What is the Kelly Criterion formula?

f* = (b × p − q) ÷ b, where f* is the fraction of your bankroll to stake, b is decimal odds minus 1 (the profit per unit), p is your estimated probability of winning, and q is 1 − p.

Should I use full Kelly or fractional Kelly?

Most professional bettors use half Kelly (50%) or quarter Kelly (25%). Full Kelly maximises long-run growth in theory but requires perfectly accurate probability estimates and produces brutal drawdowns when estimates are slightly wrong.

What does a negative Kelly result mean?

A negative Kelly result means the bet has negative expected value. The formula is telling you not to place this bet — the odds do not justify your probability estimate.

Free Tool

Kelly Criterion Calculator

Enter your edge and the odds to calculate the mathematically optimal stake as a percentage of your bankroll — at full, half, and quarter Kelly.

Your Bet Details

Decimal Odds

Enter in decimal format (e.g. 2.50 not +150)

Your Win Probability (%)

Your honest estimate of the true win probability

Bankroll

The Kelly Formula

f* = (b × p − q) ÷ b

f* — fraction of bankroll to stake
b — decimal odds minus 1 (your net profit per unit if you win)
p — your estimated probability of winning (as a decimal)
q — probability of losing = 1 − p

For an in-depth explanation and examples, see our Kelly Criterion guide.