Free betting tool
Kelly criterion calculator
Work out how much of your bankroll to stake on a bet with an edge. Enter American or decimal odds, your estimated win probability and your bankroll — the calculator does the rest.
Kelly criterion calculator
Enter the odds, your win probability and your bankroll.
Half or quarter Kelly is safer when your probability is an estimate — which it always is.
Recommended stake (half Kelly)
$41.67
4.17% of your bankroll · full Kelly is 8.33%.
Edge (EV)
+10.00%
Expected profit per unit.
Profit if it wins
$50.00
Break-even probability
45.5%
Your 50.0% beats the market's implied 45.5%.
How the Kelly criterion works
The Kelly criterion, published by John Kelly in 1956, tells you what fraction of your bankroll to bet to grow it as fast as possible over the long run. It balances two risks: betting too little wastes your edge, and betting too much risks ruin.
The stake grows with your edge and shrinks as the odds get longer, because long shots lose more often. If the odds don't beat your probability, the formula comes out at zero or below and the answer is simple: don't bet.
Worked example
You find a team at +120 (2.20 in decimal) and believe it wins 50% of the time. The odds imply only 45.5%, so you have an edge.
- b = 2.20 − 1 = 1.20, p = 0.50, q = 0.50
- f* = (1.20 × 0.50 − 0.50) / 1.20 = 0.10 / 1.20 = 8.33% of your bankroll
- With a $1,000 bankroll and half Kelly, you'd stake $41.67. The bet's expected value is +10% of the stake.
Full, half or quarter Kelly?
Full Kelly
8.33%
Maximum long-run growth, but deep drawdowns and very sensitive to estimation errors.
Half Kelly
4.17%
About three-quarters of full Kelly's growth with roughly half the volatility. A popular default.
Quarter Kelly
2.08%
Slower growth, smooth ride. Sensible when your probabilities are rough estimates.
Common Kelly mistakes
- Overestimating your edge. Kelly is only as good as your probability. If you're not sure, use a smaller fraction.
- Ignoring simultaneous bets. Ten Kelly-sized bets on one Saturday is far more exposure than the formula intends.
- Never resizing. Kelly is a share of your current bankroll, so stakes should shrink after losses and grow after wins.
- Using it without an edge estimate. If you bet a tipster's picks, a flat unit size is usually simpler and safer.
Frequently asked questions
What is the Kelly criterion?
A formula that sets your stake as a share of your bankroll based on your edge. It maximises long-run bankroll growth: bigger edges get bigger stakes, and no edge means no bet.
Is full Kelly too aggressive?
For most bettors, yes. Full Kelly assumes your win probability is exactly right, and small overestimates lead to large drawdowns. Half or quarter Kelly keeps most of the growth with far smaller swings.
What if the Kelly formula gives a negative number?
It means the odds are worse than your estimated chance of winning — there is no edge, so the right stake is zero.
How do I estimate my win probability?
A practical starting point is the no-vig probability from a sharp market, adjusted by your own analysis. The no-vig calculator on our +EV betting page does the first step for you.
Can I use Kelly for parlays or several bets at once?
For a parlay, use the combined odds and the combined probability. For several simultaneous bets, scale stakes down — Kelly for one bet assumes it's your only exposure, and correlated bets compound risk.
More betting tools
- Bankroll trackerTrack balance, units, ROI and drawdown across one or more bankrolls.
- Bet trackerLog every bet and see your record by sport, odds and outcome.
- CLV calculator & trackerMeasure whether you beat the closing line, bet by bet.
- +EV calculatorFind the expected value of a bet and the no-vig fair odds.
- Unit calculatorTurn your bankroll into a unit size and stakes for every bet.
Track every bet you size
Log your bets in the free SlipBets bankroll tracker and see your ROI, drawdown and streaks update automatically.
18+. Betting involves risk and past results don't guarantee future outcomes. Only stake what you can afford to lose. Need help? See our responsible gambling resources.