Rating period

A rating is a number intended to estimate a player's competitive strength. A rating period is the administrative window during which one particular list remains valid. In the International Chess Federation (FIDE) system, a new official rating list is prepared on the first day of each month by adding accepted rated play from the period to the previous list.

It is not the same as a tournament's length. An event may fit inside one period, cross several months, or submit interim results when it lasts more than thirty days. Under the current rules, with an exception for certain official FIDE events, the closing date is three days before the list date. A completed event can therefore appear one month later if its report misses the cutoff.

The system adds the changes produced by each result during the period. For every game, the actual score, 1 for a win, 0.5 for a draw and 0 for a loss, is compared with the expected score derived from the rating difference. The accumulated amount becomes the change published at the start of the next period.