- Enter your Bankroll, Win Probability %, and Bet Odds.
- Choose Recommended (0.25 fractional) or Full Kelly.
- Read the Recommended Stake in dollars.
Kelly Criterion Calculator
A Kelly Criterion calculator for sports betting. Enter your edge and the decimal odds to find the optimal bankroll percentage to wager.
How to Use This Calculator
What is a Kelly Criterion calculator?
A Kelly Criterion calculator determines the optimal amount of a bankroll to bet on a given wager. The formula, developed by John L. Kelly Jr. in 1956, maximizes long-term growth by balancing risk and reward based on the bettor's calculated edge. The larger the edge, the more of the bankroll the formula recommends wagering.
Formula
\[
\begin{aligned}
\text{Kelly Fraction} &= P_{win} - \dfrac{\strut P_{loss}}{\strut (\text{Decimal Odds} - 1)}
\end{aligned}
\]
Worked example
A bettor estimates a coin lands heads 60% of the time. The sportsbook offers 2.00 decimal odds (+100 American) on heads.
\[
\begin{aligned}
\text{Kelly Fraction} &= 0.60 - \dfrac{\strut 0.40}{\strut (2.00 - 1)} \\[6pt]
&= 0.60 - 0.40 = 0.20
\end{aligned}
\]
The formula recommends staking 20% of the bankroll on each flip. On a $1,000 bankroll, that is $200 per bet. Most practitioners reduce this by half or three-quarters to soften variance.
Why Kelly sizing matters for sports bettors
Bet sizing is as important as bet selection. Wager too small and growth is left on the table. Wager too large and a normal losing streak can bust the bankroll. Kelly sizing balances these two failure modes mathematically.
In practice, most professional sports bettors use fractional Kelly: half, quarter, or even tenth of the full Kelly stake. The reasons are practical. Probability estimates are imperfect, variance is real, and the psychological cost of large drawdowns is steep.
For a deeper walkthrough of the formula and how it integrates with +EV betting, read our Kelly Criterion guide. Pair this calculator with the Expected Value Calculator to first confirm a bet is +EV before sizing it.
Common mistakes
- Using full Kelly with imperfect probability estimates. Kelly assumes you know the true probability. If your estimate is off by a few percentage points, full Kelly bets can be dangerously large.
- Treating a Kelly recommendation as a green light. Kelly tells you how much to stake on a +EV bet. It does not tell you whether the bet is +EV. Confirm the edge first.
- Stacking Kelly stakes across correlated bets. Two bets that are positively correlated (for example, two legs of a parlay scenario or two player props from the same game) carry more combined risk than Kelly assumes for independent bets. Reduce stakes accordingly.
- Ignoring bankroll updates. Kelly is a percentage of current bankroll, not initial bankroll. Update the figure as your bankroll grows or shrinks.
Frequently Asked Questions
What is a Kelly Criterion calculator?
A Kelly Criterion calculator determines the optimal percentage of a bankroll to wager on a bet based on the bettor's edge and the offered odds. The formula maximizes long-term bankroll growth while balancing risk. It is the standard bet sizing tool used by professional sports bettors.
What is the formula for Kelly's ratio?
The Kelly fraction is the probability of winning minus the probability of losing divided by the decimal odds minus one.
\[
\begin{aligned}
\text{Kelly Fraction} &= P_{win} - \dfrac{\strut P_{loss}}{\strut (\text{Decimal Odds} - 1)}
\end{aligned}
\]
For a 60% chance to win at 2.00 decimal odds, the formula returns 0.20, meaning the optimal bet size is 20% of the bankroll. A negative result means the bet is -EV and should not be placed.
What is the downside of the Kelly Criterion?
The formula assumes the bettor's edge estimate is accurate. In practice, edge is rarely known exactly. Overestimating edge causes Kelly to recommend dangerously large bets. Variance can also be uncomfortable: full-Kelly drawdowns of 30% to 50% are mathematically normal. Most professionals use fractional Kelly (half, quarter, or tenth) to soften both problems.
What Kelly Criterion should I use?
Fractional Kelly is the standard professional approach. Quarter-Kelly is the most common compromise: it captures most of the growth of full-Kelly while reducing drawdowns dramatically. Half-Kelly is more aggressive; tenth-Kelly is more conservative. The right choice depends on the bettor's confidence in their edge estimates and their tolerance for variance.
Can the Kelly Criterion produce a negative bet size?
Yes. A negative Kelly fraction means the bet has negative expected value and should not be placed. Treat any negative output as a signal to pass on the bet.
Is Kelly the same as flat betting?
No. Flat betting uses the same stake regardless of edge or odds. Kelly varies the bet size based on the size of the edge and the odds, which is mathematically more efficient over the long run.
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