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Kelly Criterion Calculator
Calculate your optimal bet size given your edge and the odds. Includes full, half, and quarter Kelly options with a built-in 5% max bet cap.
Your Inputs
Your honest estimate of how often this bet wins.
Decimal: 1.909 · Breakeven win rate: 52.4%
Industry standard for serious bettors. ~75% of growth, 25% of variance.
Recommended Bet Size
What this means
Based on your inputs, Half Kelly sizing recommends $85 on this bet. This balances long-term growth against drawdown risk.
Half Kelly Stake by Win Probability
Stake size scales with your edge. Below the breakeven win rate, Kelly says don't bet at all. The 5% cap kicks in at very high edges.
Size with confidence
Don't guess your edge.
This calculator works only as well as your edge estimate. TrueLine measures your real edge from your actual bet history — closing line value, win rate by sport and market, EV vs realized performance — so you can size with Kelly using validated numbers instead of guesses.
How the math works
The Kelly Criterion formula for sports betting is:
f* = (bp - q) / b
Where f* is the fraction of your bankroll to bet, b is decimal odds minus 1 (your profit per $1 staked), p is your estimated probability of winning, and q is the probability of losing (1 - p).
This calculator multiplies the full Kelly result by your chosen fraction (1, 0.5, or 0.25), then caps the maximum at 5% of bankroll regardless of edge. The 5% cap protects against overconfidence in edge estimates — the most common reason Kelly users blow up.
If the formula returns a negative number (your win probability is below breakeven for those odds), the calculator returns 0. Kelly says don't bet when there's no edge.
All math runs in your browser. We don't log or store your inputs.
Frequently asked questions
What is the Kelly Criterion?
The Kelly Criterion is a formula that calculates the optimal bet size as a percentage of your bankroll, based on your edge and the odds. It maximizes long-term geometric growth of your bankroll given a known edge.
Should I use full Kelly?
Most professional bettors recommend fractional Kelly — typically half-Kelly or quarter-Kelly — rather than full Kelly. Full Kelly is mathematically optimal only if your edge estimate is exactly correct, which is essentially never true in sports betting.
What's the Kelly Criterion formula?
For sports betting, the formula is f* = (bp - q) / b, where f* is the fraction of bankroll to bet, b is decimal odds minus 1, p is your estimated probability of winning, and q is the probability of losing (1 - p).
Is this calculator free?
Yes, completely free. No signup, no rate limit. The math runs entirely in your browser — we don't store any of your inputs.