Work Out Your Rating After a Weekend Open

Rating lists are published monthly, but most players want to know on Sunday evening what the weekend did to their rating. The calculation is straightforward once you know the K-factor and the expected-score formula. This guide works through one junior's five-round weekend from the crosstable to the new rating, and explains the differences you may see when the official list appears.

The tournament

A 16-year-old player rated 1640 plays a five-round weekend open with a time control of 90 minutes plus 30 seconds per move, so every game is FIDE-rated as standard. The results: a win against a 1702, a draw against a 1588, a loss to an 1815, a win against a 1655 and a draw against a 1730. That is 3 points from 5, 60%.

Step 1: the K-factor

Under FIDE's rating regulations, juniors keep K = 40 until the end of the year of their 18th birthday, as long as their rating is below 2300. Our player qualifies, so every point above expectation is worth 40 rating points. The same results for an adult with K = 20 would gain half as much. Five games × 40 is 200, well below the 700 cap on K × games per rating period, so no reduction applies.

Step 2: expected scores, game by game

The Elo rating calculator works out each expectation from the rating difference. Against the 1702, 62 points higher, the expected score was 0.412, so the win was worth 40 × (1 − 0.412) = +23.5. Against the 1588, 52 points lower, the expectation was 0.574, so the draw cost 40 × (0.5 − 0.574) = −3.0. The loss to the 1815 cost only −10.7, because a 175-point gap meant an expectation of just 0.267. The win against the 1655 added +20.9 and the draw against the 1730, an expectation of 0.373, added +5.1. In total, 3 points against an expected 2.105, a surplus of 0.895, worth +35.8 rating points. The new rating is about 1676.

Step 3: the performance rating

The opponents averaged 1698. With 60%, FIDE's dp table gives +72, so the performance rating is 1698 + 72 = 1770. The linear method gives 1778 and the exact zero-change method 1772. A performance 130 points above the player's rating is excellent, but over five games it carries a large margin of error. It is a good sign, not proof that the player's true strength is 1770.

Why the official figure may differ

Three things can change the published number. First, FIDE rates all events in a rating period together, using the rating at the start of the period, so if the player plays another event in the same month, the two are combined rather than calculated one after the other. Second, FIDE's expected scores come from a table, not the logistic formula; the difference is at most about 0.01 per game, or 0.4 rating points per game at K = 40. Third, rounding: FIDE rounds rating changes at defined points in the calculation. Expect the official figure to be within one or two points of your own calculation.

Common mistakes

• Using the wrong K. Juniors under 2300 and players with fewer than 30 rated games use 40; it doubles every gain and loss.
• Updating the rating after each game. All games in a period use the starting rating.
• Ignoring the 400-point rule. Against a much weaker opponent, the difference is capped at 400 for players under 2650.
• Treating a five-game performance as a new level. Small samples vary by more than 100 points.

Checklist

1. Confirm the event is FIDE-rated and in which category: standard, rapid or blitz.
2. Work out your K-factor from your age, rated games and peak rating.
3. Enter every opponent's rating and your result.
4. Check that K × games does not exceed 700.
5. Add other events in the same rating period before predicting the list.
6. Use the performance rating as a guide to form, not a new rating.