Ratings are a measurement of your win/loss record on FIBS. All players begin with a rating of 1500, which goes up or down as you win or lose matches.

The formula for calculating ratings is a little complex, but if you are interested, here are the details:

Let D = the rating difference between the two players (higher rating - lower rating).

Let N = the match length.

The probability that the underdog wins is:

Pu =	1
	1 + 10 (D * sqrt(N) / 2000)

Let P = Pu if the player with the higher rating wins the match, let P = (1 - Pu) if the underdog wins.

For each player, let

E_p = the player's experience points.

K_p = max(1, 5 - ((E_p + N) / 100))

R_p = 4 * K_p * sqrt(N) * P

The winning player adds his R_p to his ratings, the losing player subtracts his R_p from his ratings.

The effect of experience points

Ratings change much faster for new players since K_p is very close to 5.0 for a player with zero experience points. Once you have accumulated 400 experience points, K_p will then always equal 1.0 and the effect of experience points disappears. This aspect of the formula allows new players to reach their level quickly, and is based on formulas used in similar Internet-based games such as Go and Chess.

Note that only your own experience points affect your ratings change in a match; your opponent's experience points have no influence over your ratings change for a match. Thus, you are neither penalized nor rewarded for playing opponents with fewer or more experience points than you.

For more information

Open the Match Calculator window to experiment with different values for match length, ratings, and experience points to get a feel for how this formula works.

Kevin Bastian wrote an excellent article titled The FIBS Rating System, which provides a more detailed description of the rating formula and how it works.