A hearing aid (1) comprising a receiver (2) and at least one first microphone for transforming an acoustic signal surrounding a hearing aid user into an electrical signal, where the hearing aid comprises an ear plug part prepared for fitting tightly into the car canal (11) of a hearing aid user such that an inner volume is formed in the inner part of the ear canal between the car plug part and the car drum of the hearing aid user. The car plug part comprises a second microphone (3) arranged for transforming an acoustic signal in the volume into an electrical signal. The hearing aid comprises estimating means for estimating the effective size of the air leak between the inner volume and the surroundings based on the acoustical signal detected in said volume by said second microphone (3) from a known acoustical signal below 1000 Hz generated by the receiver. The hearing aid further comprises notification means for notifying the hearing aid user if the size range of the air leak is outside a predefined range.