Method for the hot measuring, during rolling, of a transverse size of a metal profile such as a tube or a solid bar, to obtain at least the measurement of a diameter and/or a mean thickness of said metal profile, which provides to: - power with a sinusoidal current having at least two frequencies (F1, F2), a transmission element having at least two sections (14a, 14b, 14c, 14d) distinct and spatially separated from each other, disposed along a nominal axis of feed of the metal profile and operating each on the same nominal transverse size of the metal profile; - generating with said transmission element an electromagnetic field with a desired profile of the force lines; - detecting a signal relating to the variations of said electromagnetic field due to the passage of said metal profile through said sections (14a, 14b, 14c, 14d) of said transmission element by means of a reception element having one or more sections (18a, 18b) distinct and spatially separated from each other and disposed along said nominal axis in an intermediate position between adjacent sections (14a, 14b, 14c, 14d), or drowned inside a section (14a, 14b, 14c, 14d) of the transmission element wherein both the sections (14a, 14b, 14c, 14d) of the transmission element and also the reception element are spirals or coils of a cylindrical shape and each of them define a transit hole of a larger size than the diameter of the metal profile on which the measurement is performed, and inside which the metal profile is made to transit; - normalizing and non-dimensionalizing the value of said flow of electromagnetic field detected with respect to the value of said current for every frequency (F1, F2), wherein said flow of electromagnetic field is non-dimensionalized using a reference value relating to a normalized flow obtained without a metal profile; - defining a mathematical model on the basis of the normalized and non-dimensionalized values of flow; - performing a homographic transformation to determine a correspondence between real non-dimensionalized values and ideal non-dimensionalized values defined by the mathematical model in order to de-constrain said measurement of a transverse size of said metal profile from the physical sizes of said transmission element and said reception element; - processing the data obtained from the homography to calculate said diameter and/or said mean thickness of said metal profile. - power with a sinusoidal current having at least two frequencies (F1, F2), a transmission element having at least two sections (14a, 14b, 14c, 14d) distinct and spatially separated from each other, disposed along a nominal axis of feed of the metal profile and operating each on the same nominal transverse size of the metal profile; - generating with said transmission element an electromagnetic field with a desired profile of the force lines; - detecting a signal relating to the variations of said electromagnetic field due to the passage of said metal profile through said sections (14a, 14b, 14c, 14d) of said transmission element by means of a reception element having one or more sections (18a, 18b) distinct and spatially separated from each other and disposed along said nominal axis in an intermediate position between adjacent sections (14a, 14b, 14c, 14d), or drowned inside a section (14a, 14b, 14c, 14d) of the transmission element wherein both the sections (14a, 14b, 14c, 14d) of the transmission element and also the reception element are spirals or coils of a cylindrical shape and each of them define a transit hole of a larger size than the diameter of the metal profile on which the measurement is performed, and inside which the metal profile is made to transit; - normalizing and non-dimensionalizing the value of said flow of electromagnetic field detected with respect to the value of said current for every frequency (F1, F2), wherein said flow of electromagnetic field is non-dimensionalized using a reference value relating to a normalized flow obtained without a metal profile; - defining a mathematical model on the basis of the normalized and non-dimensionalized values of flow; - performing a homographic transformation to determine a correspondence between real non-dimensionalized values and ideal non-dimensionalized values defined by the mathematical model in order to de-constrain said measurement of a transverse size of said metal profile from the physical sizes of said transmission element and said reception element; - processing the data obtained from the homography to calculate said diameter and/or said mean thickness of said metal profile.