Kelly Criterion Calculator

What is the Kelly Criterion?

The Kelly Criterion is a formula for deciding how much to bet once you know you have an edge. It sizes each stake in proportion to that edge so your bankroll grows as fast as mathematically possible over the long run — while never risking the whole roll on a single bet.

The formula

For decimal odds and a win probability p:

• b = odds − 1 — the profit per $1 if the bet wins.
• q = 1 − p — the chance it loses.
• Kelly fraction = (b × p − q) ÷ b — the share of your bankroll to stake.

If that fraction comes out zero or negative, the bet has no edge and Kelly’s answer is simple: don’t bet. The formula never tells you to stake more than you have, and it never tells you to back a losing proposition.

Why fractional Kelly is safer

Full Kelly is mathematically optimal only if your probability estimate is exactly right — and in betting it almost never is. Because the downside of overbetting is far worse than the cost of underbetting, most disciplined bettors stake a fraction of full Kelly:

• Half Kelly keeps roughly three-quarters of the growth with about half the volatility.
• Quarter Kelly is gentler still — popular when your edge is uncertain or your bankroll matters a lot to you.

The risk of overbetting

Bet too big relative to your real edge and a normal losing streak can cripple your bankroll — and recovering from a big drawdown is much harder than avoiding it. When in doubt, size down. Kelly is a ceiling, not a target.

How Winora helps

Winora posts +EV bets with the edge already estimated, so you can size them sensibly with a tool like this one — and it leans conservative, because protecting the bankroll is what keeps you in the game.