Odds do two jobs. They tell you how much a bet pays, and they tell you how likely the market thinks an outcome is. Once you can read both, every line makes more sense.
American odds
American odds are built around 100.
- A minus number is how much you must stake to win 100. At -150, a 150 bet wins 100.
- A plus number is how much a 100 stake wins. At +150, a 100 bet wins 150.
The minus side is the favorite and the plus side is the underdog.
Decimal odds
Decimal odds show the total returned for every 1 staked, including your stake. At 2.50, a 10 bet returns 25, which is 15 profit plus your 10 back. Anything below 2.00 is a favorite.
Fractional odds
Fractional odds show profit against stake. At 3/2, you win 3 for every 2 staked. At 1/2, you win 1 for every 2 staked.
The same prices in every format
| American | Decimal | Fractional | Implied probability |
|---|---|---|---|
| -200 | 1.50 | 1/2 | 66.7% |
| -110 | 1.91 | 10/11 | 52.4% |
| +100 | 2.00 | 1/1 | 50.0% |
| +150 | 2.50 | 3/2 | 40.0% |
| +300 | 4.00 | 3/1 | 25.0% |
Turning odds into a probability
Implied probability is the win rate at which a bet breaks even.
- Decimal: divide 1 by the odds. 1 divided by 2.50 is 0.40, or 40%.
- American, minus: divide the number by itself plus 100. For -150, that is 150 divided by 250, or 60%.
- American, plus: divide 100 by the number plus 100. For +150, that is 100 divided by 250, or 40%.
- Fractional: divide the right-hand number by the sum of both. For 3/2, that is 2 divided by 5, or 40%.
The margin in the price
Add up the implied probabilities for every outcome in a market and the total comes to more than 100%. The extra is the bookmaker's margin, often called the vig or juice.
The standard example is a point spread priced at -110 on both sides. Each side implies 52.4%, and together they make 104.8%. The 4.8% over the top is the margin. It is also why a bettor at -110 has to win more than 52.4% of the time just to break even.
What "value" means
A bet has value when your own estimate of the chance is higher than the implied probability in the price. If you make a team 45% to win and the odds are +150, which implies 40%, the price is in your favor. If the odds are +110, which implies 47.6%, it is not, however much you like the team.
Prices differ between books, and half a point or a few cents of price adds up over a season. Comparing lines before you bet is the simplest edge available to anyone.