Abstract: An apparatus for measuring the thickness of a measurement object, preferably a measurement object in the form of a web or piece goods, in a measuring gap, with a measuring mechanism which is fitted to a machine frame, wherein the measuring mechanism for measuring the thickness comprises one or more travel measurement sensor(s) aimed at the measurement object, is characterized in that a compensation sensor which is coupled to a travel measurement sensor measures the distance to a reference rule in order to detect and compensate for a change in the measuring gap, in that the reference rule is in the form of a side of a frame-shaped reference device integrated in the measuring mechanism, and in that the reference device is configured in such a manner that the distance between the reference rule and that side of the reference device which is opposite the reference rule is known during the thickness measurement. A corresponding method for measuring the thickness is also stated.