Kelly Criterion Calculator
Free Kelly Criterion Bet Sizing Calculator
Calculate the optimal percentage of your bankroll to wager based on your edge and the odds. Maximize long-term growth while managing risk.
Kelly Formula
Bankroll Growth Simulation
⚠️ Kelly Criterion Warning
The Kelly Criterion assumes you know the true probability of winning, which is extremely difficult in sports betting. Most professionals use fractional Kelly (25% or less) to account for estimation errors and reduce variance. Never bet more than you can afford to lose.
Important for Beginners: The "risk level" refers to what PERCENTAGE of your bankroll you're betting, not the dollar amount. Even with a great edge, betting a high percentage of your bankroll is risky because one loss could devastate your funds. This is why we recommend using fractional Kelly (1/4 or 1/10) instead of Full Kelly.
What is a Kelly Criterion Calculator?
A Kelly Criterion calculator is a free betting tool that determines the optimal percentage of your bankroll to wager based on your perceived edge and the odds offered. Named after John Kelly Jr. who developed the formula at Bell Labs in 1956, it's the mathematical solution for maximizing long-term bankroll growth.
Our Kelly calculator helps serious sports bettors size their wagers scientifically rather than emotionally. Whether you're betting on NFL games, NBA spreads, or any sport where you believe you have an edge, this tool calculates the mathematically optimal stake amount.
Key Features of This Calculator:
- Visual risk meter showing volatility of your betting strategy
- Bankroll growth simulation comparing different Kelly fractions
- Edge analysis with expected value calculations
- Fractional Kelly options for risk management
- Smart recommendations based on your calculated edge
- Real-time calculations with animated results
How the Kelly Criterion Formula Works
The Kelly formula balances two competing goals: maximizing growth while avoiding bankruptcy.
The Kelly Formula Explained
Where:
- f = fraction of bankroll to bet
- b = decimal odds - 1 (net odds)
- p = probability of winning
- q = probability of losing (1 - p)
Step-by-Step Calculation
- Convert odds to decimal: +150 becomes 2.50
- Calculate net odds (b): 2.50 - 1 = 1.50
- Determine probabilities: If you think you have 45% chance, p = 0.45, q = 0.55
- Apply formula: f = (1.50 × 0.45 - 0.55) / 1.50 = 0.0167
- Result: Bet 1.67% of your bankroll
Kelly Criterion Examples for Sports Betting
Example 1: NFL Point Spread
| Scenario | Your Analysis | Bookmaker Odds | Kelly Says |
|---|---|---|---|
| Chiefs -3 | 55% cover probability | -110 (1.909 decimal) | 5.5% of bankroll |
| With 1/4 Kelly | Same 55% edge | Same -110 | 1.375% of bankroll |
Example 2: NBA Underdog Moneyline
| Bet Type | Win Probability | Odds | Full Kelly | Quarter Kelly |
|---|---|---|---|---|
| Lakers +180 | 40% | +180 (2.80) | 6.67% | 1.67% |
| Heat +250 | 35% | +250 (3.50) | 9% | 2.25% |
Example 3: When NOT to Bet
| Situation | Your Edge | Kelly Result | Action |
|---|---|---|---|
| 50% win @ -110 | -2.4% (negative) | 0% (don't bet) | ❌ No bet |
| 45% win @ +150 | +5% edge | 2% | ✅ Small bet |
| 25% win @ +500 | +8.3% edge | 10% | ✅ Use fractional |
Why Use Fractional Kelly?
Problems with Full Kelly
- Estimation errors: Small mistakes in probability estimates lead to large betting errors
- High variance: Full Kelly can suggest very large bets (25%+ of bankroll)
- Drawdowns: Even with an edge, losing streaks can devastate your bankroll
- Psychological stress: Hard to stick to strategy when betting large percentages
Fractional Kelly Benefits
| Kelly Fraction | Growth Rate | Volatility | Max Drawdown | Best For |
|---|---|---|---|---|
| Full (100%) | Maximum | Very High | ~95% | Perfect models |
| Half (50%) | 75% of max | High | ~50% | Confident bettors |
| Quarter (25%) | 50% of max | Moderate | ~25% | Most bettors |
| Tenth (10%) | 25% of max | Low | ~10% | Risk-averse |
Common Kelly Criterion Mistakes
1. Overestimating Your Edge
The biggest error is thinking you have a larger edge than reality:
- Market is efficient - true edges are rare and small
- Sample size issues - short-term results don't prove an edge
- Confirmation bias - remembering wins more than losses
2. Ignoring Correlation
Kelly assumes independent bets, but sports bets often correlate:
- Multiple bets on same game
- Similar bets across games (all favorites, all overs)
- Parlays and teasers
3. Not Adjusting for Limits
Real-world constraints Kelly doesn't consider:
- Maximum bet limits at sportsbooks
- Minimum bet requirements
- Available liquidity in betting markets
Advanced Kelly Criterion Strategies
Multiple Simultaneous Bets
When betting multiple games, adjust your Kelly percentage:
- Divide Kelly suggestion by number of concurrent bets
- Account for correlation between bets
- Never exceed 25% total exposure
Kelly with Rebates and Bonuses
Include promotional value in edge calculations:
- Cashback programs reduce effective losses
- Reload bonuses increase effective wins
- Use our EV calculator for complex scenarios
Dynamic Bankroll Management
Adjust Kelly parameters over time:
- Recalculate after significant wins/losses
- Reduce fraction during losing streaks
- Increase fraction with proven long-term edge
Kelly Criterion vs Other Betting Systems
| System | Approach | Pros | Cons |
|---|---|---|---|
| Kelly Criterion | Math-based sizing | Optimal growth | Requires edge |
| Fixed Percentage | Bet same % always | Simple | Ignores value |
| Fixed Amount | Bet same $ always | Very simple | No growth |
| Martingale | Double after loss | None | Guaranteed ruin |
Frequently Asked Questions
What is a Kelly Criterion calculator?
A Kelly Criterion calculator is a free betting tool that determines the optimal percentage of your bankroll to wager based on your edge and the odds offered. Named after John Kelly Jr., it mathematically calculates the ideal bet size to maximize long-term bankroll growth while minimizing risk of ruin.
How do you calculate Kelly Criterion?
The Kelly Criterion formula is: f = (bp - q) / b, where f is the fraction of bankroll to bet, b is the decimal odds minus 1, p is probability of winning, and q is probability of losing (1 - p). For example, with 55% win probability and +110 odds: f = (1.1 × 0.55 - 0.45) / 1.1 = 0.0136 or 1.36% of bankroll.
What is fractional Kelly betting?
Fractional Kelly betting means wagering a fraction (like 25% or 50%) of the full Kelly recommendation. This reduces variance and risk while still following optimal betting principles. Most professional bettors use fractional Kelly because full Kelly can suggest very large bets when you have a significant edge.
Is the Kelly Criterion good for sports betting?
The Kelly Criterion is excellent for sports betting IF you can accurately estimate win probabilities. It mathematically optimizes bankroll growth over time. However, most bettors should use fractional Kelly (25% or less) because it's nearly impossible to know true probabilities, and overestimating your edge leads to overbetting.
What happens if you bet more than Kelly suggests?
Betting more than Kelly suggests (overbetting) actually reduces long-term growth and dramatically increases risk of ruin. While it may increase short-term profits when lucky, the increased volatility leads to larger drawdowns and slower overall bankroll growth. Betting 2x Kelly has the same growth rate as not betting at all!
Can Kelly Criterion work with negative expectation bets?
No, the Kelly Criterion only works with positive expectation (+EV) bets. If you don't have an edge (negative expectation), Kelly correctly suggests betting 0% of your bankroll. The formula returns negative values for -EV bets, which means don't bet. This is why finding true edges is crucial for Kelly betting.
Should beginners use the Kelly Criterion?
Beginners should understand Kelly Criterion concepts but use very conservative fractions (10% Kelly or less) until they prove they can identify edges. Most beginners overestimate their edge, leading to overbetting. Start with fixed small bets, track results carefully, and only implement Kelly after demonstrating consistent profitability over hundreds of bets.
Related Calculators
- Expected Value Calculator - Calculate if bets have positive expectation
- Bankroll Calculator - Manage your betting funds effectively
- ROI Calculator - Track your betting return on investment
- Hedge Calculator - Lock in profits on existing bets
- View all 30 Free Betting Calculators
Any comments, suggestions, or corrections? Please let us know.
