Elo

The Elo system compares a player's actual result with the result expected from the rating difference between the two players. For this calculation, a win scores 1, a draw scores 0.5, and a loss scores 0. Beating a player rated above you exceeds expectation more than beating a much lower-rated player, so it normally produces a larger gain.

In the system used by the International Chess Federation (FIDE), expected score is subtracted from actual score and the difference is multiplied by the K factor. This factor is a coefficient that controls how quickly a rating can change. A loss reverses the effect, while a draw may gain or lose points depending on who was favored. Changes from several games are accumulated before the relevant rating list is published.

An Elo rating gives a relative position within a particular list and player pool. It is not a winning percentage, and by itself it does not measure knowledge, style, or the quality of one game. FIDE, national federations, and online platforms may use different formulas, update schedules, and player pools. Equal numbers from separate systems therefore do not guarantee equal practical strength.