A simple nondestructive method of measuring groove depth in lamellar gratings is presented. The method is based on processing scatterometry data for a wavelength much smaller than the grating pitch using formulas derived by means of scalar diffraction theory. The method has structural and optical limitations that are specified. The intensities of the 0th and ±1st orders reflected from silicon grating samples irradiated by a He-Ne laser, as functions of the incident angle, are measured and processed to determine the groove depths. The diffraction efficiencies calculated using the thus determined groove depths by means of rigorous diffraction theory agree with the experimental data. For the examples studied in this paper the uncertainty in the determined groove depths is as small as 10 nm.