How to Convert Odds to Probability: Step-by-Step Guide
Converting odds to probability is the foundational skill that separates analytical sports forecasting from guesswork. Once you can translate any odds number into a probability, you can compare the market's belief against your own, identify where prices may be mispriced, and track your calibration over time. This guide walks through every format: American, decimal, and fractional, and explains how to strip out the bookmaker's margin to find fair probabilities.
Why Probability Conversion Matters
Odds are not just a payout guide. They are a probability statement in disguise. When a sportsbook posts -180 on a team, it is communicating that the market believes that team wins approximately 64% of the time. If you believe the true probability is higher, the selection is analytically favorable. If you believe it is lower, the price is too short and the selection is unfavorable regardless of how “safe” the team seems.
This kind of thinking, comparing market-implied probability to your own assessment, is called expected value analysis. This is the framework used by professional sports analysts and is exactly the kind of skill that simulation platforms like OwnTheLines are designed to help you develop through regular practice.
Before reading this guide, make sure you understand the basics of how odds are formatted. See our American odds explained guide if you need a foundation in reading the numbers.
Converting American Odds
American odds use two different formulas depending on whether the number is negative (favorite) or positive (underdog):
For Negative Odds (Favorites)
-110: 110 ÷ (110 + 100) = 110 ÷ 210 = 52.38%
-150: 150 ÷ (150 + 100) = 150 ÷ 250 = 60.00%
-300: 300 ÷ (300 + 100) = 300 ÷ 400 = 75.00%
For Positive Odds (Underdogs)
+110: 100 ÷ (110 + 100) = 100 ÷ 210 = 47.62%
+200: 100 ÷ (200 + 100) = 100 ÷ 300 = 33.33%
+350: 100 ÷ (350 + 100) = 100 ÷ 450 = 22.22%
Converting Decimal Odds
Decimal odds make probability conversion the simplest of any format: just divide 1 by the odds:
1.50: 1 ÷ 1.50 = 66.67%
2.00: 1 ÷ 2.00 = 50.00%
3.50: 1 ÷ 3.50 = 28.57%
This is one reason decimal odds are popular with analytically minded bettors. The probability math is immediate and requires no sign-based branching.
Converting Fractional Odds
Fractional odds like 5/2 express profit relative to stake. To convert to probability:
5/2: 2 ÷ (5 + 2) = 2 ÷ 7 = 28.57%
1/2: 2 ÷ (1 + 2) = 2 ÷ 3 = 66.67%
7/4: 4 ÷ (7 + 4) = 4 ÷ 11 = 36.36%
The Overround and the Vig
When you calculate implied probability for both sides of a game and add them together, the total will almost always exceed 100%. That excess is the overround, the bookmaker's built-in margin.
Example: Standard -110 / -110 Market
Side A at -110: 52.38%
Side B at -110: 52.38%
Total: 104.76%, the 4.76% excess is the overround.
A lower overround is better for the bettor and means more competitive pricing. U.S. mainstream books typically run 4–5% overround on sides and totals, while sharp offshore books can be as low as 2%.
Calculating No-Vig Fair Probability
Stripping the vig gives you the market's best estimate of the “true” probability for each side. The formula is simple:
Continuing the example above: each side's vig-free probability is 52.38% ÷ 1.0476 = 50.00% which confirms the -110 / -110 market treats the game as a true coin flip after the vig is removed.
Example: Asymmetric Market
Favorite: -175 → implied 63.64% | Underdog: +155 → implied 39.22%
Total overround: 63.64% + 39.22% = 102.86%
Fair probability, Favorite: 63.64 ÷ 102.86 = 61.87%
Fair probability, Underdog: 39.22 ÷ 102.86 = 38.13%
This tells you the market believes, net of the vig, that the favorite wins about 62% of the time and the underdog about 38%. Those are the numbers you should compare against your own analysis when deciding which side to select.
Applying This to OwnTheLines Selections
When you view a game on OwnTheLines, the market lines displayed come from the same live data sources used by major sports platforms. Before you make a selection, you can now do the leg work yourself: convert the displayed odds to implied probability, strip the vig, and ask whether your own model or intuition suggests a materially different probability.
Over a full season of selections, the players who approach each pick analytically, rather than just following public popularity or team reputation, tend to show markedly different standings trajectories than those who pick at random. The math is on your side when you understand it.
For context on the three market types you will encounter on the platform, visit our overview of how sports odds work.
Frequently Asked Questions
How do you convert American odds to implied probability?
For negative odds: implied probability = |odds| / (|odds| + 100). For positive odds: implied probability = 100 / (odds + 100). For example, -200 converts to 200/300 = 66.7%, and +150 converts to 100/250 = 40%.
How do you convert decimal odds to probability?
Divide 1 by the decimal odds number. For example, decimal odds of 2.50 convert to 1/2.50 = 40% implied probability.
What is the no-vig fair probability?
The no-vig (or fair) probability is the estimated true probability of an outcome after removing the bookmaker's margin. You calculate it by dividing each side's implied probability by the total over-round (the sum of all implied probabilities). This gives a cleaner estimate of the market's true belief.
Why do the implied probabilities from both sides of a bet add up to more than 100%?
Because the sportsbook adds a margin (called the vig or juice) to both sides. This ensures total implied probability is above 100%, securing the bookmaker a profit regardless of the game result. The excess above 100% represents the edge the house holds.
What is overround in sports betting?
Overround (also called the book) is the total of all implied probabilities in a market, expressed as a percentage above 100%. A 104% overround means the market has a 4% built-in margin for the bookmaker.