Loading
Find your optimal bet size from the odds, your true win probability, and your bankroll - with an adjustable Kelly multiplier.
Find your optimal bet size from the odds, your true win probability, and your bankroll.
A multiplier of 0.25 (quarter Kelly) is a common, lower-variance choice. If expected value is negative, Kelly stakes $0.
Create your Rainbet account and enter code SPINDEX during registration.
Get StartedEstimated typical sports margin by book - illustrative industry ranges, not Spindex-measured.
A Kelly Criterion calculator works out the optimal amount to bet based on your edge. Instead of guessing whether to risk 2% or 5% of your bankroll, the Kelly formula gives you a precise, mathematically grounded stake designed to grow your bankroll as fast as possible without taking on reckless risk.
The Kelly Criterion is a bankroll-management formula that sizes each bet according to your edge - the difference between your estimated true win probability and what the odds imply. A bigger edge means a bigger recommended stake; no edge means it tells you to bet nothing. It's used by professional bettors and investors alike because, over the long run, it maximizes growth while mathematically avoiding ruin.
The formula is f = (b x p - q) / b, where b is the decimal odds minus 1, p is your win probability, and q is 1 minus p. Multiply that fraction by your bankroll for the dollar stake. For example, at +110 odds (decimal 2.10, so b = 1.10) with a 60% win probability, full Kelly is (1.10 x 0.60 - 0.40) / 1.10 = 23.6% of bankroll. Apply a 0.25 multiplier for quarter Kelly and that drops to about 5.9% - roughly $295 on a $5,000 bankroll.
Full Kelly maximizes growth in theory but is volatile and unforgiving of mistakes - if you overestimate your win probability, you'll overbet badly. That's why most bettors use fractional Kelly: a 0.5 (half) or 0.25 (quarter) multiplier that keeps most of the upside while smoothing out the swings. The multiplier field in this calculator lets you dial that in.
Enter the odds (American or decimal), your estimated true win probability, your bankroll, and a Kelly multiplier. The calculator returns the bet's expected value, the fraction of your bankroll to wager, and the exact dollar amount. If the expected value is negative, it correctly recommends a $0 stake. Pair it with a no-vig calculator to get a sharp win-probability estimate in the first place.
The Kelly Criterion is a formula for optimal bet sizing. It tells you what fraction of your bankroll to stake based on your edge - the gap between your true win probability and the odds - to maximize long-term bankroll growth while limiting the risk of ruin.
Kelly fraction = (b x p - q) / b, where b is the decimal odds minus 1, p is your win probability, and q is 1 - p. Multiply the result by your bankroll for the dollar stake. For +110 odds (b = 1.1) and a 60% win chance, full Kelly is about 23.6% of bankroll.
Most bettors use a fractional Kelly - 0.25 (quarter) or 0.5 (half) - rather than full Kelly. Full Kelly is high-variance and very sensitive to errors in your win-probability estimate, so scaling down trades a little growth for a much smoother bankroll.
Quarter Kelly means staking 25% of the amount the full Kelly formula recommends (a 0.25 multiplier). It's a popular, conservative choice that dramatically reduces swings while keeping most of the long-term growth.
Yes. If your win probability doesn't beat the odds - meaning the bet has zero or negative expected value - the Kelly formula returns 0 or a negative number, which means you should not place the bet.
It's the mathematically optimal way to size bets when you have a real edge, which makes it excellent for disciplined, +EV bettors. Its main risk is overestimating your win probability, which is why fractional Kelly is the standard in practice.
18+ only · Gamble responsibly · Tools are for informational purposes only.