Average Rating of Opponents

Average Rating of Opponents, abbreviated ARO, summarizes the nominal strength of a player's opposition. A rating is a number estimating competitive strength. ARO is found by adding the ratings of opponents actually played over the board and dividing by the number of those opponents.

The result is rounded to the nearest whole number, with an exact 0.5 rounded upward. ARO does not directly reflect whether those opponents performed well or poorly in this event. Two players facing the same set of rating values have the same ARO even if their opponents finish with very different scores.

The rules of FIDE, the International Chess Federation, require rating-based tie-breaks to be dropped when unrated players are present unless the event regulations have defined and published how to handle them. They are also not recommended when a player may receive more than one rating during the event. If used anyway, the first applicable rating is taken unless the regulations say otherwise.

ARO differs from APRO, which averages each opponent's performance in the event, and from AOB, which uses scores and schedules rather than ratings.