A method for determining the ferrite phase fraction xa after heating or when cooling a steel strip (2) in a metallurgic system. Also, a device for carrying out the method. A method by which the ferrite phase fraction in the steel strip (2) can be determined online, quickly and easily, includes measuring a width w1 and a temperature T1 of the steel strip (2), wherein the steel strip (2) comprises a ferrite phase fraction xa1 during the measurements; heating or cooling the steel strip (2); when heating the steel strip (2) a phase conversion at least in part occurs, a→y from the ferrite state a into the austenitic state y and when cooling the steel strip a phase conversion at least in part occurs, from the austenitic state y into the ferrite state a; measuring of a width w and a temperature T of steel strip (2) converted at least in part; determining the ferrite phase fraction of the formula (I), wherein T0 is a reference temperature and aa and ay are the linear heat expansion coefficients of ferrite and austenite.