Kelly Criterion Calculator
Calculate the mathematically optimal bet size to maximize long-term bankroll growth.
Odds Format
Your honest assessment of the true probability this bet wins
Recommended Bet Size
$83.33
8.33% of bankroll
Full Kelly %
8.33%
Expected Value
+12.50%
Implied Probability
40.0%
Your Edge
+5.0%
What is the Kelly Criterion?
The Kelly Criterion is a mathematical formula developed by John Kelly at Bell Labs in 1956. It calculates the optimal percentage of your bankroll to wager on a bet with positive expected value, maximizing long-term growth while minimizing risk of ruin.
The Kelly Formula
Kelly Criterion formula:
f* = (bp - q) / b
Where:
f* = fraction of bankroll to wager
b = net odds (decimal odds - 1)
p = probability of winning
q = probability of losing (1 - p)
Why Use Fractional Kelly?
- •Full Kelly maximizes growth but is volatile—big swings in bankroll
- •Half Kelly sacrifices ~25% of growth for ~50% less volatility
- •Quarter Kelly is conservative—lower growth, much smoother ride
- •Most professional bettors use Half or Quarter Kelly to account for uncertainty in their edge estimates
Important Warnings
- !Kelly assumes your probability estimates are accurate—overconfidence leads to overbetting
- !Never bet more than Kelly suggests—this reduces long-term growth AND increases risk
- !Kelly is for long-term growth—short-term results will still be volatile