by Mark Orr
created June 15, 2012; last updated Nov 30

To calculate the change in a full rating after a tournament, the difference between the player's expected and actual scores is multiplied by a number called the K-factor. This number represents the maximum possible rating change per game. Most rating systems, the ICU included, vary this number depending on a player's strength and experience.

In the case of the ICU system, the K-factor is determined as follows:

  • if the player's start rating is ≥ 2100, then 16
  • otherwise, if the player's age is < 21 years, then 40
  • otherwise, if the player's experience is < 8 years, then 32
  • otherwise, 24

Age and experience are calculated at the start of the tournament. Experience is calculated using the date the player joined the ICU or played their first rated game, whichever came first. If date of birth is unknown it defaults to 1950-01-01. If join date is unknown (which applies to some players inherited from old versions of the ICU rating system) it defaults to 1975-01-01.