American, Decimal, and Fractional Odds: A Plain-English Guide to Implied Probability
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Key takeaways
- Every price is a probability in disguise. Implied probability = 1 ÷ decimal odds.
- Both sides of a market add up to more than 100%. The excess is the sportsbook’s margin, called the vig.
- Decimal odds make the math easiest. Convert everything to decimal before you calculate stakes.
- Profitable betting means paying less than the true probability: through promos, arbitrage, or prices that beat a sharp line.
Every strategy on this site rests on one skill: looking at a price like +240 or −135 and immediately knowing what probability it implies. Once you can do that, you can tell whether a bonus bet is converting well, whether two sportsbooks disagree enough for an arbitrage, and whether a price is better than it should be.
This guide covers the three odds formats, how to convert between them, and how to see the sportsbook’s margin hiding in every market.
The three odds formats
American odds
American odds (also called moneyline odds) are built around $100.
- Positive numbers show the profit on a $100 stake. +250 means a $100 bet wins $250 profit, for a $350 total return.
- Negative numbers show how much you must stake to win $100 profit. −250 means you stake $250 to win $100, for a $350 total return.
The sign tells you who the market favors. A minus sign is a favorite; a plus sign is an underdog. A price of +100 (sometimes shown as EVEN) is a coin flip with no margin.
Decimal odds
Decimal odds show the total return per $1 staked, including the stake. 3.50 means a $10 bet returns $35: $25 profit plus your $10 back.
Decimal odds are the easiest format for math. Payout is just stake × odds, and implied probability is 1 ÷ odds. Every calculator on this site converts to decimal internally.
Fractional odds
Fractional odds, common in UK horse racing, show profit relative to stake. 5/2 means you win $5 for every $2 staked. Add 1 to the fraction to get decimal odds: 5/2 = 2.5, plus 1 = 3.50.
Converting between formats
You don’t need to memorize these. The odds converter does it instantly. But knowing the shape of the math makes the rest of this site easier to follow.
Implied probability
Implied probability is the win rate a price assumes. It’s also your break-even win rate: bet at these odds forever and you need to win this often just to stay even.
For American odds there’s a shortcut. For favorites, divide the odds by the odds plus 100: −150 → 150 ÷ 250 = 60%. For underdogs, divide 100 by the odds plus 100: +150 → 100 ÷ 250 = 40%.
Run the numbers: What is +150 in decimal, fractional, and implied probability? Open the Odds Converter
Where the sportsbook’s margin hides
Here’s a standard NFL spread:
| Side | Odds | Implied probability |
|---|---|---|
| Chiefs −3 | −110 | 52.38% |
| Bills +3 | −110 | 52.38% |
| Total | 104.76% |
Two outcomes can’t have a combined probability above 100%. The extra 4.76% is the overround, the sportsbook’s built-in margin. It’s often called the vig or the juice.
Another way to express it is hold: the share of all money bet that the book expects to keep if it takes equal action on both sides. For a −110 / −110 market the hold is about 4.55% (1 − 1 ÷ 1.0476).
Margins vary a lot:
- Main-market NFL and NBA lines at US books usually hold 4% to 5%.
- Player props and same-game parlays often hold 8% to 20% or more.
- Sharp books like Pinnacle often hold 2% to 3% on major markets.
That last point is why sharp bettors care so much about Pinnacle. A market with less margin baked in gives a cleaner read on the true probability. We cover this in detail in why Pinnacle’s line matters.
From implied probability to fair probability
If you remove the margin, you get the market’s best estimate of the true probability, called the fair or no-vig probability. The simplest method scales each side down proportionally:
A −110 / −110 market is a coin flip once the vig is gone. There are several ways to remove the vig, and they matter more on lopsided markets. Our devig guide compares them, and the no-vig calculator runs all four at once.
Why this matters for making money
The sportsbook’s margin means that picking winners at standard prices is a losing game for most people. You need to win 52.38% of −110 bets just to break even. There are three ways around the margin, and they’re what the rest of this site is about:
- Promotions. A bonus bet has no stake at risk, which changes the math completely. Hedged correctly, it becomes cash. Start with how to convert bonus bets.
- Arbitrage. When two books disagree enough, the best prices on each side add up to less than 100%. Bet both and you profit whatever happens. See our arbitrage guide.
- Positive expected value. When a US book’s price implies a lower probability than the fair probability from a sharp book, the bet is worth more than it costs. See positive EV betting explained.
All three start with the same question: what probability does this price imply, and what’s the real probability?
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Are higher odds better?
Higher odds pay more, but they also imply a lower chance of winning. What matters is whether the odds are higher than they should be for the true probability. +300 on a 30% chance is a good bet. +300 on a 20% chance is a bad one.
What does a minus sign mean in American odds?
The minus sign marks the favorite. −150 means you stake $150 to win $100 profit. A plus sign marks the underdog: +150 means a $100 stake wins $150.
What is −110 in decimal odds?
−110 is 1.909 in decimal odds. You get back $1.909 for every $1 staked, including the stake. The implied probability is 52.38%.
Why do sportsbooks show American odds in the US?
American odds are the long-standing convention at US sportsbooks, built around risking or winning $100. Most apps let you switch to decimal odds in settings, and many serious bettors do.
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ProfitDuel is a subscription toolkit for promo conversion, arbitrage, and +EV betting: live matchers, step-by-step guides, and a member community.
Start the $1 ProfitDuel trialAffiliate link. The $1, 3-day trial applies to ProfitDuel's Starter plan as listed in September 2026. 21+ and in states where online sports betting is legal.