Finding a potential value bet is only half the problem. The other half is deciding how much to stake. Bet too little and the edge has limited impact. Bet too much and normal variance can damage the bankroll before the edge has time to show.
This Kelly Criterion Calculator estimates stake size from your bankroll, offered odds and estimated win probability. It shows Full Kelly, Half Kelly, Quarter Kelly, implied probability, edge and expected value per bet.
Important: Kelly staking is only as reliable as your probability estimate. If you overestimate your edge, the formula will recommend stakes that are too large. Many bettors use Half Kelly or Quarter Kelly to reduce drawdowns and protect against model error.
Kelly Criterion Calculator
Calculate Full Kelly, fractional Kelly, EV, edge and stake size.
How to Use the Kelly Calculator
- Enter bankroll: the amount reserved for betting, not your full savings.
- Enter odds: use decimal odds, or switch to American odds if needed.
- Enter your win probability: your own estimate of the true chance of the bet winning.
- Choose Kelly fraction: Full Kelly maximizes theoretical logarithmic growth, while Half Kelly and Quarter Kelly reduce volatility.
- Read the stake result: if the calculated edge is negative, the calculator recommends no bet.
For a broader map of bankroll and risk tools, use the Bankroll Risk Calculators Hub. To check whether variance can exhaust your bankroll, use the Risk of Ruin Calculator. To model downswings across many flat-stake bets, use the Sports Betting Variance Calculator.
Kelly Criterion Formula
The standard Kelly formula is:
f* = (bp – q) / b
- f* = fraction of bankroll to stake
- b = net decimal odds, or decimal odds minus 1
- p = your estimated probability of winning
- q = probability of losing, or 1 – p
If the result is positive, the formula suggests a stake. If the result is zero or negative, the bet does not have positive expected value under your inputs.
Full Kelly vs Half Kelly vs Quarter Kelly
| Kelly fraction | Meaning | Typical use |
|---|---|---|
| Full Kelly | Uses the full calculated stake. | Maximum theoretical growth if probabilities are accurate. |
| Half Kelly | Uses 50% of the Full Kelly stake. | Common practical choice to reduce drawdowns. |
| Quarter Kelly | Uses 25% of the Full Kelly stake. | Conservative option when probability estimates are uncertain. |
Full Kelly can be volatile. If your edge estimate is noisy, fractional Kelly is usually more realistic.
Example: A 55% Edge at Even Money
Suppose a bookmaker offers 2.00 decimal odds. The market-implied break-even probability is 50%. Your model estimates the true chance at 55%.
- Bankroll: $1,000
- Odds: 2.00
- Your probability: 55%
- Full Kelly: 10% of bankroll
- Full Kelly stake: $100
- Half Kelly stake: $50
The bet has positive EV because your probability estimate is higher than the break-even probability. But if the true chance is only 51% or 50%, the Full Kelly stake would be too aggressive.
Why Accurate Probability Matters
Kelly does not create an edge. It only sizes a bet based on the edge you enter.
| Input problem | Effect on Kelly stake |
|---|---|
| You overestimate win probability. | The calculator recommends too large a stake. |
| You ignore bookmaker margin. | Your “edge” may be smaller than it looks. |
| You use stale odds. | The edge may disappear before the bet is placed. |
| You use Full Kelly with uncertain estimates. | Drawdowns can become uncomfortable or destructive. |
For no-vig probability estimates, use the No-Vig Calculator. For converting odds into break-even probability, use the Odds to Implied Probability Calculator.
When Kelly Says “Do Not Bet”
If the recommended stake is zero, the math says the offered odds are not high enough for your probability estimate.
For example, if the odds are 2.00 but your estimated win probability is 49%, the bet is negative EV. Increasing stake size cannot fix that. The correct decision is to pass or find a better price.
Frequently Asked Questions
What is the Kelly Criterion?
The Kelly Criterion is a bet-sizing formula that estimates what fraction of bankroll to stake when you know the odds and your estimated probability of winning.
What is the Kelly formula?
The formula is f* = (bp – q) / b, where b is decimal odds minus 1, p is win probability, and q is loss probability.
Why does the calculator say do not bet?
It means the probability you entered is not high enough relative to the offered odds. Under those inputs, the bet has zero or negative expected value.
What is Half Kelly?
Half Kelly means staking 50% of the Full Kelly recommendation. It reduces bankroll volatility and protects against probability-estimation error.
Does Kelly guarantee profit?
No. Kelly depends on accurate probability estimates and positive EV. If your inputs are wrong, the recommended stake can be too large.
Should I use Full Kelly?
Full Kelly is mathematically aggressive. Many bettors use Half Kelly or Quarter Kelly because sports betting probabilities are uncertain and markets move.
Responsible gambling notice: Kelly staking is a risk-sizing formula, not a guarantee of profit. Use conservative stakes, track results honestly and never risk money you cannot afford to lose.