How Betting Odds Work: Decimal, Fractional, American
Decimal, fractional, and American odds are three ways to write one price. Learn to convert them, read implied probability, and spot the bookmaker's vig.
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The short answer
Betting odds are prices, not predictions. They tell you how much a winning bet pays, and they hide a probability the bookmaker has already baked in. Decimal, fractional, and American odds are three ways of writing the same number: decimal 2.50, fractional 6/4, and American +150 all describe the exact same bet. Our parlay calculator lets you enter odds in any format and instantly see the combined payout for multi-leg bets. For single bets, use the odds payout calculator, or convert between formats with the odds converter.
One bet, three formats
Say a book prices an underdog at decimal 2.50. That identical price shows up as 6/4 in fractional odds and +150 in American odds. Here's what a $10 bet returns in each format:
- Decimal 2.50: multiply your stake by the odds to get your total return. $10 × 2.50 = $25 back, which is your $10 stake plus $15 profit.
- Fractional 6/4: the fraction reads profit over stake. You win $6 for every $4 you stake, so $10 × 6 ÷ 4 = $15 profit, $25 total.
- American +150: a positive number shows profit on a $100 stake. +150 means $150 profit per $100, so $10 wins $15 profit, $25 total.
Same bet, same payout, three notations. Decimal dominates in Europe, Canada, and Australia. Fractional is the British classic. American odds rule US books, with one twist: negative numbers flip the logic. A price of -110 means you must stake $110 to win $100 profit.
Most sportsbooks let you switch formats in the settings menu. Flip between them on a live sportsbook like wild.io's and watch the numbers change while the prices stay identical. It's a quick way to build intuition.
How do you convert between odds formats?
Every conversion runs through decimal odds, because decimal is just "total return per $1 staked." Learn these moves and you can translate any price on any screen:
Format | Example | Implied probability | Conversion rule (to decimal) |
|---|---|---|---|
Decimal | 2.50 | 1 ÷ 2.50 = 40% | Already decimal: total return per $1 staked |
Fractional | 6/4 | 40% | Divide the fraction, then add 1: 6 ÷ 4 = 1.5, plus 1 = 2.50 |
American (positive) | +150 | 40% | Divide by 100, then add 1: 150 ÷ 100 = 1.5, plus 1 = 2.50 |
American (negative) | -110 | 52.4% | Drop the minus sign, divide 100 by the number, then add 1: 100 ÷ 110 = 0.909, plus 1 = 1.909 |
Going the other way works too. Decimal to fractional: subtract 1 and write it as a fraction, so 2.50 minus 1 = 1.5, which is 3/2, listed as 6/4 by tradition. Decimal to American: if the decimal is 2.00 or higher, subtract 1 and multiply by 100 (2.50 becomes +150). If it's under 2.00, divide -100 by the decimal minus 1 (1.909 becomes -110).
Rule of thumb: when a price confuses you, convert it to decimal first. Decimal odds are the only format where bigger always means "pays more," with no fractions and no sign flips.
What is implied probability?
Implied probability is the win rate a price assumes, and it's one division away: 1 ÷ decimal odds. Our example bet at 2.50 implies 1 ÷ 2.50 = 0.40, or 40%. The bookmaker is pricing that outcome as a 40% chance.
Now take the most common price in American sports, -110. Convert it to decimal first: 100 ÷ 110 = 0.909, plus 1 = 1.909. Then 1 ÷ 1.909 = 0.524, or 52.4%.
That 52.4% is worth staring at. It means a bet at -110 only breaks even in the long run if it wins 52.4% of the time. Not 50%. Flipping a fair coin at -110 prices loses money, slowly and reliably. Where does that missing edge go? That's the next section.
What is the vig?
Vig, short for vigorish and also called juice, is the bookmaker's fee built into the odds. You never see it as a line item. It hides in prices that are slightly worse than the true probabilities deserve; odds references like the Wizard of Odds house edge guide publish the equivalent edges across betting markets and casino games.
Here's the tell. A typical point spread offers -110 on one side and -110 on the other. Each side implies 52.4%, so together they imply 52.4% + 52.4% = 104.8%. Real probabilities for two outcomes must sum to exactly 100%. The extra 4.8 points is called the overround, and it's the book's built-in margin on that market. Expressed as a house edge, the standard -110-both-ways line works out to 4.54%, the figure the Wizard of Odds sports FAQ uses for spread bets.
What does the overround mean for you in practice? Whichever side you pick, you're paying a small premium over the fair price. The book doesn't need to predict games better than you. It just needs balanced action at prices that sum past 100%, and the math does the rest.
Worth remembering: the vig works like the house edge in a casino game. It doesn't decide any single bet, but it taxes every bet, and no staking pattern or gut feel makes it go away.
How do you calculate a bookmaker's margin?
You can measure the margin on any market in under a minute. Convert every outcome to implied probability, add them up, and see how far past 100% the total lands.
Take a soccer match with three outcomes: home win at decimal 2.10, draw at 3.40, away win at 3.80. The implied probabilities are:
- Home: 1 ÷ 2.10 = 47.6%
- Draw: 1 ÷ 3.40 = 29.4%
- Away: 1 ÷ 3.80 = 26.3%
Total: 47.6 + 29.4 + 26.3 = 103.3%. That market carries a 3.3% overround, which is a fairly sharp price. Big-league moneylines often run 2-5%, while niche sports, player props, and parlays can carry margins several times higher. Same book, same sport, very different fees depending on where you click.
Realized results land in the same territory. Nevada's sportsbooks kept 7.4% of everything wagered with them across 2025, and 7.3% through April 2026, with 2025 monthly holds swinging between 6.1% and 9.1%, per Covers' Nevada revenue tracker. That's the overround plus parlay margins doing their quiet work. Competition moves these numbers too: one analysis of English football odds found Premier League overrounds fell from about 9% to 4% between the 2005-06 and 2017-18 seasons.
From board price to fair price
Push that soccer market one step further and you can see the fair odds hiding under the posted ones. Divide each implied probability by the 103.3% total: home 47.6 ÷ 103.3 = 46.1%, draw 29.4 ÷ 103.3 = 28.5%, away 26.3 ÷ 103.3 = 25.4%. Those three now sum to 100%, the way true probabilities should.
Now turn those fair probabilities back into prices with the same tool in reverse: 1 ÷ probability. The fair home price is 1 ÷ 0.461 = 2.17, not the 2.10 on the board. Run the conversions from earlier and 2.17 comes out as roughly 7/6 fractional or +117 American, against the posted 11/10 and +110. The book pays 2.10 for a bet that fair math prices at 2.17. That gap, about 3% of the fair payout, is the overround landing on your ticket.
The whole routine takes three steps, and it works on any market with any number of outcomes:
- Convert every price to implied probability: 1 ÷ decimal odds.
- Add them up. Whatever sits above 100% is the overround.
- Divide each implied probability by the total to see the fair, vig-free line.
Why does odds shopping matter?
Odds shopping means checking the same bet at more than one book, because prices vary. One book posts your side at -110, another at -105. Both are the same bet on the same game, but -105 implies 51.2% while -110 implies 52.4%.
In our experience, price-checking is the habit new bettors adopt last: they'll study team form for an hour and never spend the thirty seconds it takes to compare the same line at a second book.
That gap looks tiny on one ticket. Across a season it's the difference between needing to win 52.4% of your bets and needing 51.2% just to break even. Very few casual bettors clear either bar, but paying less vig per bet is the one improvement that costs nothing and requires zero prediction skill.
Put dollars on it. At -110 you risk $110 to win $100 profit, while at -105 you risk only $105 for the same $100. Over 200 bets at that size, winning exactly half, the -110 bettor collects $10,000 in profits and pays out $11,000 in losses, ending $1,000 down. The -105 bettor collects the same $10,000 but pays only $10,500, ending $500 down. Identical picks, identical results, half the damage, purely from taking the better price.
The same logic says slow down on parlays. Every leg stacks another slice of overround, which is exactly why books promote them so heavily.
What odds do and don't tell you
Odds tell you the market's current price for an outcome, shaped by the bookmaker's opening number and the money that followed. They're the combined opinion of the book and thousands of bettors, minus the vig. As information goes, that's genuinely useful: an implied 40% is a serious signal, not a random guess.
What odds don't offer is a prediction you can casually out-guess. Prices move as sharp money arrives, and beating the closing line consistently is hard enough that it's the standard test of professional skill. Watching one game a week doesn't clear that bar, and the vig means you start every bet slightly behind.
So treat odds as a fee schedule to read, not a puzzle to crack. Know what a price implies, know what the market charges, shop for the smaller margin, and size every stake from a plan. Our guide to bankroll management covers that last part, and it matters more than any single price you'll ever take.
Play it smart: the vig guarantees the book an edge over time, so bet for entertainment, not income. Set a budget before you open the app, stick to stakes you can afford to lose, and take a break if it stops being fun. Support and self-exclusion tools live in your sportsbook account's responsible-gambling settings. 18+/21+ depending on where you live.
Apply Odds to Specific Sports
Now that you understand how odds work, see how they play out in real markets:
- How to Bet on Football (NFL) — spreads, moneylines, and props
- How to Bet on Basketball (NBA) — point spreads and totals
- How to Bet on Soccer — 1X2, Asian handicap, and correct score
- How to Bet on Tennis — set betting and live markets
- How to Bet on UFC & MMA — fight odds and method of victory
- How to Bet on Baseball (MLB) — moneylines and run lines
Sources
- Wizard of Odds — House Edge of Casino Games Compared — retrieved July 2026
- Wizard of Odds — Ask the Wizard: Sports Betting FAQ — retrieved July 2026
- Wikipedia — Mathematics of bookmaking — retrieved July 2026
- Covers — Nevada Sports Betting Revenue Tracker — retrieved July 2026
Frequently asked questions
What does -110 mean in betting odds?
A price of -110 means you must stake $110 to win $100 profit. In decimal terms it converts to 1.909 (100 divided by 110, plus 1), which implies a 52.4% win probability. That means a bet at -110 only breaks even over the long run if it wins 52.4% of the time, not 50% — the missing edge is the bookmaker's vig.
Are decimal 2.50, fractional 6/4, and American +150 the same odds?
Yes. They are three notations for the exact same price. A $10 bet at any of them returns $25 total: your $10 stake plus $15 profit. Decimal multiplies stake by odds, fractional reads profit over stake ($6 won per $4 staked), and positive American shows profit per $100 staked. Most sportsbooks let you switch formats in settings without changing the price.
Which odds format is easiest to understand?
Decimal odds, because they simply state total return per $1 staked — bigger always means pays more, with no fractions and no sign flips like American odds have. Every conversion between formats also runs through decimal, so when a price confuses you, convert it to decimal first: it is the cleanest way to compare any two bets.
Why do bettors need to win more than half their bets to break even?
Because of the vig, the bookmaker's fee hidden in the prices. A typical spread offers -110 on both sides, and each side implies a 52.4% probability — together 104.8%, when true probabilities must sum to 100%. That extra 4.8% is the book's margin, so whichever side you take, you pay a premium and must win 52.4% of -110 bets just to break even.
How do you remove the vig to find the fair odds?
Three steps that work on any market. Convert every price to implied probability (1 divided by decimal odds), add them up — whatever sits above 100% is the overround — then divide each implied probability by that total. In a soccer market summing to 103.3%, a home price of 2.10 works out to fair odds of 2.17, showing the roughly 3% the book keeps.
Does shopping for better odds actually make a difference?
Yes, and it costs nothing. The same bet at -105 instead of -110 lowers your break-even rate from 52.4% to 51.2%. Over 200 bets winning exactly half, the -110 bettor ends $1,000 down while the -105 bettor loses only $500 — identical picks, half the damage. Paying less vig per bet is the one improvement that requires zero prediction skill.
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About the author
Betting & Casino Math Writer
Sofia writes the odds track — betting odds formats, implied probability, house edge, and RTP — and the Myth vs Math pieces that test popular casino tricks against the arithmetic.
