Kelly Criterion Calculator: Optimal Bet Size
A Kelly Criterion calculator tells you the optimal share of your bankroll to put on a bet, based on the odds and your own estimate of how likely that bet is to win. Enter the odds, your win-probability estimate, and your total bankroll, and the tool below returns the exact stake the Kelly formula recommends, plus safer half-Kelly and quarter-Kelly amounts for lower swings. It does the math that keeps you from overbetting your strongest edges or underbetting them, so every stake is sized to the value you actually have.
Kelly Criterion Calculator
What the Kelly Criterion Calculator Does
The Kelly Criterion calculator turns three inputs — the odds, your estimated win probability, and your bankroll — into a single recommended stake, shown as both a percentage of your roll and a dollar figure. It exists to answer the one question flat bettors never really solve: not which bet to make, but how much to put on it. Bet too little on a genuine edge and you leave growth on the table; bet too much and one cold streak can wipe you out.
The tool also flags whether your inputs describe a positive-expected-value (+EV) bet — one where your estimated win chance is higher than the odds imply. If the math says there is no real edge, the recommended stake drops to zero, which is the calculator’s polite way of telling you to pass. Everything it shows comes straight from the Kelly formula, so the next section breaks down exactly how that formula works.
How the Kelly Formula Works
The Kelly formula is f* = (b × p − q) / b, where b is your decimal odds minus 1 (the profit you win per $1 staked), p is your estimated chance of winning, and q is the chance of losing, which is simply 1 − p. The result, f*, is the fraction of your bankroll to wager. It was first published in 1956 by John L. Kelly Jr., a researcher at Bell Labs, and later popularized for gambling and investing by mathematician Ed Thorp, who used a scaled-down version of it to beat blackjack and the stock market.
The logic is intuitive once you sit with it. The bigger the gap between your win probability and the odds, the bigger your edge, and the more of your bankroll the formula tells you to risk. Shrink that gap and the recommended stake shrinks with it. You can read the original derivation and a clean breakdown of each term in the Corporate Finance Institute’s overview of the Kelly Criterion.
f* = (b × p − q) / b — b = decimal odds − 1, p = your estimated win probability, q = 1 − p. The output f* is the fraction of your bankroll to stake. The calculator runs this for you, so you only supply the inputs.
How to Use the Calculator
Using the calculator takes four inputs and one click: choose your odds format, enter the odds, add your win-probability estimate, and type in your bankroll. There is nothing to download and no sign-up, and the results appear the moment your numbers are in. Here is what each input means.
Step 1: Enter the Odds
Pick your odds format — American, decimal, or fractional — then type the price exactly as your sportsbook lists it. If you are using American odds, include the plus or minus sign, since +150 and -150 describe very different bets. If you only have the price in one format, our odds calculator will convert it in a click.
Step 2: Add Your Win-Probability Estimate
Enter how likely you think the bet is to win, as a percentage. This is your number, not the sportsbook’s — it is where your edge lives, and it is the input that does the most work in the formula. The section further down on estimating win probability covers how to come up with a defensible figure rather than a hopeful one.
Step 3: Enter Your Bankroll and Read the Output
Type in your total bankroll — the full amount you are betting with, not just the money set aside for this one play. The calculator then returns the Kelly percentage, the full-Kelly stake in dollars, the half- and quarter-Kelly alternatives, and whether the bet grades as +EV. The worked example below shows exactly what those outputs look like with real numbers.
A Worked Kelly Example
Say you back a +150 underdog you believe has a 55% chance to win, with a $1,000 bankroll — full Kelly recommends staking 25% of your bankroll, or $250. Here is how the formula gets there. The +150 American price converts to decimal odds of 2.50, so b = 2.50 − 1 = 1.5. Your win probability is p = 0.55, which makes q = 1 − 0.55 = 0.45. Plug those in: f* = (1.5 × 0.55 − 0.45) / 1.5 = (0.825 − 0.45) / 1.5 = 0.375 / 1.5 = 0.25, or 25% of your bankroll.
On a $1,000 bankroll, that is a $250 full-Kelly stake. Most bettors scale that down, and the table shows why the half- and quarter-Kelly numbers are so much easier to stomach when a losing streak hits.
| Kelly Level | % of Bankroll | Stake on $1,000 |
|---|---|---|
| Full Kelly | 25.00% | $250.00 |
| Half Kelly | 12.50% | $125.00 |
| Quarter Kelly | 6.25% | $62.50 |
Notice the bet still grades as +EV: a 55% win chance is better than the roughly 40% the +150 price implies, which is why the formula is willing to commit a quarter of your roll at all. If you had pegged the same underdog at only 40%, the edge would vanish and the recommended stake would fall to zero.
How to Estimate Your Win Probability
Estimate your win probability by starting from the market’s no-vig implied odds, then adjusting up or down with a model, power ratings, or a meaningful sample of past results — never a gut feeling on a single game. It is the hardest and most important Kelly input, because the formula assumes that number is accurate: if it is inflated, every stake the calculator produces is too big. There is no shortcut for this part; it is the actual skill of handicapping, and a few methods turn a gut read into something you can defend:
- Start from the no-vig implied probability. Strip the sportsbook’s margin out of the line to see the market’s true estimate, then ask whether you genuinely think the real chance is higher.
- Lean on a model or power ratings. Even a simple system based on team strength, pace, or recent form gives you a repeatable number instead of a mood.
- Use a real sample, not one game. A 55% edge means little if it is based on three results; the more outcomes behind your estimate, the more you should trust it.
The whole point is to find spots where your probability beats the price — the definition of a value bet. Our guide to value betting walks through how to spot those edges in the first place, which is the work that has to happen before Kelly sizing means anything at all.
Full Kelly vs. Fractional Kelly
Full Kelly grows a bankroll fastest in theory, but most bettors use half-Kelly or quarter-Kelly because it cuts the wild swings dramatically while keeping most of the long-run growth. The reason is that full Kelly is brutally sensitive to a bad probability estimate. Overrate your edge by even a little and full Kelly quietly turns into overbetting, which is the fastest way to torch a bankroll.
Fractional Kelly is the standard fix. Betting half or a quarter of what the formula suggests sacrifices a sliver of theoretical growth in exchange for far smaller drawdowns — and since your real edge is always an estimate, that trade is almost always worth it. Ed Thorp himself bet half Kelly. Use full Kelly only when you genuinely trust your numbers and can stomach big short-term swings; use a fraction when you are testing a new model, building your roll slowly, or simply want to sleep at night.
Drawdowns are unforgiving: a 50% loss requires a 100% gain just to get back to even. Full Kelly can produce drawdowns that deep even when your edge is real, which is exactly why disciplined bettors size down. Treat any Kelly number as a ceiling, not a target.
Sizing bets this way is only one piece of a healthy approach to your money. Our guide to bankroll management covers the rest — setting a dedicated bankroll, deciding what counts as one unit, and keeping your betting money separate from your real life.
Related Betting Tools
Pair the Kelly Criterion calculator with our other free tools to price bets and find the edges worth sizing in the first place. Kelly tells you how much to stake; the rest of the toolkit helps you decide whether a price is worth taking at all. If you spot the same game priced differently across books, our arbitrage calculator shows when you can lock in a profit no matter the result, and you can browse the full set on our sports betting tools hub.
Play Safe: Gambling should be fun, not stressful. Set limits, stick to your budget, and never chase losses. If you or someone you know has a gambling problem, call 1-800-MY-RESET or visit ncpgambling.org. For more resources, see our Responsible Gambling page.
Frequently Asked Questions
Still deciding how much to stake or whether to trust the number the calculator gives you? Here are the questions bettors ask most about Kelly sizing.
How much of my bankroll should the Kelly Criterion tell me to bet?
It depends entirely on your edge, but in practice the recommended stake is usually a single-digit percentage of your bankroll once you account for realistic win probabilities. A big edge can push full Kelly into the 10-25% range, which is exactly why most bettors scale it down to half or quarter Kelly to keep individual bets smaller and safer.
Should I use full Kelly or stick to half Kelly?
Most bettors should stick to half Kelly or even quarter Kelly. Full Kelly grows a bankroll fastest in theory, but it assumes your win-probability estimate is perfect, and it isn’t. Betting a fraction of the full amount cuts your drawdowns dramatically while keeping most of the long-run growth, which is why Ed Thorp and many pros bet half Kelly.
What happens if my win-probability estimate is wrong?
The bet size will be wrong too, because the formula trusts your number completely. If you overrate your chances, full Kelly tells you to bet more than you should and you end up overbetting, which is the fastest way to drain a bankroll. Using fractional Kelly is the standard cushion against this exact problem.
Do professional bettors actually use the Kelly Criterion?
Yes, most serious bettors use some form of Kelly staking, almost always a fractional version. It’s a proven bankroll-management framework that has been used in sports betting, poker, blackjack, and financial markets since John Kelly published it in 1956 and Ed Thorp applied it to real money.
Is the Kelly Criterion a good strategy for sports betting?
It’s one of the best frameworks for sizing bets, as long as you only use it on bets where you genuinely have an edge. Kelly doesn’t find good bets for you — it tells you how much to risk once you’ve found one. If you can’t reliably estimate your win probability, the discipline of flat staking with a fractional-Kelly cap is the safer path.