Reliability measures of SAIDI (system average interruption duration index) and SAIFI (system average frequency index) are well established industry standards used world wide. While both measures have their limitations, they give a broad indication of average reliability that allows comparison within networks and across networks world wide. No such industry standard indices exist for voltage sags. The main reason being that voltage sags are multi-dimensional, involving retained sag voltage, sag duration, number of phases effected, phase angle jumps and the time between successive sags. This paper proposes a new voltage sag index that is dimensionally the same as SAIFI having units of equivalent interruptions per year, allowing a direct comparison with SAIFI. The proposed new index called ldquoSag SAIFIrdquo has been designed to allow voltage sag comparisons between sites, within networks and across networks. In addition, Sag SAIFI provides a means to directly compare the customer impacts of voltage sags with reliability (interruptions) and can assist in optimising expenditures on networks to maximise customer benefits of both reliability and voltage sag performance in their aggregate.