Kelly
Criterion

Bet Sizing Game f* = (bp − q) / b
HOW IT WORKS +

The Kelly Criterion

The Kelly Criterion is a formula for sizing bets to maximise the long-run growth rate of your bankroll. Bet too little and you grow slowly; bet too much and variance will destroy you. Kelly finds the exact sweet spot.

f* = (b·p − q) / b
  • f* — fraction of bankroll to wager
  • b — net payout odds (e.g. 2:1 means b = 2)
  • p — probability of winning
  • q — probability of losing (q = 1 − p)

Example: a 60% win chance on a 1:1 bet gives f* = (1·0.6 − 0.4) / 1 = 20%. Bet 20% of your bankroll each round for optimal growth.

Over-betting past Kelly reduces your long-run growth and risks ruin. At 2× Kelly your expected growth rate is zero — the same as not betting at all.

How to Play

  • Each round shows a bet with a known win probability and payout ratio. Both are visible — this is a sizing game, not a prediction game.
  • The ▲ orange marker on the slider shows the Kelly-optimal bet size for that round. Drag the slider to match it.
  • Your Kelly Score tracks average deviation from the optimal fraction across all rounds. Lower is better.
  • In Bust Mode play until you go broke or cash out voluntarily.
  • In Target Mode race to grow your balance to a chosen multiplier before busting.
  • Use CASH OUT at any time to lock in your gains and see your final score.

Each round presents a bet with a known edge. Size your bet using the Kelly Criterion to maximize long-run growth. Your discipline shapes the opportunities you receive.

Bust Mode
Play until you go broke or cash out. No safety net.
Target Mode
Race to a target multiplier before you bust.