Betting odds do two jobs at once. They tell you what the market thinks the chance of an outcome is, and they tell you what you get paid when you're right. American odds (-150, +200), decimal odds (1.67, 3.00) and fractional odds (2/3, 2/1) are three ways of writing the same number — the format changes with the country and the sportsbook, the information does not.
Convert any price into a percentage and you can compare a US app against a European book against a prediction market in the same breath. That percentage is also where the sportsbook hides its cut.
What do the plus and minus signs mean in betting odds?
American odds are built around a $100 reference bet, and the sign tells you which side of that reference you're on.
- Minus odds mark the favourite. The number is what you must stake to profit $100. At -150 you risk $150 to win $150 + $100 back. At -300 you risk $300 to win $100.
- Plus odds mark the underdog. The number is what you profit on a $100 stake. +200 means a $100 bet returns $300 total — $200 profit plus your stake.
- Even money appears as -100, +100 or "EV". A winning $100 bet returns $200.
Negative odds are the format's only real quirk: the number gets bigger as the team gets more likely. -110 is close to a coin flip. -900 is a heavy favourite paying $11.11 on a $100 stake.
Decimal odds show your total return, not your profit
Decimal odds are the standard in Europe, Australia and most of Asia, and they include your stake in the number. Multiply your stake by the decimal and you have the full amount coming back.
A $10 bet at 2.50 returns $25: $15 profit plus the $10 you risked. A $10 bet at 1.40 returns $14, so $4 profit.
Two shortcuts worth memorising:
- Anything below 2.00 is a favourite; anything above 2.00 is an underdog. 2.00 is even money.
- Profit = (decimal − 1) × stake. The "− 1" is your own stake coming back.
Fractional odds show profit per unit staked
Fractional odds are the UK and Irish convention and still dominate horse racing everywhere. They describe profit relative to stake, so the stake is never included in the fraction.
- 5/2 ("five to two") pays $5 profit for every $2 staked. A $10 bet profits $25 and returns $35.
- 7/1 ("seven to one") pays $7 per $1. A $10 bet profits $70 and returns $80 — an implied chance of 12.5%.
- 8/13 is an odds-on favourite: stake $13 to profit $8. Implied chance 61.9%.
American, decimal and fractional conversion table
| American | Decimal | Fractional | Implied probability | $100 stake returns |
|---|---|---|---|---|
| -300 | 1.33 | 1/3 | 75.0% | $133.33 |
| -200 | 1.50 | 1/2 | 66.7% | $150.00 |
| -150 | 1.67 | 2/3 | 60.0% | $166.67 |
| -110 | 1.91 | 10/11 | 52.4% | $190.91 |
| +100 | 2.00 | 1/1 | 50.0% | $200.00 |
| +150 | 2.50 | 3/2 | 40.0% | $250.00 |
| +200 | 3.00 | 2/1 | 33.3% | $300.00 |
| +300 | 4.00 | 3/1 | 25.0% | $400.00 |
| +700 | 8.00 | 7/1 | 12.5% | $800.00 |
Total return includes your original stake. Implied probability rounded to one decimal place.
What implied probability actually tells you
Implied probability is the break-even win rate a price demands. It is not the true chance of the event — it's the sportsbook's number with its margin already folded in.
- Negative American odds: probability = |odds| ÷ (|odds| + 100). So -150 → 150 ÷ 250 = 60%.
- Positive American odds: probability = 100 ÷ (odds + 100). So +200 → 100 ÷ 300 = 33.3%.
- Decimal: probability = 1 ÷ decimal. So 2.50 → 40%.
- Fractional: probability = denominator ÷ (numerator + denominator). So 5/2 → 2 ÷ 7 = 28.6%.
Two -110 lines add up to 104.8%, and that gap is the vig
A fair coin-flip market would price both sides at +100 — 50% and 50%, adding to 100%. Sportsbooks instead post -110 on both sides of a spread or total. Each side implies 52.4%, and the two together imply 104.8%.
That extra 4.8 points is the overround, known as the vig or juice. On a perfectly balanced book the sportsbook keeps 4.8 ÷ 104.8 = 4.6% of everything wagered, win or lose.
Stripping it out is simple division. Take a moneyline of -140 / +120:
- Convert both: -140 → 58.3%, +120 → 45.5%.
- Add them: 103.8%.
- Divide each by the total: 58.3 ÷ 103.8 = 56.2%, and 45.5 ÷ 103.8 = 43.8%.
Those devigged numbers are the market's honest opinion. Compare your own estimate against them, not against the posted price, or you'll credit yourself with an edge that is really just the sportsbook's commission.
Shopping -110 down to -105 moves your break-even by a full point
Margin varies between books on the identical bet, and the difference is larger than it looks. At -110 you need to win 52.4% of your bets to break even. At -105 you need 51.2%.
On a $100 stake, -110 profits $90.91 and -105 profits $95.24. That $4.33 gap arrives on every single winner. Across 50 winning bets it's over $200 that required no extra handicapping — only checking a second price before clicking. Sizing those bets sensibly is a separate skill, covered in our guide to bankroll management.
Prediction market prices are quoted in cents, not odds
Prediction markets such as Kalshi list contracts that settle at $1, trading anywhere between $0.01 and $0.99. The price is the probability.
A contract at $0.65 translates to:
- Implied probability: 65%
- Decimal odds: 1.54
- American odds: -186
Compare the same market across sportsbooks and prediction markets on odds.guru.