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Kelly Criterion Calculator

Calculate the mathematically optimal bet size using the Kelly Criterion formula with adjustable fractions and real-time variance indicator.

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Kelly Criterion Calculator
Odds Format
Total Bankroll
$
Bookmaker Odds
Estimated Win Probability (%)
%
Kelly Fraction
25%
ConservativeVariance: MediumAggressive
Kelly Analysis
$20.83recommended stake (2.1% of bankroll)
🟡 Bet — Positive Edge
Edge+12.5%
Full Kelly %8.33%
Adjusted Kelly %2.08% (25% fraction)
Implied Bookie Prob40.0%
Your Probability45.0%
Expected Growth/Bet+0.2268%
Kelly Fractions Explained
Full Kelly maximises long-term growth but is extremely volatile. Quarter Kelly is recommended for most bettors — it captures 75% of the growth rate with dramatically less variance and drawdown risk.

The Kelly Criterion Formula

Kelly % = (bp − q) / b, where b = decimal odds − 1, p = your estimated win probability, and q = 1 − p. The result is the fraction of your bankroll that maximises long-term growth.

Kelly Fractions Explained

Full Kelly (100%) - Mathematically optimal but produces extreme variance. Bankroll swings of 50%+ are common.

Half Kelly (50%) - Suitable for serious professionals with strong edge estimates. Captures 75% of the growth rate with far less volatility.

Quarter Kelly (25%) - Recommended for most bettors. The best balance of growth and risk management.

Tenth Kelly (10%) - Conservative approach for beginners or when edge estimates are uncertain.

When to Use Kelly

Kelly only works when you have a genuine edge - your estimated probability must be more accurate than the bookmaker's implied probability. If your edge estimate is wrong, Kelly will tell you to over-stake, making losses worse. Start with quarter Kelly until you have confidence in your probability assessments.