American, Decimal, Fractional: Odds Formats Explained
Three formats, one concept. Here's how American, Decimal, and Fractional odds work, how to convert between them, and which format you should actually use.
Why are there three formats?
Geography. American odds dominate US sportsbooks. Decimal odds are standard in Europe, Australia, and most prediction markets. Fractional odds are traditional in UK horse racing. They all express the same thing: the ratio of payout to stake.
If you're serious about finding the best lines, you need to read all three fluently.
American odds
American odds center around $100. There are two forms:
- Positive (+150): The profit on a $100 bet. +150 means you profit $150 on a $100 wager.
- Negative (-200): How much you must bet to profit $100. -200 means you risk $200 to profit $100.
The breakpoint is +100 / -100, which is an even-money bet (2.00 decimal).
Implied probability:
- Positive: 100 / (odds + 100). So +150 implies 100/250 = 40%.
- Negative: |odds| / (|odds| + 100). So -200 implies 200/300 = 66.7%.
Decimal odds
Decimal odds tell you the total return per dollar wagered, including your stake.
- 2.50 means $2.50 back for every $1 bet ($1.50 profit + $1 stake).
- 1.50 means $1.50 back for every $1 bet ($0.50 profit + $1 stake).
Implied probability: 1 / decimal odds. So 2.50 implies 1/2.50 = 40%.
This is the cleanest format for math. Multiplying, dividing, and comparing odds is trivial in decimal. It's also what prediction markets use — a contract at $0.40 is equivalent to decimal odds of 2.50.
Fractional odds
Fractional odds show profit relative to stake.
- 3/2 means $3 profit for every $2 wagered. Equivalent to +150 or 2.50 decimal.
- 1/4 means $1 profit for every $4 wagered. Equivalent to -400 or 1.25 decimal.
Implied probability: denominator / (numerator + denominator). So 3/2 implies 2/5 = 40%.
Fractional odds are falling out of favor outside of UK racing. They're harder to compare at a glance and awkward for parlay calculations.
Quick conversion reference
| American | Decimal | Fractional | Implied Prob |
|---|---|---|---|
| +200 | 3.00 | 2/1 | 33.3% |
| +150 | 2.50 | 3/2 | 40.0% |
| +100 | 2.00 | 1/1 | 50.0% |
| -150 | 1.667 | 2/3 | 60.0% |
| -200 | 1.50 | 1/2 | 66.7% |
Which format should you use?
Decimal. Always decimal for any calculation — EV, Kelly, parlays, anything. American odds are fine for reading a US sportsbook, but convert to decimal before you do math.
The odds converter handles all three formats instantly. If you're moving between sportsbooks and prediction markets, the market converter translates between contract prices and traditional odds.
If you're comparing prices across sportsbooks and prediction markets, read sportsbooks vs prediction markets for a full breakdown of how each platform works.
Don't let format differences slow you down. The edge is in the numbers, not the notation.