How Sports Betting Odds Work: American, Decimal, and Fractional Explained
Every sportsbook shows odds in one of three formats: American, decimal, or fractional. They all describe the exact same thing — how much you can win, and how likely the book thinks an outcome is — just written differently. Once you can read one format, the others take about five minutes to learn.
American odds (the US standard)
American odds are written as a number with a plus or minus sign, always relative to a $100 bet.
- Negative numbers (e.g., -150): how much you'd need to bet to win $100. A -150 favorite means betting $150 wins you $100 profit (plus your $150 back).
- Positive numbers (e.g., +130): how much you'd win on a $100 bet. A +130 underdog means betting $100 wins you $130 profit (plus your $100 back).
You don't have to bet exactly $100 — the ratio scales. A $20 bet at +130 wins $26 profit; a $20 bet at -150 wins $13.33 profit.
Decimal odds (common outside the US)
Decimal odds show your total payout — stake included — per $1 wagered. A decimal of 2.30 means a $10 bet returns $23 total ($13 profit plus your original $10). Decimal odds are popular because the math is simple: payout = stake × decimal odds.
Fractional odds (common in the UK, especially horse racing)
Fractional odds like 5/2 mean you win $5 for every $2 wagered (plus your stake back). A $20 bet at 5/2 wins $50 profit.
Prefer not to do the math by hand? Our free odds calculator converts between all three formats and shows implied probability and payout instantly.
Quick conversion table
-150
Decimal: 1.67 · Fractional: 2/3
+130
Decimal: 2.30 · Fractional: 13/10
-110
Decimal: 1.91 · Fractional: 10/11
+200
Decimal: 3.00 · Fractional: 2/1
What odds tell you about probability
Odds aren't just about payout size — they encode the sportsbook's estimate of how likely an outcome is. This is called implied probability. For American odds:
- Favorite (negative): implied probability = odds ÷ (odds + 100), e.g. -150 → 150/250 = 60%
- Underdog (positive): implied probability = 100 ÷ (odds + 100), e.g. +130 → 100/230 ≈ 43.5%
Notice those two numbers add up to more than 100% (60% + 43.5% = 103.5%). That extra 3.5% is the sportsbook's built-in margin, often called the vig or juice — it's how books make money regardless of who wins. Recognizing the vig is the first step toward evaluating whether a bet is actually good value, not just whether a team is likely to win.
Next, put this into practice with our guide to bankroll management, or see how odds apply across different bet types like spreads and parlays.
Frequently asked questions
What's the fastest way to tell if a bet is a favorite or an underdog?
Look at the sign. A minus sign (e.g., -150) means favorite; a plus sign (e.g., +130) means underdog. In decimal odds, anything below 2.00 is a favorite and anything above 2.00 is an underdog.
Why do American odds always reference $100?
It's just a scaling convention that makes the numbers easy to compare at a glance. You're never required to bet exactly $100 — the same ratio applies to any stake.
Is decimal or American odds better for a beginner?
Neither is mathematically better. Decimal odds are arguably easier to calculate total payout from (stake × decimal odds), while American odds are the default on virtually every US sportsbook, so it's worth being comfortable with both.
Do sportsbooks ever offer true, no-vig odds?
Rarely on standard markets — the vig is how books make money. Some promotional "no-vig" or boosted odds exist temporarily as marketing offers, not as the everyday baseline.