# Kelly criterion calculator

Estimate a Kelly stake from decimal odds, your assessed probability, and bankroll.

Canonical URL: https://stats-api.com/tools/kelly-calculator

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Estimate a Kelly stake from decimal odds, your assessed probability, and bankroll. Calculations stay in your browser and are not betting advice.

Suggested stake

The Kelly criterion answers one narrow question: what fraction of a bankroll maximises its long-run growth rate, given an edge you believe in. It is a staking rule, not a way of finding value, and it is unforgiving of overconfidence.

## The formula

- Kelly fraction: `f* = (bp − q) / b`
- `b` = decimal odds − 1 (profit per unit staked)
- `p` = your estimated probability, `q = 1 − p`
- A negative result means no bet — the formula is telling you the edge is against you.

## Worked example

At decimal odds of 2.20 you believe the true probability is 50%. Then b = 1.20, p = 0.50, q = 0.50, giving f\* = (1.20 × 0.50 − 0.50) / 1.20 = **8.3%** of bankroll. On a 1,000 bankroll that is 83. At half Kelly it is 42. Note how aggressive full Kelly is for an edge of only 10%.

## Why almost nobody uses full Kelly

Kelly assumes *p* is correct. It is not — it is an estimate, and the formula is far more sensitive to overestimating your edge than to underestimating it. Overstating probability by a few points can turn optimal growth into steady ruin, and even a correctly specified full-Kelly strategy routinely draws down 50% or more. Half or quarter Kelly sacrifices a little theoretical growth for a large reduction in variance, which is why practitioners overwhelmingly use fractions.

## Doing this at scale

Kelly consumes a probability estimate, and a probability estimate consumes history: results, fixture congestion, home and away splits. Stats API provides that history through one contract, so the input to your staking rule is not itself a data-quality risk.

## Common questions

### What if the calculator returns a negative stake?

It means the price implies a higher probability than you assigned, so there is no edge. The correct stake is zero. Kelly never recommends betting against your own estimate.

### Should I use my probability or the market's?

Yours — that is the entire point. If you use the market's implied probability, the formula returns approximately zero, because a fair price offers no growth.

Related tools

- [Odds converter](https://stats-api.com/tools/odds-converter)
- [Implied probability calculator](https://stats-api.com/tools/implied-probability)
- [Poisson football score predictor](https://stats-api.com/tools/poisson-score-predictor)
- [Football kickoff time converter](https://stats-api.com/tools/kickoff-time-converter)
- [Football league table generator](https://stats-api.com/tools/league-table-generator)

Related guides

- [Split Football Results for an Honest Model Backtest](https://stats-api.com/blog/football-model-chronological-train-test-split)

Build with the API

- [API quickstart](https://stats-api.com/docs)
- [Football data API](https://stats-api.com/football-api)
- [Competition coverage](https://stats-api.com/coverage)
- [Betting model use case](https://stats-api.com/use-cases/betting-models)

For developers

## Turn the idea into an integration.

This utility is intentionally client-side and does not require an account. When your product needs real competition, team, fixture, or result data, use the documented Stats API endpoints and your own API key.

[Read the API quickstart →](https://stats-api.com/docs)

*The live page also has interactive controls that need a browser: https://stats-api.com/tools/kelly-calculator*

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