A method for determining a momentary boil-off rate for a storage tank (150, 200) for a natural gas in liquid phase and gas phase, the method comprising the steps of: - determining a volume of the gas phase of the natural gas and a volume of the liquid phase of the natural gas in the storage tank; - measuring a temperature and a pressure of the gas phase of the natural gas in the storage tank; characterised in that the method further comprises: - measuring a mass flow from the gas phase of the natural gas at a discharge pipeline of the gas phase of the storage tank; - applying a dynamical model to the measured mass flow, temperature and pressure, and to the determined volume of the gas phase and to the determined volume of the liquid phase, to determine the momentary boil-off rate in the storage tank, wherein the applying the dynamic model comprises: - determining a density of the gas phase of the natural gas in the storage tank using a thermodynamic equation of state, the temperature in the storage tank, the pressure of the gas phase of the natural gas in the storage tank and a molar mass of the natural gas; - determining a mass of the gas phase of the natural gas in the storage tank by using the determined density of the gas phase of the natural gas and the determined volume of the gas phase of the natural gas; and - determining a mass of the liquid phase of the natural gas in the storage tank by using a mass density of the liquid phase of the natural gas and the determined volume of the liquid phase of the natural gas. - determining a volume of the gas phase of the natural gas and a volume of the liquid phase of the natural gas in the storage tank; - measuring a temperature and a pressure of the gas phase of the natural gas in the storage tank; - measuring a mass flow from the gas phase of the natural gas at a discharge pipeline of the gas phase of the storage tank; - applying a dynamical model to the measured mass flow, temperature and pressure, and to the determined volume of the gas phase and to the determined volume of the liquid phase, to determine the momentary boil-off rate in the storage tank, wherein the applying the dynamic model comprises: - determining a density of the gas phase of the natural gas in the storage tank using a thermodynamic equation of state, the temperature in the storage tank, the pressure of the gas phase of the natural gas in the storage tank and a molar mass of the natural gas; - determining a mass of the gas phase of the natural gas in the storage tank by using the determined density of the gas phase of the natural gas and the determined volume of the gas phase of the natural gas; and - determining a mass of the liquid phase of the natural gas in the storage tank by using a mass density of the liquid phase of the natural gas and the determined volume of the liquid phase of the natural gas. - determining a density of the gas phase of the natural gas in the storage tank using a thermodynamic equation of state, the temperature in the storage tank, the pressure of the gas phase of the natural gas in the storage tank and a molar mass of the natural gas; - determining a mass of the gas phase of the natural gas in the storage tank by using the determined density of the gas phase of the natural gas and the determined volume of the gas phase of the natural gas; and - determining a mass of the liquid phase of the natural gas in the storage tank by using a mass density of the liquid phase of the natural gas and the determined volume of the liquid phase of the natural gas.