The Kelly criterion is a staking formula: it sizes a bet according to how large your edge is over the price. The calculator below takes the odds, your own estimate of the chance of winning, a Kelly fraction and your bankroll, and returns your edge, the full Kelly percentage, the stake percentage at the fraction you chose and a suggested stake. It is a sizing tool, not a prediction. Every number it produces rests on the probability you type in.
Kelly criterion calculator
This calculator works on the standard version of the page.
Open the calculatorHow to use the Kelly criterion calculator
There are four inputs and four outputs.
- Odds: the price you can actually get, in fractional (11/10), decimal (2.10) or American (+110) format.
- Your win probability (%): your own estimate of the chance the bet wins, as a percentage. This is not the implied probability of the price. If you type the implied probability, the edge comes out at zero.
- Kelly fraction: Full, Half or Quarter. Half is selected by default.
- Bankroll: the money you have set aside for betting, as a plain number with no currency sign.
The outputs are your edge, the full Kelly share of the bankroll, the stake as a percentage of the bankroll at your chosen fraction, and the suggested stake in the same unit as your bankroll.
The default example is odds of 2.10, a win probability of 50%, half Kelly and a bankroll of 1,000. The calculator shows an edge of 5.0%, full Kelly of 4.55%, a half Kelly stake of 2.27% and a suggested stake of 22.73. If the edge is zero or negative, the stake shows as 0, which means no bet.
The Kelly formula, step by step
The calculator works on decimal odds, so it converts the price first. Then it applies two short formulas.
- Edge = p x decimal odds - 1, where p is your win probability as a decimal. At 2.10 and 50%, the edge is 0.50 x 2.10 - 1 = 0.05, or 5.0%. This is the expected profit per unit staked if your estimate is right.
- Full Kelly = edge / (decimal odds - 1). The bottom part is the net odds, the profit per unit if the bet wins. At 2.10 that is 0.05 / 1.10 = 0.04545, shown as 4.55% of the bankroll.
- Stake % = full Kelly x the fraction. Half of 4.545% is 2.273%, shown as 2.27%.
- Suggested stake = bankroll x stake %. 1,000 x 2.273% = 22.73.
You may also see the formula written as (bp - q) / b, where b is the net odds and q is the chance of losing, 1 - p. It is the same thing: 1.10 x 0.50 - 0.50 = 0.05, divided by 1.10, gives 4.55% again.
Two features of the formula matter in practice. The stake grows with the edge, so a bigger gap between your probability and the price means a bigger bet. And for the same edge, longer odds produce a smaller stake, because the net odds sit at the bottom of the fraction. An outsider with a 10% edge gets a smaller share of the bankroll than a favourite with the same edge, since the outsider loses more often.
Kelly is also a percentage of the bankroll you have now, not the one you started with. After a losing run the bankroll is smaller and so is the next stake, which is how the method protects you from going broke on paper. That only works if you recalculate after each settled bet.
Worked examples: a football draw, an NBA underdog and a bet to skip
Start with a Premier League match where the draw is priced at 3.40. Your model rates the draw at 32%, while the price implies 1 / 3.40 = 29.4%. The edge is 0.32 x 3.40 - 1 = 0.088, or 8.8%. Full Kelly is 0.088 / 2.40 = 3.67% of the bankroll, and half Kelly is 1.83%. With a £500 bankroll, the suggested half Kelly stake is £9.17.
Next, an NBA underdog at +150, which is 2.50 in decimal and an implied 40%. You make it 44%. The edge is 0.44 x 2.50 - 1 = 0.10, or 10.0%. Full Kelly is 0.10 / 1.50 = 6.67%. On a $2,000 bankroll, quarter Kelly is 1.67%, a stake of $33.33. Full Kelly would be $133.33, four times as much on a team you expect to lose more often than it wins.
A standard point spread at -110 is 1.909 in decimal. If you rate your side at 55%, the edge is 0.55 x 1.909 - 1 = 5.0% and full Kelly is 0.05 / 0.909 = 5.50%. At half Kelly with a $1,000 bankroll, the suggested stake is $27.50.
Finally, a bet to skip. A favourite at 1.80 implies 55.6%. If you rate it at 52%, the edge is 0.52 x 1.80 - 1 = -0.064, or -6.4%. The calculator shows 0 for every stake. Kelly never tells you to bet on a price that is lower than your own fair price, and backing a favourite because it is likely to win is not the same as having an edge.
Why most bettors use half or quarter Kelly
Full Kelly assumes you know the true probability. Nobody does. Kelly is only as good as the probability estimate, and the formula is very sensitive to it.
Go back to the default example at 2.10. At 50% the half Kelly stake on 1,000 is 22.73. Raise your estimate to 55% and the edge jumps to 15.5%, full Kelly to 14.09% and the half Kelly stake to 70.45. Five points of extra confidence roughly triple the bet. If the true chance was 50% all along, you have staked three times what the edge justified. If it was 46%, the real edge was -3.4% and there was no bet at all.
This is the core problem: over-estimating your probability leads to over-betting. Bets that are too large make the swings in your bankroll much deeper, and well above full Kelly the bankroll is expected to shrink over time even when every bet has a genuine edge. Under-betting costs you some growth, but it does not carry that risk.
Fractional Kelly is the usual answer. Staking half of the full Kelly amount halves every bet and, in the standard theory, still keeps about three quarters of the long-run growth of full Kelly. Quarter Kelly is smaller again and leaves more room for errors in your estimates. Most bettors who use Kelly at all use half or quarter, and many add a cap so that no single bet exceeds a fixed share of the bankroll, such as 2% or 3%, whatever the formula says.
Where your win probability should come from
The probability field is the one input the calculator cannot check. A number typed on a hunch produces a stake that looks precise and is not. Better sources include a model you have tested on past results, a careful assessment you write down before looking at the price, or the fair price from a sharp market. The no-vig calculator removes the margin from a market so you can see its fair probability, and the implied probability guide explains how prices translate into chances.
Record every bet with your estimate, the price you took and the result. After a few hundred bets you can see whether outcomes you rated at 55% actually win around 55% of the time. Comparing your price with the closing price, covered in the guide to line movement and closing line value, gives an earlier signal than profit and loss. Until your records support your estimates, quarter Kelly or a flat stake is the sensible choice. The value betting guide covers how to build those estimates in the first place.
What the Kelly calculator does not cover
The formula treats one bet at a time with two possible results, win or lose. Real betting often breaks those assumptions.
- Several bets at once. Full Kelly on each of five bets placed on the same Saturday can add up to a large share of the bankroll at risk together. Scale stakes down when many bets are open.
- Related bets. Two bets on the same match, or a parlay and its single legs, can win or lose together. The calculator treats each bet as independent.
- Pushes. On a whole-number spread or total, a push refunds the stake. Enter your chance of winning among the bets that settle, wins divided by wins plus losses, and the result stays correct.
- Exchange commission. Commission reduces the effective odds, so enter the price after commission to avoid overstating the edge.
- Stake limits. A bookmaker may accept less than the suggested stake, and some limit accounts that win often.
Kelly also says nothing about whether a bet is legal where you live, or whether the bankroll is money you can spare.
Related guides and a checklist before you bet
Our guide to bankroll management compares flat staking, percentage staking and fractional Kelly, and explains units and drawdowns. To convert a price or work out a return, use the odds calculator. How betting odds work covers the three odds formats.
Before you place a Kelly-sized bet, check that the probability came from a written estimate rather than a feeling, that the odds are the price you can actually take, that the fraction is half or quarter unless your records justify more, and that the stake is within any cap you set. Betting laws differ by country and, in the US, by state, and India has prohibited online money games since 1 May 2026, so check the rules where you live and use a licensed operator. If the stakes stop feeling comfortable, our responsible gambling page lists tools and places to get help.
Frequently asked questions
What is the Kelly criterion in betting?
It is a formula for stake size. Full Kelly stakes the edge divided by the net odds as a share of your bankroll. At 2.10 with a 50% win chance, the edge is 5% and full Kelly is 4.55% of the bankroll.
Why does the calculator show a stake of 0?
Your edge is zero or negative, so the formula recommends no bet. At 1.80 you need a win probability above 55.6% before Kelly suggests any stake at all.
Should I use full, half or quarter Kelly?
Full Kelly assumes your probability is exactly right, which it never is. Half Kelly halves every stake and gives up only part of the growth. Quarter Kelly is more cautious again. Most bettors who use Kelly choose half or quarter, and quarter suits anyone whose estimates are not yet backed by a long record.
Can I use the implied probability as my win probability?
You can, but the edge will be zero and the stake 0, because the price already reflects that probability. The calculator is only useful when you have an independent estimate that differs from the price.
Does the Kelly criterion guarantee profit?
No. It sizes bets to grow a bankroll if your edge is real. If your probabilities are too high, Kelly will over-bet and lose money faster than a smaller flat stake would. Losing runs happen even with a genuine edge.