Look at any sportsbook and you'll see three different ways the same bet can be priced. Same matchup, same outcome, three different-looking numbers. Once you understand what those numbers mean, you can compare prices instantly and never get fooled by an "attractive" price that's actually no better than fair.
The three formats
American odds (used in the US)
American odds use either a positive or negative number, always benchmarked to $100.
Negative numbers tell you how much you must risk to win $100. -110 means you risk $110 to win $100. The favorite, in other words. The bigger the negative number, the heavier the favorite.
Positive numbers tell you how much you win on a $100 bet. +250 means a $100 wager wins $250 in profit. The underdog. The bigger the positive number, the longer the shot.
Decimal odds (used everywhere except the US)
Decimal odds tell you what $1 returns including your stake. 1.91 means $1 returns $1.91 — your $1 stake plus $0.91 in profit.
Decimal is the cleanest format for math. To find the implied probability, just divide 1 by the decimal odds: 1 / 1.91 = 52.4%.
Fractional odds (used mostly in horse racing and the UK)
Fractional odds describe profit-to-stake. 10/11 means risk $11 to win $10. 5/2 means risk $2 to win $5.
Slightly less popular than it once was, but you'll still see it on horse-racing tote boards and in UK soccer markets.
Why implied probability is the only number that matters
Forget which format the price is shown in. Convert every odds line to implied probability and you can immediately compare apples to apples.
Implied probability = 1 / decimal odds × 100
| American | Decimal | Fractional | Implied prob. |
|---|---|---|---|
| -200 | 1.50 | 1/2 | 66.67% |
| -110 | 1.91 | 10/11 | 52.38% |
| +100 (even) | 2.00 | 1/1 | 50.00% |
| +150 | 2.50 | 3/2 | 40.00% |
| +250 | 3.50 | 5/2 | 28.57% |
| +500 | 6.00 | 5/1 | 16.67% |
Why the implied probabilities don't add to 100%
If you add the implied probabilities of both sides of a bet, you'll get something like 105% or 110%. That's the "vig" or "juice" — the bookmaker's commission. A standard -110 / -110 market on both sides has 4.76% built-in margin. The book is asking the market to pay 105% in implied probability for two outcomes that, in fact, must total exactly 100%.
Your job as a bettor is to find spots where the true probability of an outcome is higher than the implied probability the book is offering. That's the entire game.
Try it: Open our odds converter in another tab. Type any odds in any format and see all four representations instantly.
Common mistakes
- Confusing -110 and -120. They look almost identical on a screen. -120 is a 4.5% worse price than -110. Across thousands of bets, this destroys recreational bettors who don't line shop.
- Treating +200 as 50/50. +200 is a 33% implied probability — the book is saying this team wins 1 in 3 times. If you think it's actually 50/50, that's massive value. If you think it's 25%, you should pass.
- Chasing big-number boosts. A "boost" from +200 to +250 is a 17% increase. A "boost" from -110 to -100 is a 4.5% improvement. The boosts that look smaller are often the most valuable.
Final thought
Mastering odds isn't about memorizing conversion tables — it's about training your brain to immediately think "What probability is this price implying?" Once you can do that, every sportsbook becomes legible. You'll see overlays and traps the casual bettor doesn't notice.
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