Betting odds tell you two things at once: how much a winning bet pays and how likely the bookmaker thinks the outcome is. Decimal odds of 2.50 return 2.50 units for every unit staked, stake included, which implies a 40% chance. Every format below says the same thing in a different way, and every price on a betting site also hides a small margin that works against you. Once you can convert prices and measure that margin, you can compare bookmakers on numbers instead of adverts.
What do betting odds actually mean?
Odds are a price. They state what a bookmaker will pay if your selection wins, and by implication what probability it assigns to that result. A price of 4.00 says “we will pay four times your stake back”, which only makes sense for the bookmaker if the event happens less than 25% of the time. Turning odds into a probability is the first habit of a disciplined bettor, because it lets you ask the only question that matters: is the real chance higher than the price suggests?
Three ideas carry through this page. The return is what you get back on a win (stake plus profit). The implied probability is 1 divided by the decimal price. The margin (also called overround or vig) is how far the implied probabilities of all outcomes add up above 100%.
How do the six odds formats compare?
Decimal odds are standard across Europe, Australia, Canada and most international sportsbooks. Fractional odds come from British and Irish racing. American (moneyline) odds use plus and minus signs around 100. Hong Kong, Malay and Indonesian odds are common on Asian handicap markets. All of them describe the same bet; the table converts two example prices so you can see each format side by side.
| Format | Underdog example | Favourite example | How to read it |
|---|---|---|---|
| Decimal | 2.50 | 1.60 | Total return per 1 unit staked |
| Fractional | 3/2 | 3/5 | Profit / stake (3/2 = win 3 for every 2 staked) |
| American | +150 | -167 | + = profit on 100 staked; minus = stake needed to win 100 |
| Hong Kong | 1.50 | 0.60 | Profit per 1 unit staked (decimal minus 1) |
| Malay | -0.67 | 0.60 | Positive up to 1.00; negative means risk that amount to win 1 |
| Indonesian | 1.50 | -1.67 | Same logic as American, divided by 100 |
| Implied probability | 40.0% | 62.5% | 1 / decimal odds |
Decimal odds
Multiply your stake by the price to get the total return. A €20 bet at 2.50 returns €50, of which €30 is profit. Decimal is the easiest format for maths, which is why the rest of this site uses it.
Fractional odds
The first number is profit, the second is the stake that earns it. At 3/2, a €20 bet makes €30 profit and returns €50. To convert, divide the fraction and add 1: 3 ÷ 2 + 1 = 2.50. Evens (1/1) equals 2.00.
American odds
A plus price shows the profit on a 100 stake, so +150 means 100 wins 150. A minus price shows what you must stake to win 100, so -167 means staking 167 to win 100. Conversion: for plus prices, decimal = 1 + price/100; for minus prices, decimal = 1 + 100/|price|. You will meet this format on NFL and NBA markets, which we cover in the American football betting guide.
Hong Kong, Malay and Indonesian odds
Hong Kong odds are decimal odds minus the stake, so 2.50 becomes 1.50. Malay odds stay between -1 and +1: a positive number is profit per unit, and a negative number such as -0.67 means you risk 0.67 to win 1. Indonesian odds mirror American odds divided by 100. Asian bookmakers quote handicaps this way because the prices cluster near even money, which you can see on the bet types page under Asian handicap.
How do you turn odds into a probability?
Divide 1 by the decimal price and multiply by 100. At 2.50 the implied probability is 40%; at 1.60 it is 62.5%; at 11.00 it is about 9.1%. That number is the bookmaker’s view plus its margin, not the true chance. If you think a team wins 45% of the time and the price implies 40%, the bet has positive expected value; if you think 35%, it does not.
Expected value per unit staked is (probability × decimal odds) minus 1. With a 45% estimate at 2.50: 0.45 × 2.50 − 1 = +0.125, a 12.5% edge. With a 35% estimate: 0.35 × 2.50 − 1 = −0.125. Small differences in your estimate swing the answer, which is why serious bettors keep records and test their numbers over hundreds of bets rather than a weekend.
What is the bookmaker margin and how do you calculate it?
The margin is the share of every market the bookmaker keeps on average. Add the implied probabilities of every outcome; anything above 100% is the overround. The margin as a share of turnover is 1 − 1/(sum of implied probabilities). Lower is better for you, and the same match can carry very different margins at different bookmakers or in different markets.
Worked example: a three-way football market
Suppose a league match is priced Home 2.10, Draw 3.40, Away 3.60 (illustrative prices, not live odds).
| Outcome | Decimal odds | Implied probability | Fair probability (margin removed) | Fair odds |
|---|---|---|---|---|
| Home | 2.10 | 47.62% | 45.43% | 2.20 |
| Draw | 3.40 | 29.41% | 28.06% | 3.56 |
| Away | 3.60 | 27.78% | 26.50% | 3.77 |
| Total | 104.81% | 100.00% |
The overround is 4.81%. As a share of money staked, the bookmaker keeps 1 − 1/1.0481 = 4.59% if bets are spread in proportion to the prices. To strip the margin with the simple proportional method, divide each implied probability by 1.0481. Real margins are rarely spread evenly (bookmakers usually shade long shots more), so the proportional method is a starting estimate, not the truth.
Worked example: a two-way tennis market
Player A at 1.80 and Player B at 2.02 imply 55.56% and 49.50%, a total of 105.06%. The margin on turnover is about 4.8%. Compare that with 1.87 and 2.05, which add up to 102.25% (a margin near 2.2%). Same match, same players: the second price list gives back more than twice as much of every euro over time. Our tennis betting guide shows how retirement rules can change what you are really buying at either price.
Why does a small price difference matter?
Because it compounds. Take 200 single bets of €20 at roughly even odds. Getting 2.10 instead of 2.00 on each winner adds €2 per winning bet; with 100 winners that is €200 over the run, on €4,000 staked. That is a 5% swing in return from price shopping alone, more than most bettors’ entire edge. Accounts at two or three bookmakers and a habit of checking prices before you bet are worth more than any tip.
Margins also vary by market. Main markets on big football leagues are usually the tightest; player props, correct score and long-term outrights usually carry far more. Accumulators multiply the margin of every leg, which is why a five-leg acca at 5% per leg has an effective margin above 22% (1 − 0.95⁵). The bonuses guide uses the same arithmetic to show what a bonus rollover really costs.
How do odds move, and what do the moves tell you?
Prices change when money arrives, when news breaks (injuries, team sheets, weather) and when sharper bookmakers move first. A price that shortens from 2.40 to 2.10 before kick-off usually reflects information or weight of money. Bettors who regularly beat the closing price, the last price before the event starts, tend to be the ones with a real edge, so recording the price you took next to the closing price is a useful self-test. In-play prices move second by second and include a delay on your bet; the live betting guide explains how that works.
What do odds comparison sites do?
Comparison sites collect prices from many bookmakers and show the best available for each outcome, often with a history of how the line moved. They are useful for spotting the outlier price and for seeing the market’s consensus. Treat them as a starting point: check that the bookmaker with the best price actually accepts customers from your country, that its limits suit your stakes, and that its licence checks out (see how to choose a bookmaker). A big outlier price is sometimes a palpable error that the bookmaker’s rules allow it to void.
Quick conversion formulas
- Fractional a/b to decimal: a ÷ b + 1.
- Decimal to fractional: (decimal − 1) as a fraction, so 2.75 becomes 7/4.
- American +X to decimal: 1 + X/100. American −X to decimal: 1 + 100/X.
- Decimal to American: at 2.00 or above, (decimal − 1) × 100 with a plus sign; below 2.00, −100 ÷ (decimal − 1).
- Hong Kong: decimal − 1.
- Implied probability: 1 ÷ decimal × 100.
Frequently asked questions
What does 5/2 mean in betting?
Fractional odds of 5/2 mean you win 5 units of profit for every 2 staked. A €10 bet returns €35 in total: €25 profit plus your €10 stake. In decimal that is 3.50, and the implied probability is about 28.6%.
What does -110 mean in betting odds?
American odds of -110 mean you stake 110 to win 100 in profit. In decimal that is about 1.91, an implied probability of 52.4%. Two sides both priced at -110 add up to 104.8%, a margin of about 4.5% on turnover.
Which odds format is best?
None changes what you are paid, so use the one you calculate fastest. Decimal is the simplest for comparing prices and working out probability, and most international bookmakers let you switch format in the account or site settings.
How do I calculate the bookmaker margin?
Convert every outcome in the market to an implied probability (1 divided by the decimal price), add them up, and subtract 100%. A total of 104.81% means a 4.81% overround, which is a margin of about 4.59% on money staked.
Do higher odds mean better value?
Not on their own. Higher odds pay more because the outcome is less likely. A bet is only good value when your estimate of the true probability is higher than the probability implied by the price, after allowing for the margin.
Why are odds different at each bookmaker?
Each bookmaker sets its own margin, takes different money and reacts to news at a different speed. That is why the same outcome can be 2.05 at one site and 2.20 at another, and why comparing prices before you bet pays off over time.
Sources
- Wikipedia: Fixed-odds betting (odds formats and conversions)
- Wikipedia: Mathematics of bookmaking (overround)
- Pinnacle betting resources: how to calculate the bookmaker margin
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