If you’ve ever opened a bookmaker’s site and wondered how odds formats work, and what 6/1, 5.00, or +400 actually means, you’re not alone. Understanding betting odds is the foundation of everything else in sports betting.
This guide breaks down how odds work, why different formats exist, how to convert between them, and how to calculate your returns — whether you’re betting online or at a UK high street bookmaker.
By the end, you’ll be able to look at any odds format and instantly understand what it represents, how much you could win, and how the bookmaker’s margin is built in.
💡 Tip: Once you understand the math, use our free Odds Converter Calculator to switch formats in seconds.
In the simplest terms, odds show the probability of an outcome happening and the payout you’ll receive if it does.
If you think of betting as predicting outcomes, odds are the bookmaker’s way of saying, “We think this is how likely each outcome is.”
There are three main odds formats used around the world:
| Format | Region Commonly Used | Example | Meaning |
|---|---|---|---|
| Fractional | UK & Ireland | 5/1 | Profit of £5 for every £1 stake |
| Decimal | Europe, Australia, Canada | 6.00 | Total return £6 for £1 stake |
| American (Moneyline) | USA | +500 / -200 | + = underdog, – = favourite |
Let’s explore each one in detail.
Fractional odds are the classic British format — you’ll still see them displayed in betting shops, horse racing markets, and some UK bookmaker sites.
Example: 5/1 (“five to one”)
This means you’ll win £5 profit for every £1 you stake, plus you get your stake back if you win.
So:
If you see 10/3, it means for every £3 staked, you’ll win £10.
Use this formula: Implied Probability=DenominatorNumerator+Denominator\text{Implied Probability} = \frac{Denominator}{Numerator + Denominator}Implied Probability=Numerator+DenominatorDenominator
Example: 5/1 → 1/(5+1)=0.16671 / (5+1) = 0.16671/(5+1)=0.1667 → 16.7% chance.
This tells you the bookmaker believes the event has roughly a 16.7% chance of happening.
💡 Try it: Use our Expected Value Calculator to see whether a set of odds offers value based on your own estimated probabilities.
Most modern UK betting sites let you toggle between fractional and decimal odds. Decimal is simpler because it shows your total return (stake included) for every £1 bet.
Example: 6.0
Total Return=Stake×Decimal Odds\text{Total Return} = \text{Stake} × \text{Decimal Odds}Total Return=Stake×Decimal Odds
| Fractional | Decimal | Profit per £1 Stake | Implied Probability |
|---|---|---|---|
| 1/2 | 1.5 | £0.50 | 66.7% |
| 5/4 | 2.25 | £1.25 | 44.4% |
| 5/1 | 6.0 | £5.00 | 16.7% |
| 10/1 | 11.0 | £10.00 | 9.1% |
That’s why most professional bettors and odds comparison sites prefer decimal format.
🔢 Tool: Use our Odds Converter Calculator to switch between formats instantly.
American odds, also called “moneyline odds”, are popular in the U.S. but occasionally appear in international comparison tools or exchange platforms.
They use positive (+) and negative (−) numbers:
+400 means you win £400 profit for every £100 bet (total return £500).
Equivalent to 4/1 fractional or 5.0 decimal.
−200 means you need to stake £200 to win £100 profit (total return £300).
Equivalent to 1/2 fractional or 1.5 decimal.
To decimal:
Example:
+250 → (250 ÷ 100) + 1 = 3.5
−200 → (100 ÷ 200) + 1 = 1.5
💡 Quick switch: try our Odds Converter Calculator and compare any format instantly.
It’s useful to know how to move between odds formats — especially if you use betting exchanges, follow tipsters, or compare odds globally.
| Convert From | To | Formula |
|---|---|---|
| Fractional | Decimal | (Numerator ÷ Denominator) + 1 |
| Decimal | Fractional | (Decimal − 1) = Profit per £1 |
| American (positive) | Decimal | (Odds ÷ 100) + 1 |
| American (negative) | Decimal | (100 ÷ |
Example:
5/1 → (5 ÷ 1) + 1 = 6.0 decimal
2.5 decimal → (2.5 − 1) = 1.5/1 fractional
Odds also contain the bookmaker’s overround — the built-in margin ensuring profit regardless of result.
To calculate true probability vs bookmaker-implied probability, sum all probabilities from the available outcomes. Anything above 100% = bookmaker’s margin.
Example (Football 1X2):
| Outcome | Odds | Probability |
|---|---|---|
| Home Win | 2.50 | 40% |
| Draw | 3.40 | 29.4% |
| Away Win | 3.10 | 32.3% |
| Total | — | 101.7% |
That extra 1.7% is the bookmaker’s edge.
📊 Learn more: Value Betting: Finding Edges vs the Market
“Evens” or “1/1” means you double your money if you win.
In decimal form, this is 2.0, and in American it’s +100.
Even money is the dividing line between favourites and underdogs: odds shorter than evens = favourite, longer than evens = outsider.
Every betting market (match result, over/under, handicaps, etc.) uses the same principles — just applied to different probabilities.
If “Over 2.5 Goals” is priced at 2.10, that’s roughly a 47.6% implied chance.
If “Under 2.5 Goals” is 1.80, that’s 55.5%.
Together, they total 103.1% — again, including margin.
Understanding how odds reflect likelihood helps you identify when bookmakers disagree, which is the basis of finding value.
Odds move when new information affects probabilities — injuries, weather, betting volume, or line movements. The more money placed on one outcome, the shorter its odds usually become.
Yes. Every licensed UK bookmaker allows you to switch between fractional, decimal, and (sometimes) American formats in your account settings or top menu.
Decimal odds are best for quick calculations and comparing prices. Fractional odds remain familiar for horse racing fans. American odds are mostly for US markets.
Bookmakers balance risk and profit, so odds aren’t pure probabilities — they include bias. Skilled bettors estimate true probabilities independently (using stats, models, or intuition) and compare them to bookmaker odds.
If your calculated probability suggests odds should be 2.0 but the bookmaker offers 2.5, that’s a value bet — because the offered price is higher than the event’s real likelihood.
🔍 Deep dive: Expected Value (EV) Guide
🔢 Tool: Expected Value Calculator
Once you’re confident reading odds, try exploring different bet types:
The maths behind odds stays the same — only the conditions change.
For practice, open your bookmaker account or an odds comparison site and translate every market you see into probabilities. You’ll start noticing how small margins separate outcomes — and where value might lie.
| Format | Example | Implied Probability | Best For |
|---|---|---|---|
| Fractional | 5/1 | 16.7% | Traditional UK bettors |
| Decimal | 6.0 | 16.7% | Quick maths & online betting |
| American | +500 | 16.7% | US markets & comparisons |
Betting odds represent the likelihood of an outcome and determine your potential return. In the UK, fractional odds show profit per stake, while decimal odds show total return. Higher odds mean a lower probability and higher potential payout.
Fractional odds (e.g., 5/1) show profit relative to stake and are traditional in UK betting shops. Decimal odds (e.g., 6.0) include your stake and are easier to calculate online. Both represent the same probability, just displayed differently.
To convert fractional to decimal, divide the numerator by the denominator and add 1. For decimal to fractional, subtract 1. You can use an odds converter calculator to switch between fractional, decimal, and American formats instantly.
American or moneyline odds use + and − values. Positive numbers (e.g., +400) show profit from a £100 stake. Negative numbers (e.g., −200) show how much you must stake to win £100. They're common in US markets.
Implied probability converts odds into a percentage chance. For fractional odds, divide the denominator by (numerator + denominator). For decimal odds, use 1 ÷ decimal. This helps you compare your own estimated chances to bookmaker odds.
