A method and equipment are provided for determining the level/height, of one or more interfaces between two or more fluid phases of different density that are contained in a vessel, tank or the like. The position of the different interfaces in a tank is determined by measuring the pressure thereby determining the density of the fluid over the height of the tank by using a pressure measuring rod, equipped with pressure sensors, positioned in the tank. A mathematical model or algorithm is used which calculates how the density and hence the pressure vary over the height of the tank as a function of the fluid properties of the respective fluids in the tank, the interface, on the basis of the density being the same within each layer of the respective fluid, as a respective point at which the density changes from one layer to the next in the tank.