The Kelly criterion is a bankroll sizing formula that sets bet size as a fraction of bankroll proportional to edge. Full Kelly maximizes log growth when the fair probability is exact, but fractional Kelly is standard practice because sportsbook prices, model error, and limits add variance.
Calculator · Free
Kelly criterion, fractional by default.
Size a positive-edge bet from fair odds, offered odds, bankroll, and Kelly fraction. The output shows full Kelly, fractional Kelly, stake, and edge.
Inputs
Fair input is your no-vig price for the outcome. Book input is the offered price. The calculator accepts American odds, decimal odds, fractional odds, and percentages.
Recommended stake
$70.45
+15.500% edge at 50.0% Kelly
| Full Kelly | 14.091% |
|---|---|
| Fractional Kelly | 7.045% |
| Recommended stake | $70.45 |
| Edge | +15.500% |
Show your work
Step 1 - parse odds
fairDec = 1.8182
bookDec = 2.1000
Step 2 - edge
b = (1 / fairDec) * bookDec - 1
= (1 / 1.8182) * 2.1000 - 1
= 15.500%
Step 3 - full Kelly
fullKelly = b / (bookDec - 1)
= 0.155000 / (2.1000 - 1) = 14.091%
Step 4 - fractional Kelly stake
fractional = fullKelly * (50.000 / 100)
= 7.045%
stake = $1,000.00 * 7.045%
= $70.45Kelly Criterion FAQ
What is the Kelly criterion?
The Kelly criterion converts edge and payout into a bankroll fraction. A larger edge at a better payout supports a larger stake, while a zero or negative edge maps to no stake.
Why use fractional Kelly instead of full Kelly?
Fractional Kelly reduces stake size while preserving the same directional logic. Bettors use half or quarter Kelly to reduce drawdowns when their fair price has estimation error.
What inputs does this calculator need?
Enter your fair odds or probability, the book's offered odds, bankroll, and Kelly fraction. Odds can be American, decimal, fractional, or a probability such as 55%.
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