Abstract: The present invention relates to a method and an apparatus for measuring the thickness d of a transparent layer (10), and in particular to a method and an apparatus for measuring the thickness d of glass plates during wet-chemical glass thinning in the manufacturing process. The method for measuring the thickness d of a transparent layer (10) includes: determining an approximate thickness dFFT based on a precisely known dispersion n(l) of the material of the layer (10) by performing Fast Fourier Transformation, FFT, analysis on Fabry-Pérot oscillation, FPO, from the layer (10) in a spectral reflectance measurement (900); and performing a FPO full-spectral fitting procedure (910-0) with the approximated thickness dFFT as starting value d0,0 to determine an initial local best fitting thickness dFPO,0.