Kelly criterion calculator for bet sizing
The Kelly criterion sizes a bet from your edge: the bigger the edge, the larger the share of your bankroll it suggests. If your win probabilities are right, staking this way grows a bankroll faster over time than any other fixed fraction, without ever risking all of it.
Full Kelly is aggressive and assumes your probability is exactly right, which it never is, so most bettors stake a fraction of it. This calculator shows full, half and quarter Kelly.
Free, no account needed. Everything is worked out in your browser and nothing you type is saved.
How to use the kelly criterion calculator
- 1Enter the odds you can bet.
- 2Enter your win probability as a percentage, usually the fair probability from a devigged sharp line.
- 3Enter your bankroll: the money set aside for betting, not your total savings.
- 4Pick full, half or quarter Kelly and read the Optimal Stake. A Kelly of 0% means the bet has no edge and should be skipped.
Worked example: +150 at a 45% win probability
You can bet +150 on a side you estimate wins 45% of the time, with a $10,000 bankroll. These are the numbers the calculator loads with.
- Net odds (b)
- 1.5
- Kelly fraction (b × p − q) ÷ b
- 8.33%
- Full Kelly stake
- $833.33
- Half Kelly stake
- $416.67
- Quarter Kelly stake
- $208.33
- Expected profit at full Kelly
- $104.17
The formula gives (1.5 × 0.45 − 0.55) ÷ 1.5 = 0.125 ÷ 1.5 = 8.33% of the bankroll. The edge is 12.5 cents per dollar staked, so a quarter Kelly bet of $208.33 is expected to make $26.04.
Frequently asked questions
What is the Kelly criterion formula?
Kelly % = (b × p − q) ÷ b, where b is the net decimal odds (decimal odds minus 1), p is your win probability and q is 1 − p. At +150 (b = 1.5) with a 45% chance, that is 8.33% of your bankroll.
Should I use full Kelly or fractional Kelly?
Fractional. Full Kelly only maximizes growth if your probability is exactly right; overestimate your edge and it overbets, with deep drawdowns. Half Kelly gives up about a quarter of the growth rate for far less volatility, and quarter Kelly is a common choice for +EV betting.
What does a 0% Kelly result mean?
The bet has no edge at those odds and that probability: b × p is less than or equal to q. Kelly never recommends a negative-EV bet, so it returns zero.
Where does the win probability come from?
From fair odds rather than a hunch. Devig a sharp book’s line with the no-vig calculator and use the fair probability it gives. A win probability that is a guess makes the Kelly stake a guess too.
Free now, free for good
Every BreadSync calculator is free to use without an account, and that is not changing. They work on the odds you type in.
BreadSync Pro’s +EV finder sizes every positive EV bet with Kelly automatically, from live sharp-book fair odds.
Related calculators
- Expected Value CalculatorCompare the odds you can bet with fair no-vig odds to see what a bet is worth on average.
- No-Vig CalculatorStrip the vig from a two-way market to get fair odds and win probability, four devig methods.
- Odds ConverterConvert between American, decimal and fractional odds and implied probability.
- Arbitrage CalculatorCheck whether two or three prices are an arb and split your stake so every outcome pays the same.
21+. Worked examples are illustrative and use made-up prices; real odds change constantly. Calculator results are estimates for information only, not betting advice or a guarantee of profit. BreadSync is not a sportsbook and does not accept wagers. If gambling stops being fun, call 1-800-GAMBLER.