American, Decimal, and Fractional Odds Explained
Three formats, one underlying idea. Learn to read all of them fluently — and to see the implied probability hiding inside every price.
American, decimal, fractional — three ways of writing the exact same thing. If you've ever been confused by a +150 next to a 2.50 next to a 3/2, you're not alone, and the good news is they're all just different dialects for one underlying idea: how much a bet pays and how likely it is to win. This guide teaches you to read all three fluently, convert between them, and — most importantly — see the implied probability every price is really quoting.
Why there are three formats
The three formats are mostly a matter of geography and tradition. American odds dominate in the United States, decimal odds are standard across Europe and Australia and on most betting exchanges, and fractional odds are the traditional British format still seen in horse racing. They all encode the same two facts — payout and probability — so learning to move between them is purely about fluency, not new math.
American odds
American odds (also called moneyline odds) are built around the number 100 and use a plus or minus sign.
- Positive odds (like +150) tell you how much profit you'd make on a $100 stake. +150 means bet $100 to win $150 profit. These are underdogs — they pay more than even money.
- Negative odds (like -150) tell you how much you need to stake to win $100 profit. -150 means risk $150 to win $100. These are favorites — you lay more than you'll win.
The sign is the quick tell: minus means favorite, plus means underdog. Even money — a true coin flip payout — is +100 (or sometimes written -100).
-200 → stake $200, win $100 profit (clear favorite)
+100 → stake $100, win $100 profit (even money)
Decimal odds
Decimal odds are the simplest to work with, which is why exchanges and most of the world use them. The number represents your total return per $1 staked, including your stake back. Multiply your stake by the decimal number and that's your total payout.
- Decimal 2.50 means a $100 bet returns $250 total — your $100 back plus $150 profit.
- Decimal 1.50 means a $100 bet returns $150 total — $100 back plus $50 profit.
- Decimal 2.00 is even money — $100 returns $200 total.
Because they include the stake, decimal odds make combining bets easy: to find the odds of a parlay, you just multiply the decimals together. That simplicity is a big reason decimal is the preferred format for anyone doing serious math on their bets.
Decimal to profit-only: subtract 1. A decimal of 2.50 pays 1.50 in profit per $1. That "decimal minus 1" number is exactly the b value in the Kelly formula.
Fractional odds
Fractional odds, the traditional British format, express profit relative to stake as a fraction. The odds 3/1 ("three to one") mean you win $3 profit for every $1 staked. The odds 1/2 ("one to two," or "two to one on") mean you win $1 for every $2 staked — a favorite.
- 5/1 → win $5 profit per $1 staked (underdog)
- 1/1 → even money, the same as decimal 2.00 or American +100
- 1/4 → win $1 profit per $4 staked (heavy favorite)
Fractional odds map cleanly onto decimals: just divide the fraction and add 1. So 3/1 becomes 3 ÷ 1 + 1 = decimal 4.00, and 1/2 becomes 1 ÷ 2 + 1 = decimal 1.50.
The number that matters most: implied probability
Here's the concept that turns odds from a payout table into a decision-making tool. Every price, in any format, implies a probability — the win rate at which that bet would exactly break even over the long run. This is the number sharp bettors actually think in.
The easiest route is through decimal odds: implied probability = 1 ÷ decimal odds. So a decimal of 2.50 implies 1 ÷ 2.50 = 40%. A decimal of 1.50 implies 1 ÷ 1.50 = 66.7%. From American odds directly, a +150 underdog implies 100 ÷ (150 + 100) = 40%, and a -150 favorite implies 150 ÷ (150 + 100) = 60%.
American +150 = Decimal 2.50 = Fractional 3/2
Implied probability: 40%
(You break even if this wins 40% of the time.)
Enter a price in any format and see all three, plus the implied probability and break-even rate, at once.
Open the Odds Converter →Why implied probability is the key skill
Once you can read the implied probability of any price, you can start asking the only question that matters: is this bet worth it? If a price implies 40% but you believe the outcome is really more like 47% likely, you've found value — you're being paid as if it's less likely than it truly is. That gap between the implied probability and the true probability is the entire basis of profitable betting.
The catch is that the raw implied probability from a sportsbook includes the vig, so it's slightly inflated. To get the honest number, you remove the margin — which is exactly what the vig-removal method does. Reading odds fluently is step one; devigging them to find the fair probability is step two.
A quick reference
- American → Decimal: for + odds, (odds ÷ 100) + 1. For − odds, (100 ÷ odds) + 1.
- Decimal → Implied probability: 1 ÷ decimal.
- Fractional → Decimal: (numerator ÷ denominator) + 1.
- Even money in each: +100 = 2.00 = 1/1 = 50% implied.
The takeaway
The three odds formats are just different languages for the same two ideas: what a bet pays and how likely it is to win. Learn to read all three — favorites versus underdogs, stake-inclusive versus profit-only — and convert between them with the simple formulas above. But the real prize is implied probability, the number underneath every price. Once you think in probabilities rather than payouts, you can start finding the gaps between what a bet pays and what it should pay — which is where every profitable bet begins. The odds converter does all the translation for you, and the vig guide shows you how to turn those implied probabilities into fair ones.
