How do betting odds work?

American odds are written as a plus or minus number, and every price is also a probability in disguise. A minus price, like -150, is the favorite: risk that many dollars to win 100. A plus price, like +150, is the underdog: risk 100 to win that many. The gap between the chance a price implies and the real chance is the difference between a good bet and a bad one.

Written by Jesse, NegativeEV. Last updated 12 August 2026.

What do the plus and minus prices mean?

A minus price is the favorite. It says what you must risk to win 100: at -150 you risk 150 to win 100.

A plus price is the underdog. It says what you win on a 100 risk: at +150 you win 150.

The bigger the minus, the heavier the favorite. The bigger the plus, the longer the shot. Nothing about the number says whether the bet is worth taking, only what it pays.

How do odds translate into a probability?

Every price carries an implied win probability.

A -150 favorite implies about 60 percent, because it has to win that often just to break even. A +150 underdog implies about 40 percent.

Add the two sides of a real market and they sum to more than 100 percent. A game priced -110 on both sides implies about 52 percent each, or roughly 105 percent together. That extra 5 percent is the rake, the book's built-in margin.

How do you know if a sportsbook line is fair?

Compare the probability the price implies to an independent estimate of the true probability.

A line is fair when those two numbers agree. It is unfair when the implied chance sits above the real one, and that is the normal case, because the rake is built in before anything else.

A second sportsbook is not an independent estimate. Books shade the same way, so a consensus of prices can be wrong together.

The same bet also carries a different price at every book, and the gap between books, measured puts a number on what the best one is worth.

Why does the implied probability matter?

Because the price only pays off if the real chance beats the chance the price implies.

A +150 underdog is a good bet only if it actually wins more than 40 percent of the time. A -150 favorite is a bad one if it wins less than 60 percent.

Reading the payout is easy. Judging whether the true probability clears the implied one is the part that decides whether you make money, and it is the part most bettors guess at.

How can you check the odds on a real bet?

Compute the true win probability from a simulation of the actual game, then compare it to the probability the price implies.

NegativeEV simulates each game thousands of times, play by play, and reports the true probability and expected value for any bet. Ten free checks a day, unlimited when signed in. When the real chance comes in below what the odds imply, the bet is negative EV, no matter how good the number looks.

What do odds not tell you?

They do not separate what the book thinks from what the crowd already bet.

A posted price mixes the book's margin, its own estimate and the money taken so far, and none of that can be unpicked from the number alone. A bet that cannot be matched to a simulated game comes back ungraded rather than guessed at.