The formula
fraction = (p × b − q) ÷ b
Where p is your win chance, q is 1 − p, and b is decimal odds minus 1.
Why quarter Kelly
Kelly assumes your probability estimate is exact. It never is. Overestimate your edge slightly and full Kelly overstakes substantially, because the stake scales directly with the number you got wrong. Quarter Kelly gives up little long-run growth for a large reduction in volatility and stays sensible when the estimate is off.
Limits worth knowing
- It assumes many repeated bets. Over a handful it says very little.
- It assumes bets are independent — six bets in one match are not.
- No stake size makes a bad bet good. Sizing comes after finding an edge, not instead.