A method is provided for calibrating a sensor used for surface weight measurement or thickness measurement on a material sheet via transmission measurement and/or reflection measurement. The method may include: providing a two-dimensional calibration sample with a known surface weight; detection of the transmission values and/or the reflection values of the calibration sample via the sensor on a plurality of different positions two-dimensionally or areally distributed across the surface of the calibration sample; and the detection of the calibration value for the sensor via calculation of average from the plurality detected transmission values and/or reflection values and the composition of the average transmission and reflection values with the known surface weight of the calibration sample. Also provided is a method for measurement of layer thickness and/or surface weight of material sheets using the calibration method, as well as an apparatus for layer thickness measurements and/or surface weight measurement.