A method for determining the fat mass of a subject includes the steps of acquiring an image of the subject through a digital device, and generating a virtual frame that contains at least in part that image. The virtual frame contains the subject, on the basis of its height or of the greater size in the case of animals, to provide an estimation of the content of the fat mass through an algorithm on the basis of at least an indicative index of the area occupied by the subject with respect to the area of the frame in which it is contained at least in part.
