A rating works as a comparison. It does not measure talent, opening knowledge, or how well you played a single pretty move. It measures how your score compared with the ratings of the people you met.
One event, one update
You enter with a pre-event rating R and a game count N. You play m games and score S. The system sums the expected score E against each opponent's pre-event rating. The raw change is K times (S − E). If the event qualifies, a bonus is added on top of that raw change. The result is then held at your rating floor if it would otherwise fall through it.
That is why a round-by-round chart on the wall is not the official update. The estimator on this site can recompute the same formula after each round so you can see the direction. The last row is the event estimate. US Chess still posts one number for the event.
New players take a different path
With 8 or fewer prior games, the special formula solves for a rating whose expected score matches what you scored, and it will not decrease your rating for a win or increase it for a loss. Age sets the starting point when there is no other rating to convert. A FIDE rating, if you have one, is converted onto the US Chess scale and treated as a handful of games of evidence.
What does not move the number
- Games that are not US Chess rated, including most casual club games and online blitz.
- A requested half-point bye, which is not a rated game.
- A game in which a player makes no move.
- Beating the same friend four times in a quad does not unlock the bonus. Repeats can turn the bonus off.