Calculate the suggested percentage of your bankroll to wager based on your estimated edge and the odds. A mathematical approach to bet sizing that aims to balance growth potential and risk.
Kelly Criterion Calculator
Bankroll Growth Simulation (Hypothetical)
The simulation assumes your estimated win probability is correct. In practice, that assumption is the most common source of error, so actual results may differ significantly.
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 are betting, not the dollar amount. Even with a perceived edge, betting a high percentage of your bankroll is risky because one loss could significantly impact your funds. This is why we recommend using fractional Kelly (1/4 or 1/10) instead of full Kelly.
Treat the Kelly recommendation as a suggested maximum stake, not a target. If you do not have a measurable betting edge, the correct Kelly stake is 0%, and the calculator will return no recommended bet.
What is a Kelly Criterion Calculator
A Kelly Criterion calculator is a betting tool that determines a suggested 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 a mathematical approach that aims to balance growth potential with risk management.
Our Kelly calculator helps serious sports bettors size their wagers based on mathematical principles rather than emotions. Whether you’re betting on NFL games, NBA spreads, or any sport where you believe you have an edge, this tool applies the Kelly Criterion sports betting approach to calculate a suggested stake based on your inputs.
Your edge comes from the difference between your estimated win probability and the market’s implied probability. If you’re unsure how sportsbooks price events, use our implied probability calculator before applying the Kelly formula.
How the Kelly Criterion Formula Works
The Kelly formula attempts to balance two competing goals: pursuing growth while managing the risk of significant losses.
The Kelly Formula Explained
f* = (bp – q) / b
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 a 45% chance, p = 0.45 and q = 0.55.
- Apply formula: f* = (1.50 x 0.45 – 0.55) / 1.50 = (0.675 – 0.55) / 1.50 = 0.0833.
- Result: suggests betting 8.33% of your bankroll at full Kelly (2.08% at quarter Kelly).
Kelly Criterion Examples for Sports Betting
Example 1: NFL Point Spread
| Scenario | Your Analysis | Bookmaker Odds | Full Kelly | Quarter Kelly |
|---|---|---|---|---|
|
Chiefs -3 |
55% cover probability (estimated) |
-110 (1.909 decimal) |
5.50% of bankroll |
1.38% of bankroll |
Example 2: NBA Underdog Moneyline
| Bet Type | Win Probability (Est.) | 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 | Estimated Edge | Full Kelly | Quarter Kelly | Action |
|---|---|---|---|---|
|
50% win @ -110 |
-2.4% (negative) |
0% (don’t bet) |
0% |
No bet |
|
45% win @ +150 |
+12.5% edge (estimated) |
8.33% |
2.08% |
Small bet |
|
25% win @ +500 |
+8.3% edge (estimated) |
10% |
2.50% |
Use fractional |
Why Most Bettors Use Quarter Kelly
Most experienced bettors prefer quarter Kelly because the full Kelly strategy assumes your estimated win probability is perfectly accurate. In sports betting, that probability is always an estimate, and even a small error can lead to staking far too much of your bankroll. Overestimating your edge by only a few percentage points can turn a sensible full Kelly recommendation into an unnecessarily aggressive bet.
Using fractional Kelly reduces that risk while still capturing much of the long-term benefit of Kelly staking. A quarter Kelly approach delivers roughly half the expected growth rate of full Kelly with approximately a quarter of the drawdown, making it a more practical choice for most bettors. Some bettors choose half Kelly when they have greater confidence in their probability estimates, but it still carries more volatility than quarter Kelly.
For these reasons, most disciplined sports bettors use quarter Kelly or an even smaller fraction. The Kelly Criterion Calculator shows both full Kelly and quarter Kelly recommendations so you can compare the suggested stake before placing a bet.
Fractional Kelly Benefits
Full Kelly can suggest very large bets (25%+ of bankroll), and even with a real edge, losing streaks can significantly impact your bankroll. Fractional Kelly lowers both the variance and the psychological strain.
| Kelly Fraction | Potential Growth Rate | Volatility | Potential Max Drawdown | Best For |
|---|---|---|---|---|
|
Full (100%) |
Highest potential |
Very High |
~95% |
Highly accurate models |
|
Half (50%) |
~75% of full |
High |
~50% |
Confident bettors |
|
Quarter (25%) |
~50% of full |
Moderate |
~25% |
Most bettors |
|
Tenth (10%) |
~25% of full |
Low |
~10% |
Risk-averse |
If you prefer a simpler bankroll management approach alongside fractional Kelly, try our bankroll calculator to plan consistent stake sizes.
Common Kelly Criterion Mistakes
1. Overestimating Your Edge
- Markets are often efficient – true edges may be rare and small.
- Sample size issues – short-term results don’t prove a long-term edge.
- Confirmation bias – remembering wins more than losses.
2. Ignoring Correlation
- Multiple bets on the same game.
- Similar bets across games (all favorites, all overs).
- Parlays and teasers.
3. Not Adjusting for Limits
- Maximum bet limits at sportsbooks.
- Minimum bet requirements.
- Available liquidity in betting markets.
One of the best ways to avoid these mistakes is to track your results over a large sample before increasing your Kelly fraction.
Advanced Kelly Criterion Strategies
Multiple Simultaneous Bets
When using Kelly bet sizing across multiple games, adjust your Kelly percentage to account for simultaneous positions and overall bankroll exposure. Divide the Kelly suggestion by the number of concurrent bets, account for correlation, and consider limiting total exposure to 25%.
Kelly with Rebates and Bonuses
Include promotional value in your edge calculations – cashback may reduce effective losses and reload bonuses may increase effective returns. Use our expected value calculator for complex scenarios, and compare current sportsbook promos to see which offers may affect the value of qualifying bets.
Dynamic Bankroll Management
- Recalculate after significant wins or losses.
- Consider reducing your fraction during losing streaks.
- Increase your fraction only with demonstrated long-term results.
Kelly Criterion vs Other Betting Systems
| System | Approach | Pros | Cons |
|---|---|---|---|
|
Kelly Criterion |
Math-based sizing |
Structured approach |
Requires accurate edge estimation |
|
Fixed Percentage |
Bet same % always |
Simple |
Ignores perceived value |
|
Fixed Amount |
Bet same $ always |
Very simple |
No adjustment for bankroll changes |
|
Martingale |
Double after loss |
None |
High risk of significant losses |
Related Calculators
Frequently Asked Questions
What is a Kelly Criterion calculator?
A Kelly Criterion calculator is a bet sizing tool that determines a suggested percentage of your bankroll to wager based on your estimated edge and the odds offered. Named after John Kelly Jr., it provides a mathematical approach to bet sizing that aims to balance growth potential and risk management.
What is the Kelly Criterion formula?
The Kelly Criterion formula calculates the optimal percentage of your bankroll to stake based on your estimated probability of winning and the odds offered. The formula is f* = (bp – q) / b, where b is the net decimal odds, p is your estimated win probability, and q is the probability of losing (1 – p). The result is the maximum suggested stake as a percentage of your bankroll when you believe you have a positive edge.
How do you calculate Kelly Criterion?
Using f* = (bp – q) / b, take 55% win probability at +110 odds (b = 1.10). f* = (1.10 x 0.55 – 0.45) / 1.10 = 0.1409, which is 14.09% of bankroll at full Kelly, or 3.52% at quarter Kelly.
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 a mathematical approach to betting. Many bettors use fractional Kelly because full Kelly can suggest very large bets when the formula indicates a significant edge.
How do I calculate the Kelly fraction?
Calculate the full Kelly stake using the Kelly Criterion formula, then apply the fraction you want to use. For example, if full Kelly recommends staking 8% of your bankroll, half Kelly would be 4% and quarter Kelly would be 2%. Most sports bettors use quarter Kelly because it provides a more conservative staking strategy while still benefiting from the Kelly approach.
Is the Kelly Criterion useful for sports betting?
The Kelly Criterion can be a useful framework for sports betting if you can reasonably estimate win probabilities. It provides a mathematical approach to bet sizing. However, most bettors should use fractional Kelly (25% or less) because it is very difficult to know true probabilities accurately, and overestimating your edge leads to overbetting.
What happens if you bet more than Kelly suggests?
Betting more than Kelly suggests (overbetting) can reduce long-term expected growth and significantly increase risk. While it may increase short-term profits when lucky, the increased volatility can lead to larger drawdowns. According to the mathematical model, betting 2x Kelly has the same expected growth rate as not betting at all.
Can Kelly Criterion work with negative expectation bets?
No, the Kelly Criterion only provides meaningful suggestions for positive expectation (+EV) bets based on your inputs. If your inputs show 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 indicates you should not bet.
Should beginners use the Kelly Criterion?
Beginners should understand Kelly Criterion concepts but use very conservative fractions (10% Kelly or less) if applying the formula. Many beginners overestimate their edge, which can lead to overbetting. Consider starting with fixed small bets, tracking results carefully, and only implementing Kelly after demonstrating consistent results over many bets.
What is the downside of Kelly Criterion?
The biggest downside of the Kelly Criterion is that it assumes your estimated win probability is accurate. In sports betting, that probability is only an estimate, so even a small error can lead to staking too much of your bankroll. That’s why most disciplined bettors use quarter Kelly or another fractional Kelly approach to reduce volatility and limit drawdowns.
How accurate is the Kelly Criterion?
The Kelly Criterion is mathematically accurate when your estimated win probability is correct. In practice, accurately estimating your edge is the difficult part. If your probability estimate is too optimistic, the Kelly recommendation will also be too aggressive. For that reason, many experienced bettors treat the Kelly output as a maximum stake rather than a target and use quarter Kelly or smaller stakes.