Betting Odds in Detail: How Betting Odds Work

Betting odds express the potential return attached to an outcome and the probability that the bookmaker’s price implies for that outcome.

For anyone learning how sports betting works, understanding odds is one of the fundamentals. You may see them written as fractions such as 5/2, decimals such as 3.50, or American odds such as +250. Although these formats look different, they can all describe the same price.

In this betting odds guide, we explain how betting odds work, how to read fractional, decimal, and American odds, how to calculate potential returns, how odds relate to probability, and why prices can differ between bookmakers.

What Betting Odds Mean?

Betting odds are the price assigned to a particular outcome in a betting market. For example, a football match-result market may contain three possible outcomes:

• Home win
• Draw
• Away win.

Each outcome is given its own odds.

In general, shorter odds indicate that an outcome is considered more likely, while longer odds indicate that it is considered less likely.

However, betting odds should not be treated as an objective forecast of what will happen. Bookmakers build a margin into their prices, so the probability implied by the odds is not necessarily the same as the real probability of the event occurring.

Odds also determine the potential return if a selection wins. That makes odds useful for answering two different questions:

1. What likelihood does this price imply?
2. What would the theoretical return be at this price?

Before looking at the calculations, it helps to understand the three main odds formats.

3 Main Types of Betting Odds

The most common betting odds formats are:

• Fractional odds
• Decimal odds
• American odds.

Fractional odds are widely used in the UK, while decimal odds are common across Europe and are also available on many UK betting sites. American odds, sometimes called moneyline odds, are mainly associated with North America.

Here is how the same price can appear in all three formats:

The format changes, but the underlying price does not.

Fractional Odds Explained

Fractional betting odds are written as two numbers separated by a slash, such as:

2/1

The first number represents the potential profit, the second represents the stake required to produce that profit.

At odds of 2/1:

• £1 corresponds to £2 potential profit
• The original £1 stake would also be returned if the selection won
• Total return would therefore be £3.

How to Calculate Fractional Odds Returns

The basic formula is:

Potential profit = stake × numerator ÷ denominator

Then:

Total return = potential profit + stake

For example, consider a hypothetical £10 stake at 5/2.

Potential profit:

£10 × 5 ÷ 2 = £25

Total return:

£25 + £10 = £35

Fractional odds show the potential profit relative to the stake, rather than the total return.

What Does 1/1 or Evens Mean?

Odds of 1/1 are commonly described as evens, even money or EVS. At evens, the potential profit is equal to the stake. For example:

• Hypothetical stake: £10
• Profit at 1/1: £10;
• Total return: £20.

Evens correspond to an implied probability of 50% before accounting for the wider market margin.

What Does Odds-On Mean?

Fractional odds are described as odds-on when the first number is smaller than the second. Examples include:

• 1/2
• 2/5
• 4/7.

With odds-on prices, the potential profit is smaller than the amount staked.

For example, at 1/2, a hypothetical £10 stake would produce £5 profit if successful, giving a total return of £15. An odds-on price implies a probability greater than 50%.

What Does Odds-Against Mean?

Odds are odds-against when the first number in the fraction is larger than the second.

Examples include:

• 2/1
• 5/2
• 10/1.

In these cases, the potential profit is greater than the stake.

For example, at 3/1, a hypothetical £10 stake corresponds to £30 profit and a £40 total return. Odds-against prices imply a probability below 50%.

Decimal Odds Explained

Decimal odds express the total potential return, including the original stake.

That makes the calculation relatively straightforward:

Total return = stake × decimal odds

For example, at decimal odds of 3.50:

£10 × 3.50 = £35

The £35 consists of:

• £25 potential profit
• £10 original stake.

This difference is important when comparing fractional and decimal odds:

• Fractional odds show potential profit relative to the stake
• Decimal odds directly represent the total potential return per unit staked.

American Odds Explained

American odds use positive and negative numbers.

Examples include:

• +150
• +300
• -120
• -250.

They are based around a reference amount of 100.

Positive American Odds

Positive odds show the profit associated with a hypothetical stake of 100 units.

For example:

+250

means 250 units of potential profit for every 100 units staked.

+250 is equivalent to:

• 5/2 fractional odds
• 3.50 decimal odds.

Negative American Odds

Negative American odds indicate how much would theoretically need to be staked to produce 100 units of profit.

For example:

-200

means 200 units of stake for 100 units of profit.

-200 is equivalent to:

• 1/2 fractional odds
• 1.50 decimal odds.

American odds are less common on UK betting sites, but understanding the format can be useful when reading US sports coverage or international betting information.

Fractional vs Decimal Odds: What Is the Difference?

Fractional and decimal odds communicate the same underlying price in different ways. The easiest distinction to remember is:

Fractional odds focus on profit. Decimal odds show total return.

Consider 5/2 and 3.50. These are equivalent prices, with a hypothetical £10 stake:

The outcome is identical. Only the way the price is displayed changes. Neither format gives inherently better odds than the other.

How to Convert Fractional Odds to Decimal Odds

To convert fractional odds to decimal odds, divide the numerator by the denominator and add 1. The formula is:

Decimal odds = (numerator ÷ denominator) + 1

For example, convert 5/2:

5 ÷ 2 = 2.5

Then add 1:

2.5 + 1 = 3.50

Therefore:

5/2 = 3.50

How to Convert Decimal Odds to Fractional Odds

To convert decimal betting odds to fractional odds:

1. Subtract 1
2. Express the result as a fraction
3. Simplify the fraction where possible.

For example:

4.00 − 1 = 3

This becomes:

3/1

So decimal odds of 4.00 are equivalent to fractional odds of 3/1.

How Do Betting Odds Relate to Probability?

One of the most useful ways to understand betting odds is to convert them into implied probability. Implied probability represents the percentage chance suggested by a particular price.

It is important to distinguish implied probability from actual probability. Bookmakers include a margin in their markets, so the percentages contained in the odds will normally add up to more than 100%.

Decimal Odds to Implied Probability

The formula is:

Implied probability = (1 ÷ decimal odds) × 100

For decimal odds of 2.00:

1 ÷ 2.00 × 100 = 50%

For odds of 4.00:

1 ÷ 4.00 × 100 = 25%

For odds of 1.50:

1 ÷ 1.50 × 100 = 66.67%

Fractional Odds to Implied Probability

For fractional odds, use:

Denominator ÷ (numerator + denominator) × 100

For 2/1:

1 ÷ (2 + 1) × 100 = 33.33%

For 4/1:

1 ÷ (4 + 1) × 100 = 20%

For 1/2:

2 ÷ (1 + 2) × 100 = 66.67%

What Is the Bookmaker Margin?

Bookmakers include a small margin in their odds to help them make a profit. For example, a football match has three possible results: home win, draw, or away win. Normally, the chances of all three results should add up to 100%. But when you convert the bookmaker’s odds into percentages, the total might be 105%. That extra 5% is called the bookmaker margin or overround. It’s built into the odds and helps explain how bookmakers make money.

Consider a hypothetical football match with these decimal odds:

Adding the implied probabilities gives:

47.62% + 29.41% + 27.78% = 104.81%

The theoretical overround is therefore:

104.81% − 100% = 4.81%

This is one reason implied probability should not automatically be interpreted as a bookmaker saying there is exactly that percentage chance of an outcome occurring.

Why Are Betting Odds Different Between Bookmakers?

Two bookmakers can display different prices for the same outcome. There are several possible reasons why.

Different Probability Assessments

Bookmakers do not necessarily calculate the likelihood of an outcome in the same way. Their models, traders, and available information may lead to slightly different assessments.

Different Margins

A bookmaker can apply a different margin to the same betting market. As a result, two sites could broadly agree on the probability of an outcome while still publishing different odds.

Market Activity

Prices can also react to activity within a betting market. If the balance of money changes significantly, prices may be adjusted.

New Information

Sporting information can affect the expected probability of an outcome. Examples may include:

• Team news
• Injuries
• Confirmed starting line-ups
• Weather conditions
• Player withdrawals
• Changes in venue or schedule.

This is why odds should be understood as prices that can change rather than fixed predictions.

Why Do Betting Odds Move?

When odds become shorter, they are sometimes said to have shortened or contracted. When odds become longer, they may be described as drifting.

For example:

3/1 → 2/1 is a shortening price.

The implied probability has increased from 25% to approximately 33.33%.

By contrast:

2/1 → 4/1 is a drifting price.

The implied probability falls from approximately 33.33% to 20%.

Odds movements can reflect new information, changes in the market or adjustments made by the bookmaker. A movement in price does not guarantee that the outcome itself has become more or less likely by the same amount.

What Are Starting Price Odds?

The term Starting Price, usually abbreviated to SP, is common in horse racing. The SP is the price determined at or immediately before the start of a race under the relevant pricing process. This differs from taking an earlier fixed price, where the odds are known before the event starts.

Some bookmakers offer Best Odds Guaranteed for horse racing. This means that if the odds are higher when the race starts than when the bet was placed, the higher odds will be used. The rules differ between best odds guaranteed sites, so check the start time and any daily cap first.

You can learn more about how this feature works in our Best Odds Guaranteed guide.

How Do Odds Work in Multiple Bets?

When several selections are combined into one multiple, their odds are combined to produce the price for the overall bet. With decimal odds, this can be illustrated by multiplication.

Suppose three hypothetical selections have odds of:

• 1.50
• 2.00
• 2.50.

The combined decimal odds would be:

1.50 × 2.00 × 2.50 = 7.50

However, every required selection normally needs to be successful for a standard accumulator to return as a winner. This creates an important relationship: adding more selections can increase the combined odds, but it also adds more outcomes that need to occur.

Different multiples and system bets work in different ways. See our Bet Types guide for an explanation of singles, accumulators and other structures.

How Do Handicap Betting Odds Work?

Handicap betting gives one team a virtual advantage or disadvantage to make the match more balanced.

For example, if Team A has a -1 goal handicap, they need to win by more than one goal for the bet to win.

The odds work the same way as in other bets. The main difference is that the handicap changes what needs to happen for the bet to win.

Common Betting Odds Terms

Understanding a few common terms makes betting prices easier to read.

Favourite
The participant or outcome given the shortest odds in a market.

Outsider
A participant or outcome with relatively long odds and therefore a lower implied probability.

Evens/EVS
Odds of 1/1 or 2.00 decimal, corresponding to a 50% implied probability before considering the market margin.

Odds-on
Fractional odds below evens, such as 1/2. They imply a probability greater than 50%.

Odds-against
Fractional odds above evens, such as 2/1. They imply a probability below 50%.

Short odds
Relatively low odds associated with a higher implied probability.

Long odds
Relatively high odds associated with a lower implied probability.

Price
Another word commonly used for betting odds.

Implied probability
The probability percentage derived mathematically from the odds.

Overround
The amount by which the combined implied probabilities in a bookmaker’s market exceed 100%.

Starting Price (SP)
A horse-racing price established at the start of a race.

Common Mistakes When Reading Betting Odds

Confusing Profit with Total Return

This is particularly common when moving between fractional and decimal odds. At 2/1, the figure describes £2 of potential profit for each £1 staked. At 3.00 decimal, the equivalent figure describes a total return of £3 for each £1 staked. They are the same price.

Assuming Longer Odds Mean a Better Bet

Higher odds mean a larger potential return but also represent a lower implied probability. The size of the number alone therefore says nothing about whether an outcome is likely to happen.

Treating Implied Probability as an Exact Prediction

An implied probability is calculated from a price. It is not proof that the outcome has precisely that chance of occurring. Bookmaker margins and differences in probability assessments need to be considered.

Ignoring the Conditions of the Market

Two selections can appear to concern the same match while actually having different settlement conditions.

For example, a match-winner market is different from a handicap market, while an accumulator is different from a single. Understanding the different types of bets is therefore just as important as understanding the odds attached to them.

Responsible Gambling

Betting always involves financial risk, and no outcome is guaranteed, regardless of the odds. Never bet more than you can afford to lose, and remember that gambling should not be seen as a way to make money. If gambling becomes difficult to control, support is available through BeGambleAware.

About the writer

Mike Brown

Sports Betting Editor

Mike Brown is the Sports Betting Editor at CaughtOffside. A former tipster and betting syndicate analyst, he writes on odds, bookmaker margins and expected value. He also reviews UK sportsbooks on withdrawals, KYC and promotional terms. He’s based in Manchester.

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