Odds formats at a glance
Decimal odds show total return per 1 unit staked (stake included). Fractional odds show profit relative to stake (stake not included). American odds use a plus/minus system: positive values show profit on a 100-unit stake; negative values show how much you must stake to win 100. The math is the same across formats—only the display differs.
Converting odds to implied probability
Implied probability helps you compare prices and find value. Use the formulas below.
Decimal → probability
Probability = 1 / Decimal. Example: 1.80 ≈ 55.56%.
Fractional → probability
Probability = Denominator / (Numerator + Denominator). Example: 5/2 → 2/(5+2) ≈ 28.57%.
American (moneyline) → probability
For positive odds (+A): Probability = 100 / (A + 100).
For negative odds (−A): Probability = A / (A + 100). Example: +150 → 100/(150+100)=40%; −150 → 150/(150+100)=60%.
If you prefer a single reference that covers multiple conversions and formats, see the consolidated tables on sports-betting overviews.
Converting between formats
Fractional → Decimal
Decimal = (Numerator / Denominator) + 1.
American → Decimal
If positive: Decimal = (A / 100) + 1. If negative: Decimal = (−100 / A) + 1.
Decimal → Fractional
Subtract 1 and express as a fraction (e.g., 2.40 → 1.40 → 7/5).
The bookmaker’s margin (overround)
When you convert all selections in a market to implied probabilities and add them up, you’ll usually get more than 100%. That excess is the margin (also called the overround or vig). Lower overround generally means better value for bettors. Example (three-way market): 2.10, 3.40, 3.60 ⇒ 1/2.10 + 1/3.40 + 1/3.60 ≈ 104.81% (a ~4.81% margin).
For a plain-English introduction to overround and why it varies by book and market, see these explainers.
Parlays/accumulators: why books multiply decimals
For multi-leg bets, sportsbooks convert each leg to decimal odds, multiply them, and then apply stake to get total return. Example: 1.80 × 1.95 × 2.10 = 7.371; a 100-unit stake would return 737.10 (profit 637.10). The same rule underpins “acca” calculations in operator and education resources.
Crypto sportsbooks: same odds, familiar toggles
Odds formats work identically on crypto books. Most let you switch display styles in settings or account preferences (e.g., Sportsbet.io, William Hill, BetMGM). Look for “Odds format” in your profile or footer.
Worked examples you can copy
Reading decimal
If you see 2.50, your total return on a 100-unit stake is 250, and implied probability is 1/2.50 = 40%.
Reading fractional
At 7/4, profit is 175 on a 100-unit stake (total return 275). Probability is 4/(7+4) ≈ 36.36%.
Reading American
At +120, profit is 120 on a 100-unit stake (probability 100/(120+100)=45.45%). At −130, you’d stake 130 to win 100 (probability 130/(130+100)=56.52%).
Quick checklist for bettors
Set your default odds format but learn all three so you can compare prices across books and regions.
Always convert to implied probability before deciding if a price is worth it.
Add implied probabilities across a market to gauge margin; lower is generally better.
Verify parlay returns by multiplying decimal odds.
FAQs
Are decimal odds easier for parlays?
Yes. Because decimal odds include stake and multiply cleanly across legs, they’re the simplest for accumulator math.
What’s the fastest way to sanity-check a price?
Convert to implied probability using the formulas above or a reputable calculator; it takes seconds and helps you spot value.
Is “overround” the same as “vig”?
They’re closely related concepts for the book’s margin; both describe how quoted prices sum to more than 100% probability.
Do crypto books use different odds logic?
No—the formats and math are identical. Many crypto sportsbooks also let you switch between decimal, fractional, and American in one click.
Bottom line
Odds formats are just languages. Translate them into implied probability, measure the margin, and confirm parlay math—then pick the format you read fastest. Whether you bet with BTC, ETH, or fiat, the same conversions and checks apply everywhere.
