Free calculator
Kelly calculator
Enter the price, your own estimate of the chance, and your bankroll. You will see what Kelly suggests staking, and what it looks like at a fraction.
your own read, not the book’s
most bettors run a fraction
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Enter your odds and your estimate.
Kelly is only as good as your probability estimate. If your read is optimistic, the stake it suggests will be too large.
Sizing is the easy half
The hard part is finding a price worth staking on at all. BetSuite watches your sportsbooks and surfaces the moments they disagree, so you are sizing real opportunities rather than hunting for them.
Download Free →What the Kelly criterion does
Kelly answers one question: given an edge, how much of your bankroll should you put behind it? Stake too little and you leave growth on the table. Stake too much and a normal losing run can take you out of the game entirely.
The formula is f = (bp - q) / b, where b is the decimal odds minus 1, p is your estimated chance of winning, and q is 1 minus p. The output is the fraction of your bankroll to stake.
Why the answer is often zero
If your estimated chance is below the break-even number the price implies, Kelly returns a negative figure, which means no bet. That is the formula working correctly. Most prices on a sportsbook do not offer an edge, because the margin is built in.
Why almost nobody uses full Kelly
Full Kelly maximises long-run growth, but only if your probability estimate is exactly right. It rarely is. Overestimate your edge by a little and full Kelly overbets by a lot, which is why half and quarter Kelly are common in practice: much smaller swings, most of the growth.
Arbitrage is a different situation. When you back both sides at prices that already line up, the return is set by the prices rather than by your estimate, so stake sizing is limited by what the books will accept rather than by Kelly. The arbitrage calculator covers that case, and our bankroll guide covers the wider picture.
Common questions
What is the Kelly criterion?
Kelly is a formula for bet sizing. Given the odds and your estimated chance of winning, it returns the fraction of your bankroll that maximises long-run growth. The formula is f = (bp - q) / b, where b is decimal odds minus 1, p is your estimated win probability and q is 1 minus p.
What is half Kelly and why use it?
Half Kelly stakes half of what the formula suggests. Full Kelly is only optimal if your probability estimate is exactly right, and estimates usually are not, so overestimating your edge leads to serious overbetting. Half and quarter Kelly keep most of the growth with far smaller swings.
Why does the calculator sometimes say no bet?
If your estimated chance of winning is below the break-even chance the price implies, the formula returns a negative number, which means the bet has no edge and Kelly suggests staking nothing. Most sportsbook prices fall into this category because the margin is built in.
Does Kelly apply to arbitrage betting?
Not in the same way. Arbitrage locks the return in through the prices themselves rather than through an estimate, so the limit on stake size is usually what the sportsbooks will accept rather than what Kelly suggests.