This paper describes an analytical method for evaluating the voltage sag performance of a power-supply distribution network. The severity of voltage sag, as a result of system fault, is quantified by its magnitude and duration, and illustrated using a voltage sag density table. Since different loads have a different tolerance level to voltage sags, different evaluation algorithms are proposed in this paper to account for each type of sensitivity. It is illustrated that once the failure rates of the power components making up a system are known, the number and severity of the ensuing voltage sag events can be readily estimated. These estimates would be useful for predicting potential impacts on the operation of sensitive loads and for planning mitigating solutions to improve or maintain the power quality of the supply system. The possibility of incorporating other influencing factors from the behavior of the load and the fault events is also discussed.