A method and system for fitting the gain of a hearing aid for a hearing impaired person is described. For at least one frequency band a loop gain of the hearing aid in the ear canal of the hearing impaired person is measured, an effective vent parameter such as a corresponding vent diameter for the hearing aid by determining a vent parameter that generates the best fit between a modelled and the measured loop gain is estimated, a vent effect value based on the estimated effective vent parameter is determined, and a corrected hearing aid gain is provided by means of the determined vent effect value.