The Kelly criterion: sizing to your edge

Flat staking is safe and simple. The Kelly criterion is the more advanced answer to a sharper question: given a real edge, how much should you stake to grow a bankroll fastest without blowing it up?

The formula

For a bet at decimal odds O that wins with probability p, Kelly says stake this fraction of your bankroll:

f = (p × (O − 1) − (1 − p)) ÷ (O − 1)

The top of that fraction is just the EV you met in the betting course; Kelly divides it by the net odds to turn "how good is the bet" into "how much of the bankroll to risk". No edge (EV ≤ 0) → Kelly says stake nothing.

A worked example

A 60% pick (p = 0.60) at odds 2.00 (O = 2.00):

f = (0.60 × 1 − 0.40) ÷ 1 = 0.20

Full Kelly says stake 20% of your bankroll on one bet. That is a lot — and it exposes Kelly's flaw.

Why almost everyone bets fractional Kelly

Full Kelly assumes your probability is exactly right. It never is. If your true edge is smaller than you think — and beginners always overestimate — full Kelly wildly over-bets and the swings become brutal. The standard fix is fractional Kelly: stake a fraction (commonly a half or a quarter) of what the formula says. In the example, quarter-Kelly stakes 5%, not 20%. You give up a little growth for a huge drop in volatility.

• Full Kelly = fastest growth if your numbers are perfect (they aren't).
• Half/quarter Kelly = far smoother ride, still edge-aware.
• Garbage p in = dangerous stake out. Kelly amplifies bad probability estimates.

Kelly sizes bets to your edge, not your excitement. Since you never know your true edge exactly, bet a fraction of what it tells you — and see Risk of ruin for why.