Disclosed herein, among other things, are methods and apparatus for vent detection in a hearing assistance device with a real ear measurement (REM) system. One aspect of the present subject matter relates to a method for estimating vent out effect for a hearing assistance device. A REM is performed to obtain a measured response for a hearing assistance device worn by a user. A first simulation of a real ear response is performed using an occluded hearing assistance device model. The REM is compared to the first simulation in a selected frequency range to determine a vent effect and a second simulation of the real ear response is performed using the determined vent effect. The REM is compared to the second simulation to derive gains that compensate for the shape and volume of the user's ear canal.