What does the Kelly Calculator do?

The Kelly Calculator helps you decide how much of your bankroll to put on a bet when you believe you have an edge. It is not trying to predict whether this one bet will win. It is trying to size the bet properly for the long run.

If your edge is strong, the suggested stake goes up. If the bet does not have positive expected value, the calculator tells you to skip it.

What numbers do you need?

You only need three inputs: the odds you can bet, your estimated chance of the bet winning, and the bankroll you are using for this strategy.

The most important input is the win probability. If that number is wrong, the stake size will be wrong too. That is why Kelly staking works best when your probability estimate comes from a proper model, sharp market comparison or reliable +EV tool, not a gut feeling.

Full Kelly vs fractional Kelly

Full Kelly is the most aggressive version. It aims for maximum long-term bankroll growth, but the swings can be uncomfortable.

Most people use a smaller fraction instead, such as 0.25× or 0.5× Kelly. This lowers the stake, smooths out the variance and makes the strategy easier to stick with when results are temporarily going against you.

How to use the result

Treat the output as a staking guide, not a guarantee. A +EV bet can still lose. The point of Kelly is to keep your bet size in proportion to your edge and bankroll, so one bad run does not wipe out your ability to keep betting.

For more on bankroll management, see variance and bankroll.