Kelly Criterion definition
Kelly staking aims to maximize long-term bankroll growth by sizing bets in proportion to the edge you believe you have. Bigger edge, bigger stake. Smaller edge, smaller stake.
Kelly Criterion formula
The standard formula is f* = (bp - q) / b. In that formula, b is decimal odds minus 1, p is your estimated win probability, and q is the probability of losing. The result is the fraction of bankroll to stake.
Kelly betting example
If you estimate a bet has a 55% chance to win and the available odds are 2.10, the formula produces a positive stake because your probability estimate is higher than the market's implied probability. If the odds shorten too far, Kelly can return zero or a negative result, which means the bet should be skipped.
Why bettors use fractional Kelly
Full Kelly can be volatile, especially when your probability estimates are imperfect. That is why many serious bettors use half-Kelly or quarter-Kelly instead.
When Kelly helps
- You have a consistent process for estimating true probability.
- You want a structured link between edge and position size.
- You want to avoid arbitrary staking decisions.
When Kelly can fail
Kelly depends on your probability estimate being reasonably accurate. If your model is overconfident, the formula can recommend stakes that are too large.
Kelly vs flat staking
Flat staking uses the same unit size for each bet. Kelly adjusts stake size to the size of the edge. That can improve long-run growth, but it also punishes bad probability estimates. A practical path is to use flat staking while you learn to price markets, then move to quarter Kelly or half Kelly once your estimates are tracked and calibrated.
FAQs
Is full Kelly too aggressive?
For most sports bettors, yes. Full Kelly assumes accurate probabilities and independent bets. Half Kelly or quarter Kelly is usually more practical.
Can Kelly Criterion create an edge?
No. Kelly only sizes an edge you already have. Use expected value and closing line value to test whether the edge is real.
What should I use first: Kelly or bankroll management?
Start with bankroll rules first. Kelly is a sizing method inside a broader bankroll plan, not a replacement for one.
Related terms
- Bankroll defines the capital Kelly is working with.
- Expected value determines whether there is any real edge to size in the first place.
- Closing line value helps audit whether your prices are beating the market.