Odds Converter
Three Notations, One Underlying Probability
American, decimal, and fractional odds are three different ways of writing down the same information: how much a book is willing to pay relative to how likely it thinks an outcome is. American sportsbooks post -110 or +150. European and Asian books tend toward decimal odds like 1.91. British tracks and bookmakers still quote 10/11. None of the three is more "correct" — they're just different notations for identical underlying odds, and converting between them is pure arithmetic.
The Formulas
Decimal odds serve as the common intermediate format for every conversion:
American → Decimal: (100 ÷ |American|) + 1, if negative
Decimal → American: (Decimal − 1) × 100, if Decimal ≥ 2
Decimal → American: −100 ÷ (Decimal − 1), if Decimal < 2
Fractional → Decimal: (Numerator ÷ Denominator) + 1
Implied Probability = (1 ÷ Decimal Odds) × 100
Whichever format is entered, this calculator converts to decimal odds first, then derives the other two formats and the implied win probability from that common value.
Where This Calculation Matters
- Cross-book comparison — a bettor comparing lines across an American book and a European exchange needs a shared unit to know which is actually offering better value.
- Understanding implied probability — converting odds to a percentage strips away the notation and shows what the market is actually saying about an outcome's likelihood.
- Spotting overround — summing implied probabilities across all outcomes in a market reveals the bookmaker's built-in margin, since it will exceed 100%.
- Historical and legacy formats — fractional odds still appear on horse racing boards and in older betting literature, so converting them to a familiar format is often necessary.
Conversion Reference Table
These figures come from running the calculator's own formulas against common American odds values:
| American | Decimal | Fractional | Implied Probability |
|---|---|---|---|
| -200 | 1.500 | 1/2 | 66.67% |
| -150 | 1.667 | 2/3 | 60.00% |
| -110 | 1.909 | 10/11 | 52.38% |
| +100 | 2.000 | 1/1 | 50.00% |
| +150 | 2.500 | 3/2 | 40.00% |
| +200 | 3.000 | 2/1 | 33.33% |
| +300 | 4.000 | 3/1 | 25.00% |
Implied probability reflects the odds as quoted and includes the bookmaker's margin — it is not a claim about the true probability of the outcome.
How to Use This Calculator
- Select the format you're converting from — American, Decimal, or Fractional Odds.
- Enter the odds value in the field(s) shown (numerator and denominator for fractional).
- Select Calculate to see the equivalent value in all three formats plus implied probability.
Related Calculations
Once you have a probability estimate, the Fantasy Football Trade Analyzer shows a similar comparison approach applied to player value instead of odds.