Kelly Criterion Calculator
Find your mathematically optimal bet size. Enter your win probability, odds, and bankroll to get full Kelly, half Kelly, and quarter Kelly stake recommendations.
Quick presets:
Your Inputs
Your estimated probability the bet wins
Enter decimal odds (must be > 1.0)
Total amount available to bet with
Implied Probability (from odds)
Your Edge
--%
Your probability minus implied probability
Expected Value (per $100)
--
Average profit per $100 wagered
Kelly Bet Sizing
Full Kelly
Maximizes long-run bankroll growth — aggressive
--%
--
Half Kelly
RecommendedReduces variance by 50%, loses only ~25% of growth rate
--%
--
Quarter Kelly
Conservative — minimal risk of ruin, slower growth
--%
--
Bankroll After Outcome
If Half Kelly Wins
--
Bankroll
--
If Half Kelly Loses
--
Kelly says: Do not bet
Your estimated win probability is not high enough to overcome the bookmaker's margin at these odds. The Kelly formula returns zero or negative — place no bet.
Saved Calculations
Understanding the Kelly Criterion
What is Kelly?
The Kelly Criterion, developed by John L. Kelly Jr. in 1956, is a mathematical formula that determines the optimal fraction of a bankroll to wager. It maximizes the long-run growth rate of wealth while avoiding ruin.
b = net odds, p = win prob, q = loss prob (1−p)
Why Half Kelly?
Full Kelly assumes your probability estimate is perfectly accurate — it almost never is. Halving the Kelly bet reduces drawdowns by roughly 50% while sacrificing only about 25% of long-run growth rate. Most professional bettors and investors use half Kelly or less for this reason.
- ✓Half the volatility
- ✓~75% of maximum growth
- ✓Tolerates estimation errors
Risk of Ruin
Over-betting is the fastest way to go broke even with an edge. Kelly guarantees you never bet more than your edge justifies, but betting multiples of Kelly dramatically increases risk of ruin. Quarter Kelly virtually eliminates ruin risk for bettors with genuine edges.
How to Use This Calculator
1. Win Probability: Enter your honest estimate for how likely the bet is to win — not the bookmaker's implied probability.
2. Odds: Enter the odds the book is offering. Switch between decimal, American, or fractional format.
3. Bankroll: Your total available betting capital — not just what you plan to bet this session.
4. Act on Half Kelly: Use the half Kelly dollar amount as your recommended stake. Adjust down if you have any doubt in your probability estimate.
Kelly % Reference Table — $1,000 Bankroll
| Win Prob. | Decimal Odds | Edge | Full Kelly | Half Kelly | Quarter Kelly |
|---|