Abstract: A method of calculating the thickness of a sheet material comprising directing a beam of photons against the sheet material from a photon source (1) located on one side of said sheet material (7), detecting reflected radiation from said sheet material in a first detector (2) located on the same side of said sheet material and detecting transmitted radiation in a second detector (5) located on the opposite side of the said sheet material, combining the signals of transmitted and reflected photon beams, measuring and processing the data from both detectors for a number of calibrating samples, obtaining the thickness as a function of the detected data and applying this function to determine the unknown thickness of a sample. Apparatus for carrying out the method of the invention is also disclosed.