Calculate expected win probability and rating adjustments
Win (1.0)
Draw (0.5)
Loss (0.0)
40 (New players/Juniors)
20 (Club players)
10 (Grandmasters/Masters)
Expected Win Probability
0%
Rating Change
0
New Rating
0
How the Chess Elo Calculator Works
The Elo rating system is the standard method used to calculate the relative skill levels of players in zero-sum games, primarily chess. This calculator uses the mathematical formula established by Arpad Elo to determine how much your rating should change after a specific match result.
The Elo Formula Explained
Two primary factors determine your new rating: the Expected Score and the K-Factor.
Expected Score (E): This is your mathematical probability of winning. If you have a higher rating than your opponent, your expected score is higher (e.g., 0.75 or 75%).
K-Factor: This represents the "weight" of the game. FIDE usually uses K=40 for players under 18 or with fewer than 30 games, K=20 for regular club players, and K=10 for professional players reaching a 2400+ rating.
Example Calculation
If Player A (Rating 1600) plays Player B (Rating 1400):
The Expected Win Probability for Player A is approximately 76%.
If Player A wins, they gain few points (since they were expected to win).
If Player B wins, they gain significantly more points (the "upset" factor).
Frequently Asked Questions
What is a good Elo rating? A rating of 1200 is considered an average club player, while 2000+ represents an Expert/Candidate Master level. Grandmasters typically have ratings above 2500.
Why did I only gain 2 points? This happens when you beat an opponent much lower rated than you. The system assumes you should win those games almost 100% of the time.
function calculateChessRating() {
var playerRating = parseFloat(document.getElementById('playerRating').value);
var opponentRating = parseFloat(document.getElementById('opponentRating').value);
var actualScore = parseFloat(document.getElementById('gameOutcome').value);
var k = parseFloat(document.getElementById('kFactor').value);
if (isNaN(playerRating) || isNaN(opponentRating)) {
alert('Please enter valid numerical ratings.');
return;
}
// Calculate Expected Score (E)
// Formula: E = 1 / (1 + 10^((RatingB – RatingA) / 400))
var exponent = (opponentRating – playerRating) / 400;
var expectedScore = 1 / (1 + Math.pow(10, exponent));
// Calculate Rating Change
// Formula: R' = K * (ActualScore – ExpectedScore)
var ratingChange = k * (actualScore – expectedScore);
var newRating = playerRating + ratingChange;
// Update UI
document.getElementById('resultsArea').style.display = 'block';
document.getElementById('winProb').innerText = (expectedScore * 100).toFixed(1) + "%";
var diffElement = document.getElementById('ratingDiff');
var displayChange = ratingChange.toFixed(1);
if (ratingChange > 0) {
diffElement.innerText = "+" + displayChange;
diffElement.style.color = "#27ae60";
} else if (ratingChange < 0) {
diffElement.innerText = displayChange;
diffElement.style.color = "#e74c3c";
} else {
diffElement.innerText = "0.0";
diffElement.style.color = "#333";
}
document.getElementById('newRating').innerText = Math.round(newRating);
}