Unaccompanied vocalists usually change the tuning unintentionally and end up with a higher or lower pitch than the starting point during a long performance. This phenomenon is called pitch drift, which is dependent on various elements, such as the skill of the performer, and the length and difficulty of the performance. In this paper, we propose a computational method for measuring pitch drift in the course of an unaccompanied vocal performance, using pitch histogram and DBSCAN clustering.