Kelly Criterion Calculator
The bet size that maximizes long-run bankroll growth for any +EV opportunity. Half- and quarter-Kelly soften the variance with only a small drag on expected growth.
What this means
The Kelly formula is f = (decimal × p − 1) / (decimal − 1), where p is your true win probability and f is the fraction of your bankroll to wager. It's the unique bet size that maximizes the long-run geometric growth rate of your bankroll.
Full Kelly has theoretical optimum growth but stomach-churning variance — drawdowns of 50% are routine. Most serious bettors use half or quarter Kelly: ~75% / ~94% of the optimal growth with dramatically less volatility.
Kelly assumes your win probability is correct. If you're overconfident, Kelly will over-bet you. When in doubt, scale down — quarter Kelly is a safe default for the real world.