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How to Read Betting Odds: American, Decimal, and Fractional
Last Updated: February 17, 2026
Odds represent the probability of an outcome and the potential payout for a correct prediction. Three formats dominate globally: American odds (used in the US), decimal odds (used in Europe and Australia), and fractional odds (used in the UK). All three express the same underlying information — they just present it differently.
How Do American Odds Work?
American odds use a baseline of $100. Negative numbers indicate favorites; positive numbers indicate underdogs.
Negative odds (-150): You must wager $150 to win $100 in profit. Total return: $250 (stake + profit).
Positive odds (+200): A $100 wager wins $200 in profit. Total return: $300 (stake + profit).
The implied probability formula for American odds:
| Odds Type | Formula | Example | Implied Probability |
|---|---|---|---|
| Negative (-X) | X / (X + 100) | -150 | 150 / 250 = 60.0% |
| Positive (+X) | 100 / (X + 100) | +200 | 100 / 300 = 33.3% |
A standard US sportsbook line on a coin-flip event would be -110 on each side (not +100/+100). Each -110 implies a 52.4% probability, summing to 104.8%. The excess over 100% is the sportsbook’s margin, known as the vig or juice.
Our dataset across sportsbooks tracks how vig varies between platforms and markets. You can see these comparisons on the Odds Reference dashboard.
How Do Decimal Odds Work?
Decimal odds express total return (stake included) per unit wagered. Decimal odds of 2.50 mean a $1 wager returns $2.50 total ($1.50 profit + $1 stake).
Converting to implied probability is straightforward: divide 1 by the decimal odds.
| Decimal Odds | Implied Probability | $100 Profit |
|---|---|---|
| 1.50 | 66.7% | $50 |
| 2.00 | 50.0% | $100 |
| 2.50 | 40.0% | $150 |
| 3.00 | 33.3% | $200 |
| 5.00 | 20.0% | $400 |
Decimal odds are the most mathematically transparent format. They are directly comparable, easy to multiply for parlays, and trivially converted to probabilities. Most prediction market prices are effectively decimal odds expressed in cents — a $0.40 contract is equivalent to decimal odds of 2.50 (1 / 0.40).
How Do Fractional Odds Work?
Fractional odds, standard in the UK, express profit relative to stake. Odds of 3/2 mean $3 profit for every $2 wagered. Total return: $5 ($3 profit + $2 stake).
To convert fractional odds to implied probability: denominator / (numerator + denominator). For 3/2: 2 / (3 + 2) = 40%.
| Fractional | Decimal Equivalent | American Equivalent | Probability |
|---|---|---|---|
| 1/2 | 1.50 | -200 | 66.7% |
| 1/1 (evens) | 2.00 | +100 | 50.0% |
| 3/2 | 2.50 | +150 | 40.0% |
| 3/1 | 4.00 | +300 | 25.0% |
| 9/1 | 10.00 | +900 | 10.0% |
Fractional odds are becoming less common in digital platforms but remain standard at UK and Irish bookmakers and in horse racing.
How Do You Convert Between Formats?
The conversion chain runs through decimal odds as the common denominator:
American to Decimal:
- Negative: 1 + (100 / abs(American))
- Positive: 1 + (American / 100)
Decimal to American:
- If decimal < 2.00: -(100 / (decimal - 1))
- If decimal >= 2.00: (decimal - 1) * 100
Decimal to Fractional:
- (decimal - 1) expressed as a fraction, simplified
Prediction market price to any format:
- Decimal: 1 / price (e.g., $0.40 = 2.50)
- American: convert via decimal
- Implied probability: the price itself ($0.40 = 40%)
This is why prediction market pricing is the most intuitive format for probability assessment. A prediction market contract at $0.65 communicates probability instantly. American odds of -186 require calculation to reach the same 65% figure.
What Is the Vig and Why Does It Matter?
The vig (vigorish, also called juice or overround) is the sportsbook’s built-in profit margin. It is the difference between the true probability of outcomes and the implied probabilities from the offered odds.
A fair coin flip would be priced at +100/+100 (50%/50%, summing to 100%). A sportsbook prices it at -110/-110 (52.4%/52.4%, summing to 104.8%). That 4.8% is the vig.
Our analysis across sportsbook data shows vig ranges from about 3% on major NFL spread markets to 10% or more on props and exotic wagers. Prediction markets carry analogous costs in the form of bid-ask spreads, though typically narrower than sportsbook vig on comparable events.
Understanding the vig is essential for evaluating whether you are getting a fair price. The Odds Reference dashboard displays effective vig/spread across platforms so you can compare costs directly.
Key Takeaways
- American odds use +/- notation around a $100 baseline; decimal odds show total return per unit; fractional odds show profit relative to stake
- All three formats encode the same information: implied probability and potential payout
- Prediction market cent-prices are the most transparent probability format — $0.65 = 65%, no conversion needed
- The vig/spread is the hidden cost: sportsbooks embed 3-10%, prediction markets typically carry 1-5% in spread
- Always convert odds to implied probability before assessing value