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Chess Elo calculator
Standard Elo formula: expected score from the rating gap, new rating from the K-factor and actual result.
—Player A win probability
—Player A new rating
—Player B new rating
How it works
Win probability comes from the standard Elo expected-score formula, 1 / (1 + 10^((opponent rating − your rating) / 400)) — every 400-point rating gap corresponds to roughly a 10x difference in expected score. Your new rating is then the old rating plus the K-factor times the difference between your actual result (1 for a win, 0.5 for a draw, 0 for a loss) and that expected score.
FAQ
- What K-factor should I actually use?
- It depends on experience and rating: FIDE uses 40 for players new to rated chess (under 30 games), 20 for most players, and 10 once a player reaches 2400+ — a higher K-factor means bigger rating swings per game.
- Why did my opponent's rating change by a different amount than mine?
- It shouldn't, if you're both using the same K-factor — the two rating changes are always equal and opposite, since one player's gain in expected score is exactly the other's loss.
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