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Fractional Kelly calculator

Updated October 2026 · by Tom Howcroft

Size a hypothetical binary bet with stated odds and probability. Free, no account. See the formula, assumptions and editable inputs.

What does this result mean?

The full Kelly fraction is a share of the hypothetical bankroll; errors in the win probability can change it substantially. Fractional Kelly scales that fraction. Net odds exclude returned stake. Binary independent bets with a known edge are assumed; this is not a poker cash-game buy-in formula.

55%
1
25%
£1,000
10.0%
Full Kelly fraction
2.5%
Selected fraction of bankroll
£25
Hypothetical stake
Example

The example inputs below are hypothetical. Change them to inspect your own study scenario. Hold the odds fixed and lower the win probability to examine the effect of an overstated edge. Fractional Kelly scales that fraction. Net odds exclude returned stake.

Estimates for study and planning only, not a profit promise. Overshove tracks your real sample so these inputs come from your own data.

How to use the answer responsibly

Treat the calculator as a planning tool, not a verdict on one session. A live poker input is usually an estimate: your win rate is noisy, your standard deviation changes by game type, and your future volume rarely matches the clean number you type into a form. The useful habit is to save the assumptions beside the result so you know what would need to change before the decision changes.

If the result pushes against what you wanted to do, do not smooth the inputs until the answer feels nicer. Make the conservative version first, using a lower win rate, a higher swing rate or fewer weekly hours. Then run the optimistic version. The space between those answers is the real decision area: where a shot needs a stop rule, where a move up needs more buy-ins, or where a good-looking hourly still needs a bigger sample.

Overshove calculators become more useful when the inputs come from your own logged sessions. Rake, tips, travel, hours, stake mix and session length all change the number. A generic benchmark can teach the shape of the problem, but a clean personal log gives the number you should actually act on.

Questions

What formula does this tool use?

Full Kelly fraction = (net win odds × win probability - loss probability) / net win odds, floored at zero. Fractional Kelly scales that fraction. Net odds exclude returned stake.

Is this tool free and does it require an account?

It runs in your browser for free with no account. The result is a study calculation under the stated assumptions, not a recommendation to play or a promise of winnings.