What Odds Actually Represent
Every set of odds encodes two things: the payout you receive if you win and the implied probability the market assigns to that outcome. American, decimal, and fractional formats are simply three notations for the same underlying number - like writing the same temperature in Fahrenheit and Celsius.
Once you can move fluently between formats and read the probability behind a price, you can compare offers across sportsbooks and spot when a line is priced in your favor.
American Odds
American (or moneyline) odds are centered on $100. Negative numbers mark favorites; positive numbers mark underdogs.
Negative (-150): stake $150 to win $100 profit
Positive (+150): stake $100 to win $150 profit
A price of -110 - the standard "vig" on a point spread - means you risk $110 to win $100. The extra $10 is the bookmaker's margin.
Decimal Odds
Decimal odds show your total return per $1 staked, including your stake. They are the easiest format for parlays because you simply multiply the legs together.
Total return = stake x decimal odds
$100 at 2.50 returns $250 total ($150 profit)
Fractional Odds
Fractional odds (common in the UK and horse racing) show profit relative to stake. 3/2 means you win $3 for every $2 staked.
Profit = stake x (numerator / denominator)
$100 at 3/2 wins $150 profit
Conversion Formulas
American -> Decimal (favorite): decimal = 1 + 100 / |american|
American -> Decimal (underdog): decimal = 1 + american / 100
Decimal -> American (>= 2.00): (decimal - 1) x 100
Decimal -> American (< 2.00): -100 / (decimal - 1)
Implied probability = 1 / decimal
Rather than doing this by hand, drop any price into the Odds Converter and it returns all three formats plus the implied probability.
Quick Reference Table
| American | Decimal | Fractional | Implied Prob. |
|---|---|---|---|
| -200 | 1.50 | 1/2 | 66.7% |
| -150 | 1.67 | 2/3 | 60.0% |
| -110 | 1.91 | 10/11 | 52.4% |
| +100 | 2.00 | 1/1 | 50.0% |
| +150 | 2.50 | 3/2 | 40.0% |
| +250 | 3.50 | 5/2 | 28.6% |
From Odds to Value: Implied Probability
Implied probability is the win chance baked into a price. If you believe the true chance is higher than the implied number, the bet has positive expected value. Because the two sides of a market add up to more than 100% (that overround is the vig), you should remove the margin first to see the fair price.
Learn the mechanics in the Calculating the Vig guide, then apply it with the True Odds Calculator.