ELO Ratings

During the provisional period the rating is the average of per-game values. Against a provisional opponent the value is the average of both ratings (using 1600 if the opponent has never played) plus 200 × outcome (−1/0/1 for loss/draw/win).

Against an established opponent the value is the opponent's rating + 400 × outcome.

For an established player: new rating = r1 + K · (w − 1/(1 + 10^((r2−r1)/400))), where w is 1/0.5/0 for win/draw/loss. K = 32 for established-vs-established.

If the opponent is provisional, K is scaled by n/20 where n is the opponent's game count — so playing a 0-game opponent cannot change your rating.

For two established players the sum of rating changes is zero. When the rating difference exceeds 719 points, the stronger player winning does not change either rating.

During the provisional period, beating a player more than 400 points below you DECREASES your rating. This prevents inflating a provisional rating by beating very weak opponents.