This paper presents a simple method to determine an area of vulnerability to voltage sags by using voltage drop equation. In general, the stochastic assessment of voltage sags is performed by using the concept of an area of vulnerability. Therefore, in order to fast assess the voltage sag performance, the area of vulnerability should be fast and accurately calculated. The voltage sag assessment provides the expected sag frequencies at sensitive load points, and the assessment results can help to establish efficient planning for the mitigation of voltage sags. This paper describes how to determine the critical buses and lines for fast determining the area of vulnerability for given voltage thresholds.