How to use this calculator
Enter the American odds from your sportsbook, your estimated true win probability (your "fair" probability, not the book's implied probability), and your total bankroll. The calculator shows your recommended bet size at each Kelly fraction.
If Kelly returns a negative number, the bet has no mathematical edge — skip it.
Formula
f* = (b x p - q) / b
where b = decimal odds - 1
p = your fair win probability
q = 1 - p (loss probability)
Stake = f* x fraction x bankroll
Example
You estimate a 55% true win probability on a -110 line (decimal 1.909, b = 0.909). Full Kelly = (0.909 x 0.55 - 0.45) / 0.909 = 5.5% of bankroll. On a $1,000 bankroll that is $55. Quarter Kelly = 1.375%, or $13.75. Quarter Kelly is the recommended starting point for most bettors.
Frequently asked questions
What is the Kelly Criterion?
The Kelly Criterion is a formula for sizing bets to maximize the long-run growth rate of your bankroll. It was derived by mathematician John L. Kelly Jr. in 1956 and has been adopted by professional gamblers and investors alike. It balances maximizing expected log wealth against the risk of ruin.
Why use Quarter Kelly instead of Full Kelly?
Full Kelly maximizes theoretical growth but produces large swings and deep drawdowns, especially when your edge estimate is slightly off. Quarter Kelly bets 25% of the full amount, dramatically reducing volatility while preserving roughly 75% of the long-run growth rate. Most professional bettors use Half or Quarter Kelly.
What if my edge estimate is wrong?
Edge estimation error is the greatest risk with Kelly. If you overestimate your true win probability, Full Kelly will over-bet and erode your bankroll. Fractional Kelly provides a built-in safety margin. As a rule: when in doubt, bet less. A conservative bet with a real edge still grows a bankroll over time.
What does a negative Kelly value mean?
A negative Kelly value means the implied probability from the odds is higher than your estimated win probability — there is no edge. Kelly says bet zero. Do not place the bet.
