A system for indicating the mass of fuel in several fuel tanks of an aircraft has a capacitive depth sensor and a permittivity cell located in each tank. Signals from the sensors and cells are supplied to a computing unit which computes the volume of fuel in each tank and its approximate density. The system has a densitometer and another permittivity cell which receive a sample of all the fuel supplied to the tanks, and supply output signals to the computing unit. The computing unit derives a correction factor by comparing an approximate value of density derived from the other permittivity cell with a more accurate measure of density derived from the densitometer. The correction factor is used to correct the approximate values of density derived from the permittivity cell in each tank. A correction factor is calculated in respect of every batch of fuel supplied to the tanks and a combined correction factor is calculated for the mixture in each tank, from the volumes and respective correction factors of the constituent fuels.