How betting odds work
Every price tells you two things: what the bet pays and how likely the book thinks it is. Once you can read both, you can compare any two prices, at any book, in any format.
American odds
Plus numbers show what you win on a $100 bet. +150 means a $100 bet wins $150. That side is the underdog.
Minus numbers show what you have to bet to win $100. −110 means you risk $110 to win $100. That side is the favorite.
+100 is even money: bet $100, win $100. To get the payout on any amount, multiply. $50.00 at +240 wins $120.00 and pays back $170.00 including your stake.
Decimal and fractional odds
Decimal odds are the total you get back per $1, stake included. 2.50 pays $2.50 for every $1, so it’s the same as +150. Fractional odds, common in the UK, show winnings over stake: 3/2 means you win $3 for every $2.
| American | Decimal | Fractional | Implied chance |
|---|---|---|---|
| +150 | 2.50 | 3/2 | 40% |
| −110 | 1.91 | 10/11 | 52.38% |
| +100 | 2.00 | 1/1 | 50% |
| −150 | 1.67 | 2/3 | 60% |
| −200 | 1.50 | 1/2 | 66.67% |
| +250 | 3.50 | 5/2 | 28.57% |
| +300 | 4.00 | 3/1 | 25% |
Implied probability
Divide 1 by the decimal odds and you get the chance the price implies. −110 is 1 ÷ 1.909, or 52.38%. That’s also how often the bet has to win for you to break even over time.
Where the vig hides
Look at a game priced −110 on both sides. Each side implies 52.38%. Add them up and you get 104.76%, not 100%. The extra is the book’s margin, called the vig or juice. On this market the book expects to keep 4.55% of the money bet.
Take the vig out and each side is a fair 50%. Comparing a price with its no-vig version shows you how much you’re paying for the bet. The lower the total over 100%, the better the prices.
Prediction market prices
Prediction markets like Kalshi and Polymarket price sports contracts in cents. A 63¢ contract pays $1 if it wins, so the market is saying 63%. As odds that’s 1 ÷ 0.63, or 1.59 decimal, about −170. Their fees are separate, so compare after fees when you shop prices. ArbCity’s calculators take these prices as a percentage, like 63%.
Converting by hand
Plus American to decimal: 1 + odds ÷ 100 (+150 → 2.50) Minus American to decimal: 1 + 100 ÷ |odds| (−110 → 1.909) Decimal 2.00 or more to American: (decimal − 1) × 100 (2.50 → +150) Decimal under 2.00 to American: −100 ÷ (decimal − 1) (1.909 → −110)
Or skip the math and type any format into the Odds Converter. It shows the rest as you type.
Why the same game has different prices
Every sportsbook sets its own lines. Some move faster on news, some shade the side their customers like, and some run promos. The result is that the same team can be −110 at one book and +100 at another. Over a season, betting the better number on every bet is one of the few edges anyone can get.
Quick rules of thumb
- The bigger the plus number, the bigger the underdog.
- The bigger the minus number, the bigger the favorite.
- Two prices on the same game that add up to under 100% are an arbitrage.
- Half a point on a spread or a few cents on a price adds up over hundreds of bets. Shop for the best number.
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