How Kelly works
Kelly outputs the fraction of your bankroll to bet. The formula is edge divided by odds: (bp - q) / b, where b is the decimal payout minus one, p is your win probability and q is 1 minus p. A real edge produces a positive fraction; no edge produces zero, which is Kelly's way of saying do not bet.
A worked example
Suppose a bet pays even money (b = 1) and you believe you win 55 percent of the time. Kelly says stake (1 x 0.55 - 0.45) / 1 = 0.10, or 10 percent of your bankroll. Because full Kelly is aggressive and your probability estimate is never exact, most bettors use half or quarter Kelly to cut swings.
Why bettors use fractional Kelly
Full Kelly maximizes growth in theory but produces stomach-churning swings and punishes overconfident estimates hard. Half Kelly keeps most of the growth with far less volatility. In practice, disciplined bettors combine a Kelly-style sizing with a simple unit cap so no single play, however confident, can put too much of the bankroll at risk.
Frequently asked questions
Is the Kelly Criterion better than flat betting?
It grows a bankroll faster when your edge estimates are accurate, but it swings harder. Many bettors use fractional Kelly or a unit system to balance growth and safety.
What does Kelly say to bet with no edge?
Zero. If a bet has no positive expected value, the Kelly fraction is zero or negative, meaning you should not place it.
Related terms
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