A device for determining the density of a fluid comprises a buoyant body, a spring which engages on the buoyant body and whose elastic deformation is a measure of the buoyancy force of the buoyant body, and a magnet which is designed to sense the elastic deformation of the spring by means of a magnetostrictive position-measuring system. The spring is compressively loaded and, if the buoyancy force of the buoyant body is greater than its weight, said spring acts on the buoyant body underneath its centre of buoyancy. The device is preferably used to determine the density of fuel, a magnetostrictive filling level measuring system which is present in a storage tank for fuel being used.