Determination of the total mass m t of the LNG contained in the tank; said method being characterised in that said algorithm further comprises, for each instant t, the following steps: C. Calculation of the mass gross calorific value GCV mass of the LNG using a function f taking as parameters the temperature and the density of the liquid according to the formula: GCV mass = f T ρ D. Calculation of the residual chemical energy E according to the formula: E = GCV mass ∗ m t C. Calculation of the mass gross calorific value GCV mass of the LNG using a function f taking as parameters the temperature and the density of the liquid according to the formula: GCV mass = f T ρ D. Calculation of the residual chemical energy E according to the formula: E = GCV mass ∗ m t wherein the function f that connects the mass gross calorific value GCV mass to the parameters T and ρ is of the form: f T ρ = A T + B ∗ ρ where: - A is a constant value for a given temperature - B is a constant independent of the composition. - A is a constant value for a given temperature - B is a constant independent of the composition. said method being carried out by a calculator intended to be connected to level, temperature, and density sensors of which said tank is provided with.