In a method and system for fitting the gain of a hearing aid (300) for a hearing impaired person, a loop gain of the hearing aid in the ear canal (350) of the hearing impaired person is measured for at least one frequency band. 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. The invention provides a method, a computer program, a system for fitting a hearing aid, a hearing aid and a computer system.