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Use the Kelly Criterion Tool by BetDiamonds to calculate the optimal stake size based on your edge and bankroll. Maximize long-term growth while managing risk with this smart betting strategy.

Kelly Criterion Calculator | Optimize Your Bet Size for Long-Term Profi

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  <title>Kelly Criterion Calculator | BetDiamonds</title>
  <style>
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  </style>
</head>
<body>
  <div class="container">
    <h1>📈 Kelly Criterion Calculator</h1>

    <div class="card">
      <form id="kellyForm">
        <div class="input-group">
          <label for="odds">Decimal Odds</label>
          <input type="number" id="odds" step="0.01" min="1" required placeholder="e.g. 2.50" />
          <div class="info-text">The bookmaker's odds (must be > 1.00)</div>
        </div>

        <div class="input-group">
          <label for="probability">Estimated Win Probability (%)</label>
          <input type="number" id="probability" step="0.1" min="0" max="100" required placeholder="e.g. 45.5" />
          <div class="info-text">Your assessment of the true probability (0-100%)</div>
        </div>

        <div class="input-group">
          <label for="bankroll">Bankroll Amount</label>
          <input type="number" id="bankroll" step="0.01" min="0" required placeholder="e.g. 1000" />
          <div class="info-text">Your total available betting bankroll</div>
        </div>

        <button type="submit">Calculate Optimal Stake</button>
      </form>
    </div>

    <div id="resultBox"></div>
  </div>

  <script>
    document.getElementById('kellyForm').addEventListener('submit', function(e) {
      e.preventDefault();

      const odds = parseFloat(document.getElementById('odds').value);
      const probability = parseFloat(document.getElementById('probability').value) / 100;
      const bankroll = parseFloat(document.getElementById('bankroll').value);
      const resultBox = document.getElementById('resultBox');

      // Validate inputs
      if (isNaN(odds) || odds <= 1) {
        showError("Please enter valid decimal odds (must be > 1.00)");
        return;
      }

      if (isNaN(probability) || probability <= 0 || probability >= 1) {
        showError("Please enter a valid probability between 0% and 100%");
        return;
      }

      if (isNaN(bankroll) || bankroll <= 0) {
        showError("Please enter a valid bankroll amount (> 0)");
        return;
      }

      // Calculate Kelly Criterion
      const kellyFraction = ((probability * (odds - 1)) - (1 - probability)) / (odds - 1);
      const kellyPercent = kellyFraction * 100;
      const stake = kellyFraction * bankroll;

      // Build results
      let result = `
        <div class="result-item">
          <div>Kelly Fraction:</div>
          <div class="result-value ${kellyFraction > 0 ? 'positive' : 'negative'}">
            ${kellyFraction.toFixed(4)} (${kellyPercent.toFixed(2)}%)
          </div>
        </div>
      `;

      if (kellyFraction <= 0) {
        result += `
          <div class="result-item">
            <div>Recommended Action:</div>
            <div class="result-value negative">Do not bet on this outcome</div>
          </div>
          <div class="summary danger-summary">
            The Kelly Criterion suggests this is a negative EV bet. You should avoid betting on this outcome.
          </div>
        `;
      } else {
        result += `
          <div class="result-item">
            <div>Optimal Stake:</div>
            <div class="result-value positive">${stake.toFixed(2)}</div>
          </div>
          <div class="summary good-summary">
            This stake (${kellyPercent.toFixed(2)}% of bankroll) maximizes long-term growth while managing risk.
          </div>
        `;

        if (kellyPercent > 20) {
          result = result.replace('good-summary', 'bad-summary');
          result += `
            <div class="summary warning-summary">
              ⚠️ Caution: This suggests betting a large portion of your bankroll. 
              Many bettors use a fractional Kelly approach (e.g., 1/2 or 1/4 Kelly) 
              to reduce volatility.
            </div>
          `;
        }
      }

      showResult(result);
    });

    function showError(message) {
      const resultBox = document.getElementById('resultBox');
      resultBox.innerHTML = `<div class="error">${message}</div>`;
      resultBox.style.display = 'block';
      resultBox.scrollIntoView({ behavior: 'smooth', block: 'nearest' });
    }

    function showResult(content) {
      const resultBox = document.getElementById('resultBox');
      resultBox.innerHTML = content;
      resultBox.style.display = 'block';
      resultBox.scrollIntoView({ behavior: 'smooth', block: 'nearest' });
    }
  </script>
</body>
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