Last Updated on 02/04/2026 by Andy Clark
Betting odds tell you two things: how likely a bookmaker thinks an outcome is and how much you will win if your bet comes in. Understanding both is essential to making informed betting decisions – and to identifying when a bookmaker’s odds represent genuine value.
This guide covers everything from reading fractional and decimal odds to calculating implied probability, understanding the bookmaker’s margin and spotting value bets.
What Are Betting Odds?
Odds are a numerical expression of probability. They represent the bookmaker’s assessment of how likely a given outcome is and determine the return you receive if your bet wins.
The core relationship is simple: the more likely an outcome, the lower the odds. The less likely an outcome, the higher the odds.
A heavy favourite in a football match might be priced at 1/4 (very short odds, high chance of winning). A massive underdog might be priced at 10/1 (long odds, low chance of winning). The odds reflect the implied probability of each outcome happening.
The Three Odds Formats
Odds are expressed in three formats worldwide. All three say the same thing – they are just different ways of presenting the same information.
Fractional Odds (UK Standard)
Fractional odds are the traditional format used in UK and Irish bookmakers and are still the default on most UK betting sites and racecourses.
Fractional odds are expressed as two numbers separated by a slash – for example, 4/1 (read as “four to one”) or 5/2 (read as “five to two”).
How to read them: The number on the left is your profit. The number on the right is your stake. So:
- 4/1 means for every £1 you stake, you win £4 profit (plus your £1 stake back = £5 total return)
- 5/2 means for every £2 you stake, you win £5 profit (plus your £2 stake back = £7 total return)
- 1/2 means for every £2 you stake, you win £1 profit (plus your £2 stake back = £3 total return)
Calculating your return on fractional odds:
Total return = (Stake ÷ right number) × left number + stake
Example: £10 bet at 7/2
(10 ÷ 2) × 7 + 10 = 5 × 7 + 10 = 35 + 10 = £45 total return (£35 profit)
Odds-on vs odds-against:
When the left number is smaller than the right (e.g. 1/2, 2/5, 4/7), the selection is odds-on – the bookmaker considers it more likely to win than not. You must stake more than you profit.
When the left number is larger than the right (e.g. 2/1, 5/2, 10/1), the selection is odds-against – the bookmaker considers it less likely to win. You profit more than you stake.
When both numbers are equal (1/1), the selection is evens – your profit equals your stake.
Decimal Odds (European Standard)
Decimal odds are the default format on most European betting sites and exchanges. Many UK punters switch to decimal because they are easier to calculate and compare.
Decimal odds are expressed as a single number – for example, 5.0, 3.50 or 1.50.
How to read them: Multiply your stake by the decimal odds to get your total return (including your stake).
- 5.0 – a £10 bet returns £50 total (£40 profit + £10 stake)
- 3.5 – a £10 bet returns £35 total (£25 profit + £10 stake)
- 1.5 – a £10 bet returns £15 total (£5 profit + £10 stake)
The key difference from fractional: Decimal odds always include your stake in the return. Fractional odds express profit only.
Evens in decimal is 2.0 – anything below 2.0 is odds-on, anything above 2.0 is odds-against.
Converting between fractional and decimal:
Decimal to fractional: subtract 1, then express as a fraction
- 4.0 → 4.0 – 1 = 3.0 → 3/1
- 3.5 → 3.5 – 1 = 2.5 → 2.5/1 → multiply both by 2 → 5/2
Fractional to decimal: divide the left number by the right, then add 1
- 5/2 → 5 ÷ 2 = 2.5, + 1 = 3.5
- 7/4 → 7 ÷ 4 = 1.75, + 1 = 2.75
American Odds (Moneyline)
American odds are the standard format in the United States and are expressed as positive or negative numbers relative to a $100 baseline. You will not encounter them often on UK betting sites, but they appear frequently in American sports coverage and on some international platforms.
Positive moneyline (+): Shows how much profit a $100 stake returns.
- +250 means a $100 stake wins $250 profit ($350 total return)
- +150 means a $100 stake wins $150 profit ($250 total return)
Positive moneyline = odds-against selection.
Negative moneyline (-): Shows how much you need to stake to win $100 profit.
- -200 means you must stake $200 to win $100 profit ($300 total return)
- -150 means you must stake $150 to win $100 profit ($250 total return)
Negative moneyline = odds-on selection.
Converting American to decimal:
- Positive: (American odds ÷ 100) + 1. So +250 → 2.5 + 1 = 3.5
- Negative: (100 ÷ American odds) + 1. So -200 → 0.5 + 1 = 1.5
Odds Comparison Table
The same selection expressed in all three formats:
| Fractional | Decimal | American | Implied probability |
|---|---|---|---|
| 1/4 | 1.25 | -400 | 80% |
| 1/2 | 1.50 | -200 | 66.7% |
| Evens (1/1) | 2.00 | +100 | 50% |
| 6/4 | 2.50 | +150 | 40% |
| 2/1 | 3.00 | +200 | 33.3% |
| 5/2 | 3.50 | +250 | 28.6% |
| 4/1 | 5.00 | +400 | 20% |
| 9/1 | 10.00 | +900 | 10% |
| 99/1 | 100.00 | +9900 | 1% |
Implied Probability – What the Odds Really Mean
Every set of odds implies a probability. Converting odds to implied probability is one of the most useful skills in betting – it tells you what the bookmaker actually thinks will happen.
Calculating implied probability from decimal odds:
Implied probability = 1 ÷ decimal odds × 100
- Odds of 2.0 → 1 ÷ 2.0 × 100 = 50%
- Odds of 4.0 → 1 ÷ 4.0 × 100 = 25%
- Odds of 1.5 → 1 ÷ 1.5 × 100 = 66.7%
Calculating implied probability from fractional odds:
Implied probability = right number ÷ (left number + right number) × 100
- Odds of 3/1 → 1 ÷ (3+1) × 100 = 25%
- Odds of 1/2 → 2 ÷ (1+2) × 100 = 66.7%
- Odds of 9/4 → 4 ÷ (9+4) × 100 = 30.8%
Understanding implied probability is the first step to identifying value – if you believe the true probability of an outcome is higher than the bookmaker’s implied probability, the bet represents value.
The Bookmaker’s Margin (Overround)
Here is the part most betting guides leave out – and it is arguably the most important thing to understand about how odds work.
Bookmakers do not simply price up the true probability of each outcome. They add a margin – also called the overround or vig – which ensures they make a profit regardless of the outcome.
How it works:
In a true 50/50 coin flip, both heads and tails would be priced at 2.0 (evens). The implied probabilities would add up to exactly 100%.
But if you tried to bet on a coin flip at a bookmaker, both heads and tails would likely be priced at 1.90 or 1.91. The implied probabilities would add up to more than 100%:
1 ÷ 1.90 = 52.6% for heads 1 ÷ 1.90 = 52.6% for tails Total = 105.3%
That extra 5.3% is the bookmaker’s margin. Across millions of bets, it generates their profit.
Calculating the overround on a football match:
A typical Premier League three-way market (home win, draw, away win):
- Home win: 2.0 (implied 50%)
- Draw: 3.4 (implied 29.4%)
- Away win: 4.2 (implied 23.8%)
- Total: 103.2%
The overround is 3.2%. The lower the overround, the better value the market offers to punters.
Betting exchanges like Betfair typically operate with overrounds of 1-2% compared to 5-10% at traditional bookmakers, which is why exchange odds are consistently better. We have a Matchbook sign-up offer that is a betting exchange.
What Is Value Betting?
Value betting is the practice of placing bets where the true probability of an outcome is higher than the implied probability in the odds.
Example:
A bookmaker prices Arsenal to win a match at 2.0 (implied probability: 50%).
You analyse the match using form, xG data, team news and head-to-head records and conclude Arsenal’s true probability of winning is 60%.
At odds of 2.0, you are being offered a 50% probability on a 60% event. That is a value bet – the odds are too long relative to the true probability.
Over a large sample of bets, consistently finding value beats consistently backing favourites at odds that accurately reflect their true chance.
The long-term maths:
Expected value (EV) = (Probability of winning × profit) – (Probability of losing × stake)
Arsenal at 2.0, £10 stake, true probability 60%: EV = (0.60 × £10) – (0.40 × £10) = £6 – £4 = +£2 expected value
A positive expected value means the bet is profitable in the long run. A negative EV means the opposite.
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How Odds Work in Different Bet Types
Single bets
The odds you see in the market are exactly what you receive. Stake multiplied by decimal odds gives your total return.
Accumulators
Odds from multiple selections are multiplied together. A three-fold acca at 2.0, 3.0 and 4.0 gives combined decimal odds of 2.0 × 3.0 × 4.0 = 24.0. A £10 stake returns £240.
The bookmaker’s margin is compounded with each additional leg, which is why accumulators with many legs offer progressively worse value.
Each-way bets
Each-way bets are two separate bets – one on the win at the advertised odds, and one on a place at a fraction of the win odds (typically 1/4 or 1/5). See our each-way betting guide for full details.
Best Odds Guaranteed (BOG)
Several bookmakers offer BOG on horse racing – if you take a fixed price and the Starting Price (SP) at race time is bigger, you are paid at the bigger SP. This removes the risk of taking a price that shortens before the race. See our Best Odds Guaranteed guide for the full list of BOG bookmakers.
Favourite vs Underdog – Reading the Market
In any betting market, the selection with the shortest odds is the favourite – the outcome the bookmaker considers most likely. The selection with the longest odds is the underdog – the outcome considered least likely.
In a Premier League match:
- Manchester City at 1.4 = heavy favourite (71.4% implied probability)
- Draw at 4.5 = second option (22.2% implied probability)
- Opposition at 9.0 = underdog (11.1% implied probability)
The favourite wins more often than the underdog – but because the odds are short, the returns are small. Backing underdogs consistently at long odds only becomes profitable if you identify ones that win more often than the implied probability suggests.
Odds Movement – What Price Changes Mean
Odds are not fixed. Bookmakers move prices in response to betting activity, new information and their own risk management.
Odds shorten (price falls, implied probability rises) when:
- Heavy money is placed on a selection
- Positive team news emerges (key player returns from injury)
- A late change in conditions favours the selection (going in horse racing, weather in football)
Odds drift (price rises, implied probability falls) when:
- Money comes for opposing selections
- Negative news emerges (star player injury, selection withdrawn)
- The market reassesses the likely outcome
In horse racing, significant market support from professional gamblers or connections’ money is often the most reliable signal. Watching for odds movements in the hour before a race is a legitimate analytical tool.
Betting Odds FAQs
What do odds of 4/1 mean?
At 4/1 you win £4 for every £1 you stake. A £10 bet at 4/1 returns £50 (£40 profit plus your £10 stake back).
What does odds-on mean?
Odds-on means the selection is considered more likely to win than not. The profit from an odds-on bet is less than the stake. Examples: 1/2, 4/6, 2/5.
What is evens in betting?
Evens (1/1 in fractional, 2.0 in decimal) means your profit equals your stake. A £10 bet at evens returns £20 (£10 profit + £10 stake).
What does it mean when odds change?
Odds change when bookmakers receive significant betting on one outcome, when new information emerges (injuries, team news) or when the bookmaker adjusts their margin. Odds shortening usually indicates a selection is attracting support.
What is implied probability?
Implied probability is the probability of an outcome suggested by the odds. It is calculated by dividing 1 by the decimal odds and multiplying by 100. Odds of 4.0 imply a 25% probability.
What is the bookmaker’s margin?
The bookmaker’s margin (overround) is the percentage by which the sum of all implied probabilities in a market exceeds 100%. It represents the bookmaker’s built-in profit. A lower overround means better value for the punter.
Which odds format is easiest to use?
Decimal odds are generally considered the easiest for calculating returns quickly. Multiply your stake by the decimal odds to get your total return. Most UK bookmakers allow you to switch between formats in your account settings.
What does SP mean in horse racing
SP stands for Starting Price – the official odds at the time a race begins, set by the market. If you take the SP rather than an early price, you receive whatever the market settles at when the race starts.
What is value in betting?
Value means the odds offered are higher than the true probability justifies. If you assess a selection has a 40% chance of winning but the bookmaker’s odds imply only 25%, the bet has positive expected value.
More Betting Guides
- Each-Way Betting Explained
- Acca Insurance Explained
- Draw No Bet Explained
- Over/Under Goals Explained
- Handicap Betting Explained
- Half Time/Full Time Explained
- Non-Runner No Bet Explained
- Best Odds Guaranteed Guide
- Football Tips
- Horse Racing Tips
- Free Bets Hub
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Andy is the founder, owner and editor of thatsagoal.com, with over 20 years of experience in betting on sports. He has a keen eye for stats, particularly when looking at players to be carded, and these form a large part of the bet builder tips you see on the site. As well as creating daily football tips, Andy also keeps thatsagoal updated with all the best bookmaker promotions and offers for our readers.