The Kelly criterion sizes each bet by its edge: the bigger your advantage over the price, the larger the share of bankroll. Bet nothing when there is no edge.
The bet
%
The stake
%
Hard cap, % of bankroll (blank = none)Kelly fractionMost bettors use ¼–½ Kelly
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Full Kelly
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Your stake %
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Stake
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Expected profit
Not sure the bet has an edge at all? Check its value first →
The formula
f* = (b × p − q) / b where b = decimal odds − 1 (net odds),p is your win probability and q = 1 − p. f* is the share of bankroll to stake. Zero or negative means no bet.
Example: 2.10 with a 55% estimate gives (1.10 × 0.55 − 0.45) / 1.10 = 14.1% of bankroll.
Why fractional Kelly
- Full Kelly maximises long-run bankroll growth only if your probability is exactly right. Estimates never are, and overestimating your edge makes full Kelly overbet.
- Half Kelly keeps roughly three quarters of the growth with far smaller drawdowns; quarter Kelly is a common default for sports betting.
- A hard cap (e.g. 5% of bankroll) stops one strongly-rated bet from dominating the book.