Abstract: A method and device for measuring the shape and/or the planarity of a longitudinal fiber of a running strip are provided. Readings are taken of the heights (y.sub.i) of a set of n points of index (i) of the said fiber, and a function (y(l)) depending on the shape of the said fiber is determined. When readings are taken of the height of the point of index (i), readings are taken simultaneously of the heights of at least two other points which are close to and located on either side of it, the curvature (K(x).sub.i) of the said fiber in the vicinity of the said point of index i is calculated approximately, and then a function (k(l)) representing the change in the curvature between the said points of index 1 and index n is determined, and, by successive integrations of (k(l)), a function (Y(l)) representing the actual shape of the said fiber between the said points of index I and index n, and the length L of the said fiber, from which its elongation ratio is calculated, are determined.