Enter your bankroll, decimal odds and your own estimated probability of winning to calculate a Kelly Criterion stake estimate.
Enter your own probability estimate between 0% and 100%. The calculator cannot determine the true probability of an outcome.
This is a mathematical Kelly Criterion result based entirely on the probability estimate you entered. It is not a recommendation or prediction.
Important: The Kelly Criterion is a mathematical staking formula, not a prediction system. Its result depends heavily on the accuracy of the probability estimate entered by the user. The calculator cannot determine the true probability of an outcome, guarantee a profit or prevent losses. 18+ only. Gamble responsibly.
How to Use the Kelly Criterion Calculator
Enter your bankroll, the decimal odds offered for the bet and your own estimated probability of winning.
Then choose Full Kelly, Half Kelly or Quarter Kelly and select Calculate Kelly Stake.
The calculator will show the bookmaker’s implied probability, your estimated probability difference, the Full Kelly percentage and the stake produced by your selected Kelly strategy.
What Is the Kelly Criterion?
The Kelly Criterion is a mathematical formula used to calculate a stake size based on a bankroll, betting odds and an estimated probability of winning.
Unlike a normal payout calculator, the Kelly Criterion requires you to provide your own probability estimate.
This is important because the calculator cannot determine the true probability that a team, player or other outcome will win.
If your probability estimate is inaccurate, the calculated stake can also be inappropriate.
How Is the Kelly Criterion Calculated?
The basic Kelly formula is:
Kelly % = (bp − q) ÷ b
Where:
b = decimal odds minus 1
p = your estimated probability of winning
q = probability of losing (1 − p)
The result represents the fraction of the bankroll produced by the Full Kelly calculation.
Kelly Criterion Example
Suppose you enter:
Bankroll: R1,000
Decimal odds: 2.50
Estimated win probability: 50%
Decimal odds of 2.50 have an implied probability of:
1 ÷ 2.50 × 100 = 40%
Using a 50% personal probability estimate, the Full Kelly calculation produces approximately:
16.67% of the bankroll
For a R1,000 bankroll, that equals approximately:
Full Kelly: R166.67
If Half Kelly is selected, the calculated stake becomes approximately:
R83.33
Quarter Kelly would produce approximately:
R41.67
These figures are mathematical outputs based on the probability estimate entered. They are not predictions or guarantees of profit.
What Is Full Kelly?
Full Kelly uses the complete percentage produced by the Kelly Criterion formula.
For example, if the formula produces a Kelly percentage of 10%, Full Kelly would allocate 10% of the entered bankroll.
Because the result depends heavily on the accuracy of the probability estimate, the calculator should not be interpreted as determining how much someone must bet.
What Is Half Kelly?
Half Kelly uses 50% of the Full Kelly calculation.
For example, if Full Kelly produces 10% of the bankroll, Half Kelly produces 5%.
The calculator allows you to switch between Full, Half and Quarter Kelly so you can see how the mathematical stake changes.
What Is Quarter Kelly?
Quarter Kelly uses 25% of the Full Kelly calculation.
If Full Kelly produces 12% of the bankroll, Quarter Kelly produces 3%.
Again, this is simply a mathematical adjustment to the Full Kelly result.
Why Does the Calculator Sometimes Show R0?
A Kelly calculation can produce a zero or negative result when the probability estimate entered does not provide a positive Kelly value at the odds entered.
Our calculator displays R0.00 rather than suggesting a negative stake.
For example, if the bookmaker’s decimal odds are 2.50 and you estimate the probability of winning at only 35%, the calculation does not produce a positive Kelly stake.
Implied Probability vs Your Estimated Probability
Implied probability is calculated from the bookmaker’s decimal odds.
For example:
Decimal odds 2.00 = 50% implied probability
Decimal odds 2.50 = 40% implied probability
Decimal odds 4.00 = 25% implied probability
Your estimated probability is different. It is the probability figure you enter yourself.
The calculator does not verify whether your estimate is accurate.
If you only want to convert betting odds and see their implied probability, use our Odds Converter South Africa.
Frequently Asked Questions
Does the Kelly Criterion guarantee profit?
No. The Kelly Criterion is a mathematical staking formula. It cannot guarantee that a bet will win or that a bettor will make a profit.
Does the calculator know the real probability of winning?
No. You must provide the estimated win probability yourself. The calculator performs the calculation using the number you enter.
Why is my Kelly percentage 0%?
This happens when the inputs do not produce a positive Kelly value. In that situation, the calculator displays a zero suggested stake.
What is the difference between Full Kelly and Half Kelly?
Full Kelly uses the entire percentage calculated by the Kelly formula. Half Kelly uses 50% of that percentage.
Can I use South African rand?
Yes. Enter your bankroll in rand and the calculated stake and remaining bankroll will also be displayed in rand.
Can I use fractional or American odds?
The calculator uses decimal odds. You can convert fractional or American odds using our Odds Converter South Africa before entering them.
Gamble Responsibly
Betting involves financial risk. The Kelly Criterion does not predict winners, guarantee profit or eliminate the possibility of losing money.
The calculation depends on the probability estimate entered by the user, which may be incorrect. Only bet with money you can afford to lose. 18+ only. Gamble responsibly.