Betting Odds Converter
Three Notations, One Underlying Probability
American, decimal, and fractional odds are three different ways of writing the exact same information — the implied probability of an outcome and the payout it carries. American odds dominate U.S. sportsbooks, decimal odds are standard across Europe and Australia, and fractional odds remain common in UK racing and betting markets. This calculator converts between all four directions without rounding errors compounding along the way.
The Formulas
Decimal to American: American = (Decimal − 1) × 100 if Decimal ≥ 2, else −100 / (Decimal − 1)
Fractional to Decimal: Decimal = 1 + (Numerator / Denominator)
Implied Probability = (1 / Decimal) × 100
Common Odds, Side by Side
| American | Decimal | Approx. fractional | Implied probability |
|---|---|---|---|
| −200 | 1.500 | 1/2 | 66.67% |
| −110 | 1.909 | 10/11 | 52.38% |
| +100 | 2.000 | 1/1 | 50.00% |
| +150 | 2.500 | 3/2 | 40.00% |
| +300 | 4.000 | 3/1 | 25.00% |
Negative American odds (favorites) always convert to a decimal below 2.00 and an implied probability above 50%; positive American odds (underdogs) always convert to a decimal above 2.00 and an implied probability below 50%.
Where This Gets Used
- Comparing lines across sportsbooks — different books sometimes default to different notations, and converting to a common format makes lines directly comparable.
- Spotting the vig (bookmaker margin) — summing the implied probabilities on both sides of a two-outcome market and seeing how far the total exceeds 100% reveals the book's built-in edge.
- Reading international odds formats — converting fractional odds from a UK source into the decimal or American format you're used to.
How to Use This Calculator
- Choose a conversion direction: American to Decimal, Decimal to American, Fractional to Decimal, or Decimal to Fractional.
- Enter the odds value(s) required for that direction.
- Select Calculate to see the converted odds and implied probability.
Related Calculations
Turn converted odds into a bet size with the Kelly Criterion Calculator, or compute the house edge behind any payout with the House Edge Calculator.