Learn · Betting basics
How Betting Odds Work
Try the Odds Converter calculatorSame odds. Three ways to read them.
Betting odds are simply a way of expressing two things at once: how much a winning bet pays out, and how likely that outcome is considered to be. The three main formats — American (+150, −110), decimal (2.50, 1.91), and fractional (3/2, 10/11) — all describe the exact same information in different shapes. Once you can read one format, converting between them is just arithmetic, and understanding that arithmetic is the key to understanding what a “price” is actually telling you.
The three odds formats, explained
American odds
American odds are built around a $100 reference stake. A minus sign (−) shows how much you’d need to risk to win $100 profit; a plus sign (+) shows how much profit you’d win on a $100 stake.
- −110 means you’d stake $110 to profit $100.
- +150 means a $100 stake profits $150.00.
- −200 means you’d stake $200 to profit $100.
- +300 means a $100 stake profits $300.
Decimal odds
Decimal odds (common in the UK, Europe, and Australia) show your total payout — stake plus profit — for every $1 staked. A decimal of 1.91 (equivalent to −110) means a $1 stake returns $1.91 total. A decimal of 2.50 (equivalent to +150) means a $1 stake returns $2.50 total. Decimal odds are arguably the easiest format for quick mental math because the number already includes your original stake.
Fractional odds
Fractional odds, traditional in UK horse racing and still widely used for outright markets, show profit relative to stake. 10/11 means you’d profit $10 for every $11 staked (the fractional equivalent of −110). 5/2 means you’d profit $5 for every $2 staked (equivalent to +250 in American odds, or 3.50 in decimal). 3/2 is the fractional form of +150, and is also sometimes written as 6/4 depending on the market.
Implied probability: what the odds are really saying
Every price, in any format, can be converted into an implied probability — the chance of that outcome happening according to the odds. This is the single most useful concept in understanding odds, because it lets you compare prices in different formats on equal footing.
For example:
- −110 implies a 52.38% chance.
- +150 implies a 40.00% chance.
- −200 implies a 66.67% chance.
- +300 implies a 25.00% chance.
- Evens (+100 / 2.00 / 1/1) implies exactly 50.00%.
Shorter odds (like −200) mean a higher implied probability and a smaller payout. Longer odds (like +300) mean a lower implied probability and a bigger payout. This inverse relationship is the whole logic of pricing: the less likely an outcome is considered, the more it pays if it happens.
Worked example: converting and calculating a payout
Say a market is priced at +150 in American odds. Converting formats:
- Decimal: 2.50
- Fractional: 3/2
- Implied probability: 40.00%
If you stake $100 at +150, your profit is $150.00, for a total payout of $250.00 (your original $100 stake plus the $150.00 profit).
Compare that with a shorter price: $100 staked at −110 profits $90.91, for a total payout of $190.91. The shorter the odds, the smaller the profit on the same stake — because the implied probability of winning is higher.
In fractional terms, a £10 stake at 5/2 profits £25.00, for a total payout of £35.00 — the same relationship, just expressed in pounds and fraction form instead of dollars and American odds.
If you’re working across formats regularly, it’s much faster to run the numbers through a converter than to do it by hand every time — our odds converter handles American, decimal, fractional, and implied probability in one step. For the most common prices already worked out, from heavy favorites to long shots, see our betting odds chart.
Why probabilities add up to more than 100%: the vig
If odds purely reflected true chance, the implied probabilities of all outcomes in a market would add up to exactly 100%. In practice, they don’t — they add up to slightly more. That extra percentage is the vig (short for vigorish), also called the overround or margin, and it’s how the price-setter builds in a mathematical edge.
Take a simple two-way market priced at −110 on each side. Both sides individually imply 52.38%, but added together that’s 104.76% — the market’s overround (4.76 percentage points above 100%). The theoretical hold/vig on balanced action is 4.55% of total stakes, even though the fair (no-margin) probability for each side, if the match were truly a coin flip, is exactly 50.00%.
| American | Decimal | Fractional | Implied probability |
|---|---|---|---|
| −110 | 1.91 | 10/11 | 52.38% |
| +150 | 2.50 | 3/2 | 40.00% |
| −200 | 1.50 | 1/2 | 66.67% |
| +300 | 4.00 | 3/1 | 25.00% |
| +100 (evens) | 2.00 | 1/1 | 50.00% |
| +250 | 3.50 | 5/2 | 28.57% |
Understanding the vig matters because it explains why the “fair” chance of something happening and the number baked into the odds are rarely identical. For a deeper look at this specific topic, see our guide on what is vig in betting.
Common mistakes
- Mixing up plus and minus in American odds. A minus sign means you stake more than you win; a plus sign means you win more than you stake. Mixing these up leads to miscalculated payouts.
- Forgetting decimal odds include the stake. New readers sometimes treat 2.50 as “profit” when it’s actually total payout — the profit at 2.50 (equivalent to +150) on a $100 stake is $150.00, not $250.00.
- Assuming implied probability equals “fair” probability. As the −110/−110 example shows, the market’s total (104.76%) is higher than a fair 100% because of the built-in margin — the quoted number isn’t a pure probability estimate.
- Comparing fractions and decimals without converting first. 5/2 and 3/2 look similar on the page but represent very different probabilities (28.57% vs. 40.00%) — always convert to the same format before comparing.
- Not checking how odds formats relate across bet types. The same conversion logic applies whether you’re reading a moneyline, a spread, or a totals market — see our guides on moneyline betting, spread betting, and over/under betting for format-specific context.
Final word
Odds are, at their core, just probability dressed up in different notations — American, decimal, and fractional all answer the same question in a different accent. Learning to convert between them, and to read the implied probability underneath, turns an odds board from a wall of symbols into something genuinely understandable. If you want to check a specific number rather than convert by hand, our odds converter does it instantly. Always gamble responsibly, and only if you’re 18+ (21+ in some US states).