How to Read Sports Betting Odds: American, Decimal, and Fractional
American moneyline odds, decimal odds, fractional odds — learn what they all mean, how to convert between them, and how to calculate implied probability.
Why odds formats matter
Sportsbooks around the world display odds in three different formats: American (moneyline), decimal, and fractional. Understanding all three — and crucially, the implied probability each encodes — is the foundation of finding value in any betting market.
American odds (moneyline)
American odds are expressed as a positive or negative number around 100.
- Negative numbers (favourites): The number tells you how much you must risk to win $100. −150 means you bet $150 to win $100 profit.
- Positive numbers (underdogs): The number tells you how much you win on a $100 bet. +130 means you bet $100 to win $130 profit.
Implied probability from American odds
Favourite: implied prob = |odds| / (|odds| + 100)
Underdog: implied prob = 100 / (odds + 100)Examples:
- −150: 150 / (150 + 100) = 60.0% implied
- +130: 100 / (130 + 100) = 43.5% implied
- −110 (standard vig): 110 / 210 = 52.4% implied
Decimal odds
Decimal odds represent the total return per unit staked, including your stake back. They are used in Europe, Australia, and Canada.
- 2.00 = even money (you double your money on a win)
- 1.909 = the decimal equivalent of −110 (return $1.909 for every $1 bet)
- 2.30 = equivalent to +130
Implied probability = 1 / decimal odds
Example: 1 / 1.909 = 52.4%Decimal odds make it easy to compare markets directly — a lower decimal means a bigger favourite. SherlockPicks uses decimal odds internally for all EV calculations.
Fractional odds
Fractional odds are traditional in the UK. They show profit relative to stake as a fraction.
- 6/4 = for every $4 you stake, you profit $6 (return $10 total)
- 1/2 = for every $2 you stake, you profit $1 (return $3 total) — a strong favourite
Implied probability = denominator / (numerator + denominator)
Example: 4 / (6 + 4) = 40%The vig: why implied probabilities exceed 100%
If you add the implied probability of both sides of a standard −110/−110 bet, you get 104.8% — not 100%. That extra 4.8% is the sportsbook's margin. It means the "true" implied probability of each side is actually lower than what the odds suggest. This is why finding genuine positive EV requires a model that is more accurate than the market, not just picking winners at random.
Converting between formats
| American | Decimal | Fractional | Implied % |
|---|---|---|---|
| −200 | 1.50 | 1/2 | 66.7% |
| −110 | 1.909 | 10/11 | 52.4% |
| Even / −100 | 2.00 | 1/1 | 50.0% |
| +120 | 2.20 | 6/5 | 45.5% |
| +200 | 3.00 | 2/1 | 33.3% |
Frequently Asked Questions
Negative 110 means you must bet $110 to win $100 profit (total return $210). It is the standard vig applied to most spread and totals bets in the US. The −110 on both sides implies a combined 104.8% probability, with the extra 4.8% being the sportsbook's margin.
For favourites (negative odds): divide the absolute value by (absolute value + 100). Example: −150 → 150/250 = 60%. For underdogs (positive odds): divide 100 by (odds + 100). Example: +130 → 100/230 = 43.5%.
American moneyline odds show profit relative to $100 (negative = favourite, positive = underdog). Decimal odds show the total return per $1 staked including your stake back. −110 American equals 1.909 decimal. Decimal odds are easier for direct comparison and EV calculations.
Because sportsbooks build in a margin (the vig or juice). On a standard −110/−110 line, both sides imply 52.4%, totalling 104.8%. The extra 4.8% is the house edge on that market. Finding positive EV means finding bets where your true probability estimate exceeds the implied probability after accounting for the vig.
See EV and edge live
SherlockPicks calculates all of this automatically for every game, every day.