It is the benchmark in bankroll management, provided you understand its main weakness: Kelly assumes you know the true probability. You don't know it — you estimate it.
The formula
where p is the true probability and f the fraction of bankroll to stake. On odds of 2.20 with a true probability of 50%, Kelly recommends 8.3% of the bankroll — €83 on €1,000 of capital.
That is enormous. It is also why almost nobody plays full Kelly.
Why play a fraction of Kelly
Full Kelly is optimal if your probability estimate is exact. As soon as it is a few points too high — which happens constantly — full Kelly becomes over-staking and the bankroll takes violent losing runs.
- Half Kelly keeps roughly 75% of the theoretical growth for half the variance. It is the most common compromise among professional bettors.
- Quarter Kelly suits an uncertain probability estimate, which is the normal case in value betting.
- Many bettors prefer a flat stake of 1 to 2% of the starting bankroll: less optimal in theory, far easier to stick to in practice.
Kelly and stake limits at French bookmakers
A point too often forgotten: on the French market the constraint is not only mathematical. Licensed operators limit winning accounts, and an €83 stake on a second-division market draws attention far faster than a €20 one.
In practice, stake size settles on the minimum between Kelly's recommendation and what your account can absorb without being noticed.
Frequently asked questions
Do you have to recompute your stake for every bet?
It can be automated. Stats&Bet alerts include the suggested stake derived from your bankroll, your Kelly fraction and the detected EV — the calculation is redone on every opportunity without you lifting a finger.
Can Kelly return a negative stake?
Yes, and it simply means the bet has no positive expectation under your assumptions: don't play it. The calculator then brings the stake back to zero.
Should you recompute your bankroll after every bet?
Kelly assumes so. In practice most bettors recompute their unit once a week or once a month: it avoids cutting stakes at the worst point of a losing run, and makes the whole thing far easier to follow.
How do you handle several simultaneous bets?
Applying Kelly independently to bets running at the same time leads to overall over-staking. The simple fix is to cap total exposure — say 10% of the bankroll in play at once — cutting each stake proportionally if needed.