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Kelly criterion bet size calculator

Enter your bankroll, the odds and your win probability. See the Kelly stake and a simulation of 2,000 bankroll paths.

Kelly calculator illustration

The formula

f* = (b × p − q) ÷ b

f* is the share of bankroll to stake, b = decimal odds − 1, p the win probability and q = 1 − p.

If the result is zero or negative, the bet has no edge and Kelly says skip it.

Full Kelly grows a bankroll fastest only if your probability is exactly right. Because it never is, most bettors use a fraction: half Kelly gives up about a quarter of the growth for far smaller swings, and quarter Kelly is calmer still.

Worked example

Worked example: Bankroll $1,000, odds +100, win probability 55%

  1. b = 2.00 − 1 = 1
  2. Full Kelly: (1 × 0.55 − 0.45) ÷ 1 = 10%, so $100

Half Kelly stakes $50 and quarter Kelly $25.

The simulation below the stakes replays the same bet 100 times on 2,000 paths, so you can see the spread of outcomes at each fraction before you pick one. The Kelly guide explains why the swings are so large.

Frequently asked questions

What is the Kelly criterion?

A formula for the share of your bankroll to stake on a bet with an edge so the bankroll grows fastest over time. It stakes more when the edge is bigger and nothing when there is no edge.

Should I bet full Kelly?

Usually not. Full Kelly assumes your win probability is exact and produces big drawdowns. Quarter to half Kelly is the common choice.

What does Kelly say for a negative EV bet?

Zero. If b × p − q is negative, there is no stake that grows the bankroll, so the answer is not to bet.

Read the guide: Kelly criterion and bankroll sizing for sports bets

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