How to Convert Betting Odds: American, Decimal, Fractional
Odds formats can feel like a foreign language when you're first getting into sports betting. American odds, decimal odds, and fractional odds all express the same thing in different ways. Once you understand how to convert betting odds between formats and how they connect to implied probability, you can evaluate any line on any sportsbook quickly and confidently.
The Three Odds Formats, Briefly Explained
Before diving into conversions, here's a quick orientation on each format.
American odds are the standard in the United States. They're expressed as a positive or negative number relative to $100. A negative number shows how much you need to bet to win $100. A positive number shows how much you win on a $100 bet.
Decimal odds are common in Europe, Canada, and Australia. They represent your total return per $1 wagered, including your stake. A decimal of 2.50 means a $1 bet returns $2.50 total, so $1.50 in profit.
Fractional odds are traditional in the UK and Ireland, especially in horse racing. A fraction like 5/2 means you win $5 for every $2 wagered, not counting your returned stake.
All three formats describe the same relationship between risk and reward. The math just packages it differently.
How to Convert Betting Odds: American, Decimal, and Fractional Formulas
American to Decimal
For positive American odds:
Decimal = (American / 100) + 1
For negative American odds:
Decimal = (100 / |American|) + 1
Example: The Kansas City Chiefs are listed at +135. Convert to decimal:
(135 / 100) + 1 = 2.35
Example: The favorite is listed at -180. Convert to decimal:
(100 / 180) + 1 = 0.556 + 1 = 1.556
American to Fractional
For positive American odds:
Fractional = American / 100 (then simplify)
For negative American odds:
Fractional = 100 / |American| (then simplify)
Example: +150 becomes 150/100, simplified to 3/2
Example: -200 becomes 100/200, simplified to 1/2
Decimal to American
If the decimal is 2.00 or higher (equivalent to a positive American line):
American = (Decimal - 1) x 100
If the decimal is below 2.00 (equivalent to a negative American line):
American = -100 / (Decimal - 1)
Example: Decimal odds of 3.40:
(3.40 - 1) x 100 = +240
Example: Decimal odds of 1.65:
-100 / (1.65 - 1) = -100 / 0.65 = -154
Decimal to Fractional
Fractional = (Decimal - 1) expressed as a fraction
Example: 2.50 - 1 = 1.5 = 3/2
Fractional to Decimal
Decimal = (Numerator / Denominator) + 1
Example: 7/4 odds: (7 / 4) + 1 = 1.75 + 1 = 2.75
Fractional to American
If the fraction is greater than 1 (meaning more profit than stake):
American = (Numerator / Denominator) x 100
If the fraction is less than 1:
American = -100 / (Numerator / Denominator)
Example: 5/2 = 2.5, which is greater than 1, so: 2.5 x 100 = +250
Example: 1/4 = 0.25, so: -100 / 0.25 = -400
Implied Probability: The Number That Actually Matters
Every set of odds carries an implied probability, the win percentage the sportsbook is pricing in. It's the most important conversion you'll make, because it lets you judge whether a bet has value.
Converting Odds to Implied Probability
From American odds (positive):
Implied Probability = 100 / (American + 100)
From American odds (negative):
Implied Probability = |American| / (|American| + 100)
From decimal odds:
Implied Probability = 1 / Decimal
From fractional odds:
Implied Probability = Denominator / (Numerator + Denominator)
Example: A team is priced at +200. The implied probability is:
100 / (200 + 100) = 100 / 300 = 33.3%
Example: A team is priced at -150. The implied probability is:
150 / (150 + 100) = 150 / 250 = 60%
Why Implied Probability Adds Up to More Than 100%
Add the implied probabilities of both sides of a bet and you'll typically get something like 104% to 108%. That excess is the vig, also called juice. It's built-in margin for the sportsbook. This is why betting both sides of a game at -110 is not a break-even proposition — you're paying vig on every wager.
If you believe a team's true win probability is higher than what the odds imply, that's a positive expected value bet. If it's lower, the book has the edge.
Using the Line Whale Odds Converter
If you'd rather skip the manual math, the Odds Converter handles all of this instantly. Enter any odds in American, decimal, or fractional format and it converts to all three formats while showing you the implied probability.
For example, if a European book shows 1.91 on a side and a US book shows -115 on the same side, plug both into the converter to compare implied probabilities directly: 1.91 converts to 52.4% and -115 converts to 53.5%. The European book is offering better value on that wager.
Practical Example: Shopping Lines Across Sportsbooks
You're looking at an NBA game. Book A shows the Lakers at +3.5 (-108). Book B shows the same line in decimal format as 1.93.
Convert Book B to American: -100 / (1.93 - 1) = -100 / 0.93 = -107.5
Book B is offering a slightly better price. It's not a dramatic edge on a single bet, but shopping for the best line consistently adds up over hundreds of wagers. The Sportsbook Rankings page can help you identify which books offer competitive lines for the sports you bet most.
Key Takeaways
- American, decimal, and fractional odds all express the same thing: the relationship between risk and reward.
- Implied probability is the most actionable number. It tells you what win rate a price assumes and whether you have an edge.
- The sum of implied probabilities across both sides of a market exceeds 100% because of vig. A perfectly fair market would sum to exactly 100%.
- Converting between formats is straightforward with the formulas above, but the Odds Converter makes it instant.
- Always compare implied probability when shopping lines. A half-point or a few cents of vig difference is real money over time.