Development coefficient

The development coefficient, usually called the K-factor, controls the sensitivity of a FIDE rating. FIDE is the International Chess Federation, and a rating is a numerical estimate of competitive strength. Before a game, the system calculates an expected score from the rating difference. It then compares that expectation with the actual score: 1 for a win, 0.5 for a draw, and 0 for a loss. The accumulated difference is multiplied by K.

A larger K produces faster changes from the same performance. A smaller K makes the rating more stable. K is not a measure of playing strength and is not a fixed number of points awarded for a win. Beating a much stronger opponent and beating a much weaker one involve different expectations.

Main values in FIDE standard rating

• K = 40 for a player new to the rating list until completed events contain at least 30 games.
• K = 20 while the rating remains below 2400, unless a K = 40 rule applies.
• K = 10 once a published rating reaches 2400; it remains 10 even if the rating later falls.
• K = 40 through the end of the calendar year of the player's 18th birthday, provided the rating remains below 2300.

A rating-period cap also applies. A rating period is the set of games processed together for one published list. If the number of games multiplied by K exceeds 700, the largest whole-number K keeping the product at 700 or less is used. This limits the potential swing when many games are processed together.