This method is designed for calculating a quantification indicator for quantifying a dermal reaction on a skin having several chromophores. The method includes illumination of a zone to be characterized on the skin, the skin reaction being included in the zone to be characterized; and measurement of the spectrum of a back scattered radiation coming from the skin after illumination of said zone to be characterized. The method also includes determination, according to the measured spectrum, of an absorption coefficient value for the zone to be characterized, and calculation, according to the absorption coefficient value, of each chromophore concentration. The method includes calculation of the quantification indicator according to each calculated chromophore concentration. The method includes determination, according to the measured spectrum, of a diffusion coefficient value of the zone to be characterized, and the quantification indicator is further calculated according to the diffusion coefficient value.