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Kelly Criterion Calculator

The Kelly criterion is a staking formula that computes the optimal fraction of your bankroll to bet: Kelly % = (decimal odds x true win probability - 1) / (decimal odds - 1). It maximizes long-term bankroll growth, recommends zero when you have no edge, and most bettors stake half the Kelly amount to cut variance.

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Enter the odds, your true win probability, and bankroll. Get the Kelly criterion bet size that grows a bankroll fastest, the safer half Kelly stake, and the bet's expected value and edge.

Odds (American, decimal or fractional)
Your true win %
Bankroll ($)
Don't know your true win %? Paste two book prices
Your side price
Other side price
Fair win % (vig removed): --
Implied (break-even) %
--
Your edge
--
Expected value (per $100)
--
EV %
--
Full Kelly stake
--
Half Kelly (recommended)
--
Positive EV means the bet wins long term; Kelly sizes it
If your true win chance beats the implied (break-even) number, the bet is +EV. The Kelly criterion turns that edge into the stake that grows a bankroll fastest. Most pros bet half Kelly to cut variance. Everything here depends on an honest win probability, which is the hard part, and what our tools find.
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What is the Kelly criterion?

The Kelly criterion is a formula for how much of your bankroll to stake on a +EV bet to grow it as fast as possible without risking ruin. Bet too little and you leave money on the table; bet too much and variance can wipe you out. Kelly finds the mathematically optimal middle. It only recommends a bet when you actually have an edge; with no edge, Kelly says stake nothing.

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The Kelly formula: bet size and half Kelly

Kelly fraction = (decimal odds x true win % - 1) / (decimal odds - 1). Multiply by your bankroll for the bet size. Because real win probabilities are estimates, most sharp bettors use half Kelly (or quarter Kelly): half the recommended stake. That keeps almost all of the growth while sharply reducing the swings, which is why this tool shows both.

Expected value comes first, then Kelly

Kelly only sizes a bet that already has positive expected value (+EV). Your edge per $1 staked is (true win % x decimal odds) - 1, and the Kelly fraction is that edge divided by (decimal odds - 1). No edge means no bet. For the full walkthrough of expected value, including EV from a de-vigged sharp line and what a +EV bet earns over many bets, use the EV calculator.

Worked example. You make a bet 55% to win and the book offers +100 (decimal 2.00). EV per $1 = (0.55 x 2.00) - 1 = +$0.10, a 10% edge. Kelly % = (2.00 x 0.55 - 1) / (2.00 - 1) = 10% of bankroll. With a $1,000 bankroll that is a $100 full Kelly stake, or $50 at half Kelly.

The Wise Guy Team way

We bet to the edge, not to a hunch. Honest probabilities, disciplined unit sizing, and fractional Kelly are how a bankroll compounds instead of busting. The whole game is finding bets where your real win rate beats the implied price, then sizing them with a clear head.

Frequently asked questions

How does expected value relate to the Kelly criterion?

Kelly only sizes bets with positive expected value. The Kelly fraction is your edge per $1 divided by (decimal odds minus 1), so no edge means no bet. To work out a bet's EV first, use the EV calculator.

What is the Kelly criterion?

A formula for the bankroll fraction to stake on a positive EV bet to maximize long-term growth without risking ruin. No edge means no bet.

What is the Kelly criterion formula?

Fraction = (decimal odds x true win % minus 1) divided by (decimal odds minus 1), times your bankroll for the stake.

Should I use full or half Kelly?

Most bettors use half Kelly because true probabilities are estimates. Half Kelly keeps most of the growth with far less variance.

Does the Kelly criterion work?

Yes, when your win probabilities are accurate. Its weakness is bad inputs: overestimate your edge and Kelly overstakes, which is why fractional Kelly is standard.

EV Calculator → No-Vig Calculator → Odds Converter → Line Shopping tool →

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