Kelly Criterion Calculator — Bet Sizing
Finding a +EV bet is only half the job — how much to stake decides whether you grow a bankroll or blow it up. The Kelly criterion sizes each bet to the size of your edge. Enter your bankroll, the odds, and your win probability to get a recommended stake.
The formula
b = decimal − 1 f* = (b × p − (1 − p)) / b stake = bankroll × max(0, f*) × multiplier
Worked example
At +150 (b = 1.5) with a 45% win probability, full Kelly is 8.33% of bankroll. On a $1,000 bankroll that's $83.33 at full Kelly, or $41.67 at half Kelly. At -110 with a 55% estimate, full Kelly is 5.5%.
Practical advice
- Use half or quarter Kelly unless you're highly confident in your probabilities.
- Get probabilities from the no-vig calculator and confirm the edge with the EV calculator.
- Kelly assumes bets are independent and repeated; it's a sizing guide, not a guarantee.
Informational and educational only — not betting advice. Gamble responsibly.
FAQ
What is the Kelly criterion?
The Kelly criterion is a formula that gives the fraction of your bankroll to wager so as to maximize long-run growth, given the odds and your estimated probability of winning.
What is the Kelly formula for betting?
f* = (b × p − q) / b, where b is the net odds (decimal odds minus 1), p is your win probability, and q is 1 − p. At +150 (decimal 2.50) with a 45% win chance, b = 1.5 and f* = (1.5 × 0.45 − 0.55) / 1.5 = 8.33% of your bankroll.
Why do people use half or quarter Kelly?
Full Kelly is very volatile and assumes your probability is exactly right. Because real estimates are noisy, most bettors scale down to half or quarter Kelly, which sacrifices a little growth for far smaller drawdowns.
What does 0% mean?
If your win probability is at or below the break-even probability for the odds, the bet has no edge and Kelly recommends staking nothing. This calculator never outputs a negative stake.