A method of estimating a real-ear-to-coupler-difference (RECD) in a hearing is provided. The hearing aid comprises an input transducer; an output transducer; and an ITE-part configured to define a residual volume when mounted in an ear canal of a user. The method comprises providing an earpiece configured to fit tightly to walls of an ear canal of the user and to provide said residual volume; that the earpiece comprises at least one ventilation channel or opening; a sound outlet allowing sound from said output transducer to be played into said residual volume; characteristics of the at least one ventilation channel or opening; a frequency dependent feedback path estimate from said output transducer to said input transducer through said ventilation channel or opening; and estimating a low-frequency and high frequency RECD values in dependence said feedback path estimate; and determining estimated frequency dependent RECD values in dependence of said estimated low-frequency RECD value, and said estimated high-frequency RECD value.