Convert any sports betting odds between American, Decimal, Fractional, and Implied Probability formats. Understanding the relationship between these formats is fundamental to evaluating any bet — and to comparing prices across sportsbooks that use different formats.
Why odds format matters
The same probability is expressed three different ways across global sports betting markets. American odds (-110, +200) dominate US sportsbooks. Decimal odds (1.91, 3.00) are standard in Europe, Australia, and most international books. Fractional odds (10/11, 5/2) survive primarily in UK horse racing.
For the bettor, the math is identical regardless of format — but cross-book comparison requires conversion. If you're shopping the same NFL spread at DraftKings (showing -110 American) and a European book (showing 1.909 decimal), you need to know they're the same price. The decimal format makes parlay multiplication visual; American is easier for at-a-glance moneyline reads; fractional is the language of UK racing.
The fourth format — implied probability — is the universal currency. Every American/Decimal/Fractional price corresponds to a specific implied probability that the outcome will happen. Implied probability is what you compare to your own probability estimate to determine whether a bet has positive expected value.
The math behind each conversion
American → Decimal
For positive American odds: decimal = 1 + (odds / 100). Example: +200 → 1 + 200/100 = 3.00.
For negative American odds: decimal = 1 + (100 / |odds|). Example: -150 → 1 + 100/150 = 1.667.
Decimal → American
If decimal ≥ 2.0: American = (decimal - 1) × 100. Example: 2.5 → 1.5 × 100 = +150.
If decimal < 2.0: American = -100 / (decimal - 1). Example: 1.50 → -100 / 0.50 = -200.
Fractional → Decimal
Decimal = (numerator / denominator) + 1. Example: 5/2 → 2.5 + 1 = 3.50.
Decimal → Implied Probability
Probability = 1 / decimal. Example: 1.91 → 1/1.91 = 52.4%.
American → Implied Probability
For negative odds: |odds| / (|odds| + 100). Example: -110 → 110/210 = 52.4%.
For positive odds: 100 / (odds + 100). Example: +200 → 100/300 = 33.3%.
Worked example: cross-book comparison
You see a sportsbook spread at -105, another at -110, a European book at 1.952. Which is the best price? Convert all three to implied probability:
- -105 → 105/205 = 51.22% implied
- -110 → 110/210 = 52.38% implied
- 1.952 (decimal) → 1/1.952 = 51.23% implied
The -105 and 1.952 prices are essentially equivalent. The -110 is the worst of the three by about 1.15 percentage points. If your fair line says the spread should be 53%, all three are +EV — but the first two are more profitable than the third.
Common odds-conversion mistakes
- Confusing decimal odds with payout multiplier. Decimal odds include the stake — 2.0 returns $2 for $1 ($1 profit), not $2 profit. Bettors used to American odds sometimes mis-read decimal as "double my money" when it's actually break-even.
- Thinking +200 and 2/1 are the same. They are equivalent (both mean 33.3% implied), but +200 is American (showing profit per $100) and 2/1 is fractional (showing profit per $1). 5/2 fractional = +250 American = 3.50 decimal = 28.6% implied.
- Mixing formats during parlay calculation. Always convert all legs to decimal before multiplying. Parlay math fails if you try to mix formats.
- Ignoring the vig when computing fair probability. Implied probability includes the operator's margin. To find true fair probability, normalize by removing the vig — see our vig and no-vig pricing guide.
Frequently asked questions
What's the difference between American and Decimal odds?
American odds show profit relative to a $100 stake (-110 means risk $110 to win $100; +200 means risk $100 to win $200). Decimal odds show total return per $1 staked, including stake (1.91 means a $1 stake returns $1.91 total = $0.91 profit). Math is identical; presentation differs.
Why do American odds use 100 as the reference?
Historical convention from US horse racing and bookmaker traditions. The -100/+100 split point represents 50/50 odds (an even-money bet).
Are fractional odds still used?
Primarily in UK horse racing. Major US sportsbooks have abandoned fractional in favor of American or decimal. International soccer markets use decimal universally.
What does implied probability mean?
The win probability the odds suggest. -110 implies 52.4% (you'd need to win 52.4%+ of the time to break even). Comparing implied probability to your own estimate is the foundation of EV-based betting.
How do I find the no-vig probability?
Sum the implied probabilities of both sides of a market. The total typically exceeds 100% — that's the vig. Divide each side's implied probability by the total to get the no-vig (fair) probability for that side.
What's the fastest way to compare prices across sportsbooks?
Convert everything to implied probability or no-vig probability. The format of each book's display doesn't matter; the underlying probability is what determines value. Use our calculator above.
Can I use this calculator on my phone?
Yes. The page is mobile-responsive. All format conversions work identically on mobile and desktop.
Why doesn't the converter accept '4-1' format?
Standard fractional notation uses a forward slash: 4/1, 5/2, 10/11. We accept the slash format universally; dash-separated notation is non-standard.