Kelly Criterion Calculator

Eliminate the risk of ruin. Use the formula trusted by Wall Street and professional gamblers to determine the exact mathematical stake size for maximum bankroll growth.

Wes Frank
Wes FrankFounder, EdgeSlip Analytics
Updated Dec 25, 2025
Fact Checked

Kelly Criterion Calculator

Bankroll Management

1. Your Bet

2. True Probability

Don't guess. Use "Sharp Odds" for accuracy.

3. Risk Tolerance

0.5x Kelly

Optimal Wager +EV FOUND

Bet Amount
Click to Copy
$23.73
2.27% of Bankroll0.5x Kelly
Expected Value
+5.00%
True Win Prob.
50.00%

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What is this tool?

The Kelly Criterion is a mathematical formula used to determine the optimal size of a series of bets. In sports betting, it balances your 'Edge' (Expected Value) against the 'Odds' offered to calculate a stake percentage that maximizes the logarithm of wealth. Put simply: It tells you to bet more when you have a big advantage, less when you have a small advantage, and nothing when you have no advantage.

How to use it

Enter your current Bankroll, the Betting Odds offered by the sportsbook, and your estimated Win Probability. Ideally, use a 'Fair Probability' derived from a sharp bookmaker (No-Vig) rather than a gut feeling. The calculator will output the 'Kelly Percentage'—the exact slice of your bankroll you should wager. Use the 'Multiplier' slider to adjust your risk tolerance (e.g., Half Kelly).

Why use the Kelly Criterion?

If you bet too little, your bankroll grows slowly (Arithmetic Growth). If you bet too much, you eventually go bust due to variance (Geometric Decay).

The Kelly Criterion finds the mathematical peak between these two extremes. It is the only strategy proven to maximize wealth over the long run.

Flat BettingSafe but slow growth
Kelly BettingMaximal Geometric Growth
OverbettingInevitable Bankruptcy

The Formula

The formula was developed by J.L. Kelly Jr. at Bell Labs in 1956. While the derivation involves complex calculus and information theory, the practical version for sports betting is simple:

f* = (bp - q) / b
f*Fraction of bankroll to bet
bDecimal odds - 1 (Net Odds)
pProbability of winning

Full vs. Fractional Kelly

While "Full Kelly" is mathematically optimal for growth, it assumes your edge calculation is 100% perfect and that you have infinite utility for money. In reality, bettors overestimate their edge, and losing 50% of a bankroll hurts psychologically.

To counter this volatility, professional bettors use Fractional Kelly (usually Half or Quarter). This dramatically reduces variance while still retaining a huge portion of the growth potential.

Full Kelly

The raw formula output (1.0x).

  • • Max Growth Rate (100%)
  • • Max Volatility (100%)
  • • High Risk of Ruin if edge is wrong

Half Kelly

Betting 50% of the recommendation.

  • • 75% of Max Growth Rate
  • • 50% of Max Volatility
  • • The Professional Standard

Quarter Kelly

Betting 25% of the recommendation.

  • • 44% of Max Growth Rate
  • • 25% of Max Volatility
  • • Extremely Safe

Common Mistakes

  • Overestimating the Edge: If you think you have a 60% win rate but really have 53%, Full Kelly will advise you to bet too much, leading to ruin. This is why Fractional Kelly is vital.
  • Ignoring Simultaneous Bets: If you place 10 bets at once using the Kelly calculator for each individually, you are over-leveraging your bankroll. You must adjust the bankroll size for subsequent bets or use a "Simultaneous Kelly" formula.
  • Betting on Negative EV: If the calculator returns 0 or a negative number, DO NOT BET. It means the odds offered are worse than the true probability.

Frequently Asked Questions

What inputs do I need for the Kelly Criterion?▼
You need three things: 1) Your current bankroll size, 2) The odds offered by the sportsbook, and 3) The true probability of the bet winning. Finding the 'True Probability' is the hardest part and is usually done by 'devigging' sharp market lines.
Does Kelly betting guarantee profit?▼
No. It guarantees efficient bankroll growth *if and only if* your probability assessment is accurate. If you consistently overestimate your edge, the Kelly Criterion will accelerate your losses.
Why does the calculator sometimes say $0?▼
This happens when the bet has Negative Expected Value (-EV). The Kelly formula effectively says 'If the house has the edge, the optimal bet size is zero.' You should only bet when you have an advantage.
What is a good Kelly Multiplier to use?▼
For most serious bettors, 0.25 (Quarter Kelly) to 0.50 (Half Kelly) is recommended. It drastically reduces variance and stress while sacrificing a relatively small amount of long-term growth potential.
Can I use this for stock trading?▼
Yes, the Kelly Criterion is widely used in finance for portfolio allocation. However, sports betting is binary (win/loss), while stocks have continuous outcomes, so a modified continuous Kelly formula is often used for markets.
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