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The Kelly Criterion calculates the mathematically optimal bet size to maximize long-term growth. Enter your win probability and the odds offered.
Kelly Percentage
—
Optimal Bet
—
Half Kelly (Safer)
—
Quarter Kelly (Safest)
—
Edge
—
⚠️ No edge detected. Kelly says don't bet. Your win probability doesn't justify the odds offered.
For informational purposes only. Not legal gambling advice. Must be 21+ in your jurisdiction.
Gamble responsibly. If you or someone you know has a gambling problem, call 1-800-522-4700.
// Calculator
function americanToDecimal(odds) {
const o = parseFloat(odds);
if (isNaN(o) || o === 0) return null;
return o > 0 ? (o / 100) + 1 : (100 / Math.abs(o)) + 1;
}
function calculate() {
const winP = parseFloat(document.getElementById('winProb').value) / 100;
const dec = americanToDecimal(document.getElementById('odds').value);
const bankroll = parseFloat(document.getElementById('bankroll').value);
if (isNaN(winP) || !dec || isNaN(bankroll)) return;
const b = dec - 1; // decimal odds - 1 = payout ratio
const q = 1 - winP; // loss probability
// Kelly formula: f* = (bp - q) / b
const kelly = (b * winP - q) / b;
const edge = ((winP * dec) - 1) * 100;
document.getElementById('edge').textContent = edge.toFixed(2) + '%';
document.getElementById('edge').className = 'result-value ' + (edge > 0 ? 'green' : 'red');
if (kelly <= 0) {
document.getElementById('kellyPct').textContent = '0%';
document.getElementById('optimalBet').textContent = '$0';
document.getElementById('halfKelly').textContent = '$0';
document.getElementById('quarterKelly').textContent = '$0';
document.getElementById('noEdge').style.display = 'block';
} else {
const kellyPct = (kelly * 100).toFixed(2);
const optimal = bankroll * kelly;
const half = bankroll * kelly * 0.5;
const quarter = bankroll * kelly * 0.25;
document.getElementById('kellyPct').textContent = kellyPct + '%';
document.getElementById('optimalBet').textContent = '$' + optimal.toFixed(2);
document.getElementById('halfKelly').textContent = '$' + half.toFixed(2);
document.getElementById('quarterKelly').textContent = '$' + quarter.toFixed(2);
document.getElementById('noEdge').style.display = 'none';
}
document.getElementById('results').classList.remove('hidden');
// Increment smart bets counter
if (window.incrementSmartBets) window.incrementSmartBets();
}
document.addEventListener('input', function(e) {
if (['winProb', 'odds', 'bankroll'].includes(e.target.id)) {
const r = document.getElementById('results');
if (!r.classList.contains('hidden')) calculate();
}
});
calculate();
