Kelly criterion
The Kelly Criterion sizes each stake from your bankroll, the odds and your win probability, staking more when the edge is bigger. Most pros use half or quarter Kelly to smooth out the swings.
Full Kelly is mathematically optimal but volatile: a string of losses can gut your bankroll fast. Half or quarter Kelly keeps most of the growth with a fraction of the swings.
How the Kelly Criterion works
Kelly is a formula for the stake that grows your bankroll fastest over the long run without risking ruin. It stakes a share of your money in proportion to your edge: f = (b·p − q) ÷ b, where b = odds − 1, p is your win probability and q = 1 − p. The bigger your edge, the bigger the recommended stake.
Why fractional Kelly. Full Kelly assumes your probability estimate is exactly right. It never is. Overestimate your edge even slightly and full Kelly tells you to bet far too much. Halving or quartering the stake costs a little long-term growth but massively reduces the chance of a brutal drawdown. That's why almost every serious bettor uses half or quarter Kelly.
The danger of overbetting. Stake more than full Kelly and your expected bankroll growth turns negative even with a real edge: the variance eats you alive. If the formula returns zero or less, there's no edge at these odds and the maths says don't bet at all.
This is a maths tool, not a promise. Kelly only helps if your probability estimate is genuinely good, and no staking plan turns a losing system into a winning one. Only ever bet money you can afford to lose.
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