What is the Kelly criterion?
The Kelly criterion is a formula for figuring out how much of your bankroll to stake on a single bet, given your perceived edge and the offered odds. It was developed by John Kelly Jr. at Bell Labs in 1956 as a method to maximize long-term geometric growth of capital. In betting and investing, it answers a deceptively simple question: how much should I bet to grow my bankroll fastest, without going broke?
The answer it gives is mathematically optimal in the long run — but it comes with significant practical caveats that most beginners miss.
The formula
For a binary bet (win or lose), the Kelly fraction is: f* = (bp − q) / b, where b is the net odds (decimal odds − 1), p is your estimated probability of winning, and q = 1 − p is the probability of losing.
If the result is positive, that's the fraction of your bankroll you should stake. If it's zero or negative, don't bet — you have no edge.
Run any specific bet through our Kelly criterion calculator to skip the algebra.
A real example
You think Manchester City has a 60% chance of beating Arsenal. Bookmaker offers 2.00 (even money). Your edge:
— b = 2.00 − 1 = 1
— p = 0.60, q = 0.40
— f* = (1 × 0.60 − 0.40) / 1 = 0.20
So Kelly says stake 20% of your bankroll. On a €1,000 bankroll, that's €200 — a massive single bet by most standards. This is exactly why pros use Half Kelly or Quarter Kelly instead.
Why pros use fractional Kelly
Full Kelly maximizes long-run growth, but the variance is brutal. With a real edge of 3-5%, Full Kelly produces drawdowns of 50% with surprising regularity. Most bettors psychologically can't handle that and start deviating from the strategy — which destroys the whole point.
Half Kelly captures about 75% of the optimal growth with much lower variance. Quarter Kelly captures about 60% with very low variance. Our Kelly fraction comparison tool shows the trajectories side by side, and our drawdown calculator quantifies the variance you'll actually face.
The biggest practical mistake
Beginners overestimate their edge. They think they have 60% on a coin-flip when they actually have 51%. Kelly is exquisitely sensitive to this error — if you input the wrong p, you can stake far more than is wise.
Verify your edge first with our Closing Line Value calculator over 100+ bets before trusting Kelly outputs. Many "winning" bettors are actually losing once you account for true no-vig fair odds.
Bottom line
Use Kelly as a sizing guide, not gospel. Half Kelly is the practical default for skilled bettors. Verify your edge with the tipster reality check before scaling stakes. And no matter what the formula says, never bet more than you're emotionally OK with losing — discipline beats math every time.