How much to bet when you have an edge. Full Kelly maximizes long-run growth; almost everyone should bet a fraction of it.
The Kelly criterion formula
The Kelly criterion sizes a bet to the size of your edge: stake the fraction of bankroll equal to (b × p − q) ÷ b, where b is the profit per $1 at the odds, p your win probability, and q = 1 − p. No edge, no bet — Kelly goes to zero exactly when EV does.
Why half Kelly is the default
Full Kelly assumes your probability estimate is perfect. It never is, and overbetting a wrong estimate is how bankrolls die. That's why half Kelly (or quarter) is the standard: most of the growth, a fraction of the variance. The calculator defaults to half.
Common questions
What is the Kelly criterion formula for betting?
Stake percentage = (b × p − q) ÷ b, where b is decimal odds minus 1, p is your win probability, and q is 1 − p. At +120 with a 48% edge estimate: (1.2 × 0.48 − 0.52) ÷ 1.2 = 4.7% of bankroll at full Kelly.
Why use half Kelly instead of full Kelly?
Because your win probability is an estimate. Half Kelly gives up about a quarter of the theoretical growth rate while cutting variance dramatically — the trade every professional takes.
