How Do Bookmakers Set Horse-Racing Odds? An On-Course Bookmaker Explains
When you look at the betting for a horse race, every runner has a price beside its name.
One horse may be offered at 2/1, another at 5/1 and an outsider at 25/1. As the start approaches, those prices can change repeatedly.
But who sets horse-racing odds? How does a bookmaker decide whether a horse should be 3/1 or 10/1? And why does a price sometimes shorten dramatically before a race?
At Kevin Myles, we have been involved in independent on-course bookmaking since 1989. In this guide, we explain how horse-racing odds are formed, why prices move and how an on-course bookmaker manages a betting market.
What Do Horse-Racing Odds Represent?
Horse-racing odds communicate two important pieces of information:
The potential return if the horse wins.
The market’s assessment of the horse’s chance of winning.
For example, fractional odds of 4/1 mean that a successful £1 win bet would produce £4 profit, with the original £1 stake also returned.
A £10 win bet at 4/1 would therefore produce:
£40 profit + £10 stake = £50 total return
Odds are also connected to probability.
A price of 4/1 is equivalent to decimal odds of 5.00. Its basic implied probability is:
1 ÷ 5.00 = 20%
This does not necessarily mean a horse has precisely a 20% chance of winning. It means that 20% is the probability represented by that individual price before considering the bookmaker’s margin and the remainder of the market.
For a complete introduction, read How Do Horse-Racing Odds Work? A Bookmaker Explains.
Who Sets the Odds for a Horse Race?
Major horse-racing markets will normally begin with prices produced by professional odds compilers and trading teams.
They assess every runner and create an initial set of prices—sometimes called an opening show or opening market.
Once those prices are released, the market begins to develop.
Bookmakers, traders, betting exchanges and customers all contribute to that process. Prices are compared across the industry, bets are placed, new information emerges and individual bookmakers adjust their odds.
By the time the race starts, the market may look considerably different from the opening prices.
At the racecourse, on-course bookmakers assess the wider market, the prices displayed by other bookmakers and the bets being placed in the ring. They then decide which prices they are prepared to offer.
It is a live and constantly changing marketplace.
What Information Is Used to Calculate Horse-Racing Odds?
Before a market opens, an odds compiler can consider a wide range of information about each horse.
This may include:
Recent form.
Previous performances over the same distance.
Performances on similar ground.
The racecourse and track configuration.
The horse’s official rating.
The weight being carried.
The draw in Flat racing.
The trainer’s recent form.
The jockey booking.
Previous course form.
Age and racing experience.
Equipment such as blinkers or a tongue-tie.
The expected pace of the race.
The strength of the opposition.
Market information and previous prices.
Not every factor carries equal importance in every race.
The draw may be highly relevant in a large-field sprint but much less influential in a different type of contest. Jumping experience may be critical in a chase, while previous performance over the distance may be particularly important in a staying race.
The aim is to assess each horse’s chance in relation to every other runner.
How Does a Bookmaker Turn Probability Into Odds?
Suppose an odds compiler estimates that a horse has a one-in-four chance of winning.
That represents a probability of 25%.
The fair decimal price would be calculated as:
1 ÷ 0.25 = 4.00
Decimal odds of 4.00 are equivalent to fractional odds of 3/1.
Other examples include:
Estimated probabilityFair decimal oddsFair fractional odds50%2.00Evens40%2.506/433.3%3.002/125%4.003/120%5.004/110%10.009/15%20.0019/1
These are simplified fair-price examples. A real bookmaker’s market also includes a margin.
What Is a Bookmaker’s Margin?
If every horse were offered at its precise mathematical fair price, the combined implied probabilities would total 100%.
A bookmaker’s displayed market will normally total more than 100%. The amount above 100% is commonly known as the overround or bookmaker’s margin.
For a very simple example, imagine a race containing five horses with identical chances.
A perfectly fair market would make each horse a 20% chance, equivalent to 4/1.
Five runners at 20% would total exactly 100%.
A bookmaker is unlikely to offer a market with no margin. The displayed prices may instead add up to more than 100%.
This margin helps compensate the bookmaker for taking on risk and operating the betting service.
However, an overround does not mean the bookmaker is guaranteed to make that percentage as profit. The actual outcome depends on the bets accepted, the prices offered, the liabilities created and which horse wins.
Does a Bookmaker Simply Try to Balance the Book?
You will sometimes hear that bookmakers shorten one horse and lengthen another until the same profit is guaranteed regardless of the result.
That is an oversimplification.
A balanced book may be desirable, but it is not always possible—or necessarily the bookmaker’s exact objective.
Customers do not distribute their stakes evenly across every runner. Some horses attract significantly more support than others, and a bookmaker must decide whether to accept that liability, change the price or manage the position in another way.
An on-course bookmaker may hold more money on one runner than another and will therefore have different financial results depending on which horse wins.
The skill lies in pricing the market, understanding the liabilities and making informed decisions as bets are accepted.
Why Do Horse-Racing Odds Change?
Horse-racing odds move because the market is continually responding to betting activity and new information.
A horse might open at:
8/1
It could then shorten to:
6/1 → 5/1 → 4/1
Alternatively, it could drift:
8/1 → 10/1 → 12/1
Several factors can cause these movements.
Money Being Bet on a Horse
If a bookmaker accepts a significant amount of money on one runner, the potential payout—or liability—on that horse increases.
The bookmaker may respond by shortening its price.
A lower price can reduce the potential return on subsequent bets and may encourage customers to consider other runners available at larger odds.
Movement Across the Wider Market
Bookmakers do not operate in isolation.
Prices are visible across racecourses, betting websites and exchanges. If a horse shortens across the wider industry, individual bookmakers will assess whether they should adjust their own price.
A bookmaker offering a noticeably larger price than the rest of the market may attract considerable betting activity on that horse.
New Information
Prices may change following information about:
Going conditions.
A change in the weather.
A jockey replacement.
A horse becoming a non-runner.
A trainer or jockey interview.
Travel or veterinary concerns.
A horse’s behaviour before the race.
How the track appears to be riding.
Results from earlier races on the card.
Not every piece of information will produce a significant movement, but the market is capable of reacting quickly.
Racecourse Betting Activity
As the race approaches, more people begin placing bets.
This increased activity can make the final few minutes particularly lively. On-course bookmakers monitor their own liabilities while watching how prices are changing throughout the ring and the wider market.
This is one of the defining features of racecourse bookmaking: prices are not static signs. They are part of a live market.
What Does It Mean When a Horse Is “Backed”?
When a horse attracts support and its odds shorten, racing commentators may say that it has been backed or is well backed.
For example:
10/1 → 8/1 → 6/1
The smaller price means a lower potential return for anyone betting after the move.
A well-backed horse is not guaranteed to win. Its price movement tells you what has happened in the betting market, not what will definitely happen in the race.
What Is a Drifter in Horse Racing?
A drifter is a horse whose odds become larger.
For example:
3/1 → 4/1 → 5/1
This may happen because customers are supporting other runners, bookmakers are comfortable accepting more bets on that horse or the wider market has reassessed its chance.
A drifting horse can still win.
Market movements are useful information, but they should never be treated as certainty.
Does Every Bookmaker Offer the Same Odds?
No.
Different bookmakers can offer different prices on the same horse.
One bookmaker may display a horse at 5/1 while another offers 11/2 or 6/1. Differences can occur because bookmakers have:
Accepted different bets.
Built different liabilities.
Formed different opinions.
Adjusted their prices at different times.
Adopted different approaches to the market.
This competition is one of the reasons customers compare prices before placing a bet.
The price available can make a meaningful difference to the final return.
For example:
£20 at 5/1 = £120 total return
£20 at 6/1 = £140 total return
Both examples assume the horse wins and no deductions or special settlement conditions apply.
How Do On-Course Bookmakers Set Their Prices?
Modern on-course bookmakers have access to considerably more technology and market information than their predecessors.
Prices can be displayed electronically and updated rapidly. Bookmakers can view market movements, record bets and calculate liabilities using specialist systems.
However, the fundamental decisions remain familiar:
What price should be offered?
How much money has been accepted?
What is the potential liability?
How is the rest of the market moving?
Should the price be shortened or lengthened?
Is the bookmaker willing to accept another bet at the displayed price?
An on-course bookmaker must make these decisions in a busy environment, often within seconds.
The technology may have changed, but judgement, experience and an understanding of the market remain important.
How Has Racecourse Bookmaking Changed Since 1989?
When Kevin Myles began bookmaking in 1989, the racecourse betting ring was a very different environment.
Prices were communicated through boards, voices and traditional hand signals. Information travelled more slowly, and experienced bookmakers relied heavily on their knowledge of the ring and the activity taking place around them.
Today, digital displays, specialist software, betting exchanges and near-instant market information allow prices to change across the industry within seconds.
Customers can also compare markets from mobile devices while standing at the racecourse.
Despite this technological transformation, the central principles of bookmaking remain:
Assess the probability.
Offer a price.
Accept bets.
Monitor liabilities.
Respond to the market.
Settle the result correctly.
Having operated through both eras, Kevin Myles combines traditional on-course bookmaking experience with modern betting technology.
What Happens When a Horse Is Withdrawn?
A non-runner changes the shape of a race.
The remaining horses now face one fewer opponent, and the removal of a short-priced runner can significantly improve their mathematical chances.
Bookmakers will therefore reform the market.
Bets placed after the revised market is published will use the new prices. Bets placed before the withdrawal may be subject to a Rule 4 deduction, depending on the price of the withdrawn horse and the applicable terms.
The purpose of the deduction is to account for the improved chances of the remaining runners.
How Is the Starting Price Determined?
As the race begins, an official Starting Price, or SP, is returned for each horse.
The SP records the industry price available at the start and is used to settle qualifying bets where a customer selected Starting Price rather than accepting fixed odds.
It is independently produced under rules overseen by the Starting Price Regulatory Commission rather than being decided after the race by an individual bookmaker.
Read our full guide: What Is Starting Price in Horse Racing? A Bookmaker Explains.
How Do Odds Work on an Each-Way Bet?
An each-way bet contains two parts:
A bet on the horse to win.
A bet on the horse to finish within the advertised places.
The win portion uses the horse’s full odds. The place portion is settled at a fraction of those odds, subject to the number of places being offered.
For example, a horse priced at 10/1 with place terms of one-fifth of the odds would have its place portion calculated at 2/1.
A £5 each-way bet costs £10 because £5 is placed on each part.
Read What Does Each-Way Mean in Horse Racing? A Bookmaker Explains for a complete explanation.
Do Short Odds Mean a Horse Will Win?
No.
A short price means the market considers the outcome more likely than a horse offered at longer odds. It does not make the result certain.
A 1/2 favourite can lose, while a 20/1 outsider can win.
Odds represent an assessment of probability and determine the potential return. They are not a promise or prediction of a guaranteed outcome.
Uncertainty is fundamental to both horse racing and betting.
Can Bookmakers Get the Odds Wrong?
Odds are opinions expressed as prices.
A market can underestimate or overestimate a horse’s chance. New information can emerge, customers can disagree with the available price and the eventual result may be very different from market expectations.
Bookmakers use information, mathematics, technology and experience, but no system can predict every race correctly.
That is why prices move—and why different bookmakers and customers can hold different opinions about the same runner.
How Bookmakers Set Odds: The Simple Version
If you remember nothing else from this guide, remember:
Bookmakers assess the chance of each horse winning.
Those probabilities are converted into odds.
A margin is included within the overall market.
Prices can change as bets are accepted and new information emerges.
On-course bookmakers monitor both their own liabilities and the wider market.
A shortening price means the odds are becoming smaller.
A drifting price means the odds are becoming larger.
The official Starting Price is returned when the race begins.
No price guarantees that a horse will win.
Bookmaking is not simply about picking the winner. It is about pricing every possible winner and managing the financial risk created by the bets accepted.
Independent On-Course Bookmakers Since 1989
Kevin Myles is a long-established independent on-course bookmaking business with a proud heritage in British racing.
Since 1989, we have experienced the betting ring’s evolution from traditional boards and racecourse communication to modern electronic displays and technology-driven markets.
Our A Bookmaker Explains series uses that experience to make betting terminology easier to understand.
Continue reading:
What Does Each-Way Mean in Horse Racing? A Bookmaker Explains
What Is Starting Price in Horse Racing? A Bookmaker Explains
Request a Bet With Kevin Myles
Whether you meet us in the betting ring or use our remote betting service, Kevin Myles combines traditional bookmaking experience with a modern way to request a bet.
Open an account with Kevin Myles and request your next bet.
Kevin Myles Ltd is licensed and regulated in Great Britain by the Gambling Commission under account number 100277.
18+ only. Please gamble responsibly.