Method for determining at least one concentration of at least one absorbing gas in a gas mixture to be measured by means of a spectroscopic measuring device, which comprises a light source, a measuring cell comprising an optical resonator, a detector and a computer unit, wherein the gas mixture is arranged in the measuring cell and a light beam is sent to the measuring cell through an entrance of the measuring cell by means of the light source, wherein wavelength-dependent measurement values for a light intensity of light leaving from an exit of the measuring cell are obtained by means of the detector, wherein a wavelength-dependent shape of the light intensity is represented as a wavelength-dependent measurement value function and wherein a curve fitting calculation between a theoretical wavelength-dependent function and the wavelength-dependent measurement value function is performed, wherein the theoretical function contains a wavelength-dependent calibration parameter for taking into account the state of the measuring device as well as at least one concentration of the at least one absorbing gas contained in the gas mixture as a parameter to be fitted during the curve fitting calculation, characterized in, that for performing the curve fitting calculation, the calibration parameter is defined as a function of a predetermined wavelength-dependent device parameter characterizing a path length of the light which the light beam travels in the measuring cell and a wavelength-dependent correction factor, wherein the correction factor is defined as a function of at least one concentration, wherein a cycle comprising a sequence of steps is performed several times in a row, wherein a) in a first step of the sequence, a numerical value for the correction factor is calculated from a stipulated assumed value for the at least one concentration using the function defining the correction factor, b) in a second step of the sequence, the theoretical function is determined, wherein the calibration parameter is calculated from the numerical value for the correction factor calculated in the first step, c) in a third step of the sequence, a value of the at least one concentration is obtained by a curve adjustment calculation between the theoretical function determined in the second step and the measurement value function and is stipulated as new assumed value for the concentration, wherein during the curve adjustment calculation the at least one concentration included in the theoretical function as freely selectable parameter is adjusted in order to adjust the theoretical function, which has been determined in the second step, as closely as possible to the measurement value function, wherein the assumed value obtained in the third step of the last cycle is output as a measured value of the at least one concentration. a) in a first step of the sequence, a numerical value for the correction factor is calculated from a stipulated assumed value for the at least one concentration using the function defining the correction factor, b) in a second step of the sequence, the theoretical function is determined, wherein the calibration parameter is calculated from the numerical value for the correction factor calculated in the first step, c) in a third step of the sequence, a value of the at least one concentration is obtained by a curve adjustment calculation between the theoretical function determined in the second step and the measurement value function and is stipulated as new assumed value for the concentration, wherein during the curve adjustment calculation the at least one concentration included in the theoretical function as freely selectable parameter is adjusted in order to adjust the theoretical function, which has been determined in the second step, as closely as possible to the measurement value function,