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Kelly Criterion Bet Size Calculator

Stake the amount that grows your bankroll fastest

How it is calculated

Inputs are your win probability in percent, the net payoff b (profit per 1 unit staked when you win) and your bankroll. The tool uses f = (b × p − q) / b, where p = win probability / 100 and q = 1 − p. The result is the fraction of the bankroll to stake. If the edge is negative, the fraction is set to 0 rather than shown as a negative number, meaning do not bet. The stake is bankroll × f, and a half-Kelly stake (bankroll × f / 2) is shown as a more cautious alternative. Payoff is net odds, so even-money bets use 1.

Worked example

Win probability 55 %, even-money payoff (b = 1), bankroll 10,000. Then p = 0.55 and q = 0.45. Kelly fraction f = (1 × 0.55 − 0.45) / 1 = 0.10, which is 10.00% of the bankroll. Full-Kelly stake = 10,000 × 0.10 = $1,000.00. Half Kelly is $500.00. If you lowered the win probability to 50 % the edge would vanish and the fraction would drop to zero.

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Common mistakes

  • Entering decimal odds such as 1.9 as the payoff: the field wants net odds (profit per unit staked), so decimal odds 1.9 means 0.9.
  • Overestimating your win probability: the result is only as good as that estimate, and full Kelly punishes small errors with large swings.
  • Staking the Kelly amount on several simultaneous bets as if each were the only one.
  • Entering the probability as 0.55 instead of 55: it is read as 0.55 %, which gives zero.
  • Treating a result of 0.00% as an error: it means no edge at those odds, so Kelly says to skip the bet.

About this calculator

The Kelly criterion calculates the fraction of your bankroll to stake on a bet that maximizes the long-run growth rate of your capital, given your true edge. The formula is f* = p − q/b, where p is your win probability, q is the loss probability (1 − p), and b is the net odds received on the bet (payout multiple minus 1). A positive f* means the bet has positive expected value at those odds; a zero or negative result means you should not bet at all.

Sports bettors, poker players and some systematic traders use the Kelly criterion because betting more than the Kelly fraction increases the risk of large drawdowns and ruin even when the bet has a real edge, while betting less than Kelly (often "half Kelly" or "quarter Kelly" in practice) trades some growth for a smoother equity curve. Full Kelly is aggressive and assumes your probability estimate is accurate, which is rarely perfectly true, so many practitioners deliberately stake a fraction of the calculated amount.

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