What is the Kelly Criterion?

The Kelly Criterion is a mathematical formula that determines the optimal bet size to maximize long-term bankroll growth. It balances risk and reward by betting more when you have a larger edge and less when your edge is smaller. The formula: Kelly % = (bp - q) / b, where b = decimal odds - 1, p = win probability, q = loss probability.

What is fractional Kelly and why use it?

Fractional Kelly means betting a fraction of what full Kelly recommends (e.g., half-Kelly = 50%). While full Kelly maximizes growth rate mathematically, it creates large bankroll swings. Most bettors use 25-50% Kelly to reduce variance while still capturing most of the edge. It's a smoother ride with slightly slower growth.

What happens if Kelly suggests 0% or negative?

A zero or negative Kelly fraction means you have no edge or negative expected value. Don't place the bet. Kelly only recommends betting when you have a positive edge. If you're consistently getting 0% Kelly, your probability estimates may be off or the odds simply aren't in your favor.

How accurate does my win probability need to be?

Very accurate. Kelly is highly sensitive to probability estimates. Overestimating your edge by even a few percentage points can lead to overbetting and ruin. This is another reason to use fractional Kelly - it provides a buffer against estimation errors. When uncertain, be conservative.

Should I use Kelly for every bet?

Kelly works best for repeated bets with known edges, like +EV sports betting. For one-off bets or when you're unsure of true probabilities, Kelly can be too aggressive. Many sharp bettors use Kelly as a ceiling and bet less based on confidence in their edge estimate.

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Kelly Criterion Calculator

Find the best amount to bet based on your advantage and how much money you have. Bet less to be safer.

Inputs

Enter fraction (0-1) or percentage (0-100) to scale the full Kelly stake.

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How to Use This Calculator

Enter the odds being offered
Enter your estimated edge/probability
Enter your bankroll size
Calculator shows optimal bet size using Kelly Criterion

Understanding Kelly Criterion

The Kelly Criterion is a formula that determines optimal bet sizing to maximize long-term bankroll growth while minimizing risk of ruin. It tells you exactly what percentage of your bankroll to wager based on your edge.

Most sharp bettors use "fractional Kelly" (25-50% of full Kelly) to reduce variance. Full Kelly is mathematically optimal but extremely volatile—a few bad beats can wipe out significant bankroll. Quarter Kelly grows slower but sleeps better.

Kelly only works if your edge estimate is accurate. Overestimating your edge leads to overbetting and potential ruin. When uncertain, bet smaller. It's better to leave money on the table than go broke.

Kelly % = (Edge / Odds) × 100 or (bp - q) / b where b=odds, p=win prob, q=lose prob

Use Kelly as a sizing ceiling, not a guarantee

Kelly sizing depends on your probability estimate. If the true win probability is lower than your input, the recommended stake can be too aggressive even when the formula is working correctly.

Many bettors use fractional Kelly, such as half Kelly or quarter Kelly, to create a buffer against bad estimates and normal variance. Treat the output as a ceiling, then apply bankroll rules and confidence limits before betting.

A 0% Kelly result means the price does not clear your edge requirement. In that case, passing is part of the strategy, not a calculator error.

Use Kelly sizing with value-finding tools

Pair bankroll sizing with Upside tools that help compare fair prices, optimizer workflows, and positive-EV betting opportunities.

Sports Betting OptimizerCompare Upside's optimizer workflow for finding +EV bets across sportsbooks.Game Lines OptimizerFind +EV spreads, totals, and moneylines by comparing prices to fair odds.+EV Props OptimizerFind positive expected value player props across DFS apps and sportsbooks.

Frequently Asked Questions