Fair Odds Calculator
Convert probability percentages to fair betting odds. Calculate what the odds should be without the bookmaker's margin to understand true value in betting markets.
Free Fair Odds Calculator
Table of Contents
How to Use the Fair Odds Calculator
Our fair odds calculator converts probability percentages into fair odds across all formats. This helps you understand what the odds should be in a perfectly balanced market without the bookmaker's margin. Compare these to actual odds using our betting odds calculator.
Enter Probability
Enter the probability of the outcome occurring as a percentage. This should be your assessed true probability based on analysis, research, or models. For example, if you believe an outcome has a 60% chance, enter 60.
View Fair Odds
The calculator instantly displays the fair odds in all three formats: American (+150, -200), Decimal (2.50, 1.50), and Fractional (3/2, 1/2). These are the odds that would exist in a fair market without any bookmaker margin.
Compare to Bookmaker Odds
Compare the calculated fair odds to the bookmaker's actual odds. If the bookmaker's odds are better than the fair odds, you've found value. If they're worse, the bet has negative expected value.
Use for Value Betting
Fair odds help you identify value betting opportunities. If you assess a 40% probability but the bookmaker offers odds better than the fair odds for 40%, you have positive expected value and should consider the bet.
What are Fair Odds?
Fair odds are the betting odds that would exist in a perfectly balanced market without any bookmaker margin or vigorish. They represent the true probability of an outcome, where the fair odds directly and accurately reflect the likelihood of the event occurring.
In a fair market, if an outcome has a 50% probability, the fair odds would be +100 (even money) or 2.00 decimal. If an outcome has a 25% probability, the fair odds would be +300 or 4.00 decimal. Fair odds are what the odds should be if there were no built-in profit margin for the bookmaker. Use our fair odds calculator to convert any probability to fair odds.
Understanding fair odds is essential for value betting. By comparing bookmaker odds to fair odds, you can identify situations where the bookmaker is offering better odds than they should based on the true probability, creating positive expected value opportunities. Use our odds value calculator to determine if a bet offers positive expected value, and our implied probability calculator to see what bookmaker odds really mean.
Fair Market
In a fair market, the sum of all possible outcomes' probabilities equals exactly 100%. There's no built-in margin, and the odds accurately reflect the true likelihood of each outcome. This is what fair odds represent—the theoretical odds in a perfectly balanced market.
Real Bookmaker Markets
In real betting markets, bookmakers build in a margin (vigorish) to ensure profit. This means the sum of implied probabilities exceeds 100%, typically by 5-10%. Bookmaker odds are less favorable than fair odds, giving the bookmaker a built-in edge.
How to Calculate Fair Odds
The formula for calculating fair odds from probability depends on the odds format. Here are the formulas for each format:
From Probability to Decimal Odds
Decimal Odds = 1 / (Probability / 100)
Or more simply: Decimal Odds = 100 / Probability
Example: 40% probability
Decimal Odds = 100 / 40 = 2.50
From Probability to American Odds
If Probability < 50%:
American Odds = +((100 / Probability) - 100)
Example: 40% probability
American Odds = +((100 / 40) - 100) = +(2.50 - 100) = +150
If Probability > 50%:
American Odds = -((Probability / (100 - Probability)) × 100)
Example: 60% probability
American Odds = -((60 / (100 - 60)) × 100) = -((60 / 40) × 100) = -150
From Probability to Fractional Odds
Fractional Odds = (100 - Probability) / Probability
Example: 40% probability
Fractional Odds = (100 - 40) / 40 = 60 / 40 = 3/2
Example: 60% probability
Fractional Odds = (100 - 60) / 60 = 40 / 60 = 2/3
Fair Odds vs Bookmaker Odds
Understanding the difference between fair odds and bookmaker odds is crucial for identifying value betting opportunities. These two concepts represent different things in the betting market.
Fair Odds
Fair odds represent the true probability without any margin. They're what the odds should be in a perfectly balanced market. Fair odds directly reflect the likelihood of an outcome, with all probabilities summing to exactly 100%. They represent the theoretical "true" odds.
Bookmaker Odds
Bookmaker odds include a built-in profit margin (vigorish). They're typically 5-10% less favorable than fair odds to ensure the bookmaker profits. The sum of implied probabilities from bookmaker odds exceeds 100%, with the excess being the bookmaker's edge.
Finding Value
When bookmaker odds are better than fair odds, you've found value. For example, if fair odds for a 40% probability are +150, but the bookmaker offers +160, you have positive expected value. The bookmaker is offering better odds than they should based on the true probability.
Example Comparison
True probability: 50%
Fair odds: +100 (2.00 decimal)
Bookmaker odds: -110 (1.91 decimal)
The bookmaker's odds are less favorable, giving them a built-in edge. You'd need to win 52.38% of bets at -110 to break even, but the true probability is only 50%.
Fair Odds Examples
Example #1: Even Probability
An outcome has a 50% probability of occurring. What are the fair odds?
Decimal: 100 / 50 = 2.00
American: +((100 / 50) - 100) = +100 (even money)
Fractional: (100 - 50) / 50 = 50 / 50 = 1/1
In a fair market, a 50/50 outcome should pay even money. If a bookmaker offers -110, they're taking a margin.
Example #2: Underdog
An underdog has a 30% probability of winning. What are the fair odds?
Decimal: 100 / 30 = 3.33
American: +((100 / 30) - 100) = +(3.33 - 100) = +233
Fractional: (100 - 30) / 30 = 70 / 30 = 7/3
If a bookmaker offers better than +233 for this outcome, you've found value. If they offer worse, the bet has negative expected value. Use our odds value calculator to determine if a bet offers positive expected value.
Example #3: Favorite
A favorite has a 70% probability of winning. What are the fair odds?
Decimal: 100 / 70 = 1.43
American: -((70 / (100 - 70)) × 100) = -((70 / 30) × 100) = -233
Fractional: (100 - 70) / 70 = 30 / 70 = 3/7
Fair odds for a 70% favorite are -233. If a bookmaker offers -200, they're giving you better odds than fair, creating value. If they offer -250, the bet has negative expected value. Check value with our odds value calculator.
Example #4: Using Fair Odds for Value Betting
You assess that a team has a 45% chance of winning. The fair odds for 45% are +122 (2.22 decimal).
You check the bookmaker and they're offering +150 (2.50 decimal).
Since +150 is better than the fair odds of +122, you've found value. The bookmaker is offering better odds than they should based on the true probability, giving you positive expected value. Verify this with our odds value calculator, and determine optimal bet sizing with our Kelly Criterion calculator.
Frequently Asked Questions About Fair Odds
Fair odds are the betting odds that would exist in a perfectly balanced market without any bookmaker margin. They represent the true probability of an outcome, where the odds directly reflect the likelihood of the event occurring. Fair odds don't include the bookmaker's built-in profit margin (vigorish), so they're what the odds should be in a fair market. For example, a 50% probability has fair odds of +100 (even money) or 2.00 decimal. Use our fair odds calculator to convert any probability to fair odds, and compare to actual odds using our odds value calculator.
To calculate fair odds from probability: For decimal odds, divide 100 by the probability (100 / probability). For American odds: if probability < 50%, use +((100 / probability) - 100). If probability > 50%, use -((probability / (100 - probability)) × 100). For fractional odds, use (100 - probability) / probability. Our calculator does this automatically, so you just enter the probability and get all formats.
Fair odds represent the true probability without any margin, while bookmaker odds include a built-in profit margin (vigorish). Bookmaker odds are typically 5-10% less favorable than fair odds to ensure the bookmaker profits. For example, if fair odds for a 50% probability are +100, bookmaker odds might be -110, giving the bookmaker a built-in edge. The bookmaker's margin ensures they profit regardless of the outcome.
Use fair odds to identify value betting opportunities. Calculate the fair odds based on your assessed probability, then compare them to the bookmaker's actual odds. If the bookmaker's odds are better than the fair odds, you've found value with positive expected value. If they're worse, the bet has negative expected value. For example, if fair odds for 40% are +150 but the bookmaker offers +160, you have value.
Bookmaker odds differ from fair odds because bookmakers build in a profit margin (vigorish) to ensure they make money regardless of the outcome. This margin is typically 5-10% and is built into the odds by making them slightly less favorable than fair odds. For example, where fair odds might be +100, bookmaker odds might be -110, giving the bookmaker a built-in edge. This is how bookmakers ensure profitability in the long run.
Yes, fair odds are essential for finding value bets. By calculating fair odds based on your assessed probability and comparing them to bookmaker odds, you can identify situations where the bookmaker is offering better odds than they should. When bookmaker odds are better than fair odds, you have positive expected value. This is the foundation of value betting and profitable long-term betting strategy. However, you need accurate probability assessments for this to work.
