How much of my bankroll should this bet be?

Size a bet from your win chance and the odds with the Kelly criterion, at full, half or quarter Kelly.

How it works

• Kelly fraction = (b × p − (1 − p)) / b, where p is your win chance and b is decimal odds − 1.
• Zero when the bet has no edge: Kelly never bets a negative expected value.
• Most bettors use a quarter or half Kelly, because a win chance that is slightly too high makes full Kelly overbet. Betting twice Kelly gives no growth at all.
• Your biggest edges deserve the most suspicion: they are the likeliest to be model error. Shrink a large edge toward the market before sizing it.

Questions

Where does my win chance come from?

A fair price you trust, such as Pinnacle without its margin, or a model with a tested record. A guessed chance makes Kelly dangerous.

Why not bet full Kelly?

Full Kelly grows a bankroll fastest only when your win chances are exact. With any error, the swings are severe; half Kelly gives up a little growth for much smaller drawdowns.

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