A method and a device that performs the method for measuring the flatness of a metal product traveling on a path, the method includes measuring a first longitudinal tension measurement value (T1) with a measuring roller, determining a model of stress over the thickness of the metal product as a function of plastic or elastoplastic deformation of the product, calculating a correction factor for the longitudinal deformation according to the stress model, calculating a corrective value (T1′, T2′) for the first longitudinal tension measurement value (T1) at at least one evaluation point (M1, M2) as a function of the longitudinal deformation correction factor (Z1), and calculating a corrected flatness measurement value (PC) at at least one of the evaluation points.