The full Kelly criterion maximizes long-run bankroll growth, if you know your exact edge. Real bettors never do. Fractional Kelly (betting a set fraction of what Kelly recommends) is the standard fix, and understanding why will make you a calmer, longer-lasting bettor. (Kelly criterion basics.)
The growth curve
Long-run growth rate depends on how much of the Kelly stake you bet:
| Bet size (× Kelly) | Growth rate (% of max) |
|---|---|
| 0.25 | ~44% |
| 0.5 | ~75% |
| 1.0 | 100% |
| 1.5 | ~75% |
| 2.0 | ~0% |
| 2.5+ | negative |
The curve is symmetric around 1, but the consequences are not. Underbetting by half costs you a quarter of your growth. Overbetting by double costs you all of it, with far bigger swings along the way.
The edge-uncertainty problem
Suppose your screen says a bet has a 4% edge, but the true edge is 2%. Full Kelly on 4% is actually 2× Kelly for the real edge, the zero-growth point. Quarter Kelly on 4% becomes half Kelly for the real edge, still a healthy bet.
Because every edge you measure has some error, fractional Kelly acts like insurance: when your estimates are right, you grow a bit slower; when they are wrong, you survive.
Drawdowns
Under full Kelly, the chance of your bankroll at some point halving is about 50%. Under half Kelly it falls dramatically (to roughly 12.5% in the idealized model), and under quarter Kelly it becomes small (under 1%). The numbers in practice are worse than theory because edges are uncertain, which strengthens the case for small fractions.
Choosing your fraction
| Situation | Suggested fraction |
|---|---|
| Main lines, fresh multi-source fair line | ¼ to ½ Kelly |
| Player props | ⅛ to ¼ Kelly |
| Live betting | ⅛ Kelly or flat small stakes |
| New process without CLV history | ⅛ to ¼ Kelly |
| Long CLV track record | up to ½ Kelly |
Add a hard cap on any single bet (1–2% of bankroll is common) so a mismeasured huge edge cannot produce a huge stake.
Example
Bankroll $5,000. A bet at +120 (decimal 2.20) with a conservatively measured edge of 3%.
- Full Kelly = 0.03 ÷ 1.20 = 2.5% → $125
- Half Kelly → $62.50
- Quarter Kelly → $31.25
- Cap of 2% = $100, so quarter Kelly stands
Simultaneous bets
Kelly assumes one bet at a time. If you place 15 bets on a Sunday, they are all at risk together. Many bettors scale each stake down when total open exposure exceeds a threshold (say 10% of bankroll), and treat correlated bets on the same game as one combined position.
Frequently asked questions
What is fractional Kelly?
Betting a fixed fraction (like half or a quarter) of the stake the Kelly formula recommends, to reduce volatility and protect against overestimated edges.
Is half Kelly or quarter Kelly better?
Quarter Kelly is safer and more common for +EV bettors whose edges are estimates; half Kelly suits bettors with very reliable edges and a long track record.
How much growth do you lose with half Kelly?
About a quarter of the maximum long-run growth rate, in exchange for roughly half the volatility.