ACF Ratings Formula

  Rp = Rc + Window_Size * (W - L) / N  

where Rp = performance rating,
Rc = average rating of competition played,
W = number of rounds won,
L = number of rounds lost,
N = number of rounds played,
and Window_Size = method-of-scoring constant.

Note: Ratings are computed by how many games were played in each round and then averaged with the last 128 games.

  The Window_Size Constant
The Window_Size constant indicates whether a tournament is scored per game or per round. Window_Size is set at 400 for tournaments that score by four-game rounds. For tournaments scored per game with two games per round, Window_Size is set at 670. This Window_Size produces the same average rating and distribution of ratings for tournaments scored by game that a Window_Size of 400 produces for tournaments scored by round. This is because (W - L) / N will be larger in a tournament scored by round than in a tournament scored by game.

  Win Rate
Win Rate is defined as (W / N) and is summed over all rounds played. A player with a 200 point rating advantage over an opponent (ie: Rp = Rc + 200) is expected to have a 75 percent win rate against that opponent when playing four-game rounds scored by round.

For example: Suppose a player wins three rounds and loses one round. That player has a Win Rate of W / N = (3 wins) / (4 total rounds), or 75 percent. Plugging W, L, and N into the ratings formula, we have Rp = Rc + 400 * (3-1) / 4 = Rc + 200.

Similarly, if your win rate is 50 percent, then your rating would be equal to the average rating (Rc) of your opponents. (See if you can compute this as above.)

Note: Most players have a win rate of around 50 percent but play in different tournaments and divisions.

  Special Cases
If the difference between your rating and that of your opponent's is greater than 75 percent of the Window_Size, then his / her rating will be treated as if it was your rating + (3 / 4) * Window_Size. This forces wins to increase your rating and losses to decrease your rating.

For example: Suppose your rating is 2000, and your opponent's rating is 1650. Then we have (2000 - 1650) / 400 = 87.5 percent. For the purposes of calculation for that round, your opponent's rating will be computed as 1650 + (3 / 4) * 400 = 1950.

This anomaly occurs in less than ten percent of all rounds played. Most instances occur in small tournaments.

Note: An opponent's rating is always assumed to be 1000 or above.

  The Game-Weighted Average
Ratings are calculated as a game-weighted average. To allow the average to change more rapidly, the sum of all previous games is limited to 128. Thus if you played 150 games this year, your rating would be [(This Year's Rating) * 150 + (Last Year's Rating) * 128] / (150 + 128). Once the 128 game limit is reached, your rating cannot drop by more than 100 points from your maximum rating.

  Temporary Ratings
In order have a rating, you must earn at least a draw with one rated person. Players with 15 or fewer rounds of rated tournament experience are considered to have a temporary rating. This indicates that their rating is not very stable.