How Bookmakers Make Money | StableBet

If bookmakers simply paid out fair odds on every race, they’d go bust within a week. Every betting shop, every app, every on-course bookie builds a profit margin into their prices. Understanding how this works won’t turn you into a winning punter overnight, but it will help you make smarter decisions about where and when to place your bets.

The short answer: bookmakers make money by paying out slightly less than they should based on true probability. The mechanism they use is called the overround – and once you understand it, you’ll never look at a betting market quite the same way again.

What is the Overround?

The overround (sometimes called the “vig,” “juice,” or “margin”) is the bookmaker’s built-in profit. It’s the percentage by which the total implied probability of all outcomes exceeds 100%.

In a perfectly fair market, probabilities add up to exactly 100%. If a coin flip were priced fairly, heads would be evens (2.0 decimal, 50% implied probability) and tails would be evens (2.0 decimal, 50% implied probability). That’s 50% + 50% = 100%.

But bookmakers don’t offer fair markets. They might price that same coin flip at 10/11 on heads and 10/11 on tails. Let’s see what that does:

10/11 = 1.91 decimal = 52.4% implied probability
10/11 = 1.91 decimal = 52.4% implied probability
Total: 104.8%

That extra 4.8% is the overround. It’s the bookmaker’s edge. No matter which side of the coin lands face up, they’ve built in a margin that, over thousands of bets, guarantees them profit.

How the Overround Works in Horse Racing

Horse racing markets are more complex than a coin flip, but the principle is identical. Let’s look at a simple three-horse race:

Fair odds (100% book):

Horse	True Probability	Fair Decimal Odds
Horse A	50%	2.00
Horse B	30%	3.33
Horse C	20%	5.00
Total	100%

Bookmaker odds (with overround):

Horse	Offered Odds	Implied Probability
Horse A	1.83 (5/6)	54.6%
Horse B	2.90	34.5%
Horse C	4.00 (3/1)	25.0%
Total		114.1%

The overround here is 14.1%. Every horse’s odds have been shortened slightly from what they “should” be. If you backed all three horses proportionally to guarantee a return regardless of the winner, you’d lose money. That’s by design.

Typical Overrounds by Market Type

Not all betting markets carry the same margin. Bookmakers adjust their overround based on how much action they expect and how competitive the market is.

Market Type	Typical Overround
Major race win markets (e.g., Cheltenham Gold Cup)	102-105%
Standard UK racing win markets	110-120%
Small field races (5 runners or fewer)	108-115%
Large field handicaps (15+ runners)	120-135%
Each-way betting (place portion)	125-140%
Forecasts and tricasts	130-160%
Ante-post markets	130-150%

The pattern is clear: the more outcomes, the higher the margin. A 20-runner handicap gives bookmakers more room to hide their edge across all those prices. A two-horse match bet offers nowhere to hide, so margins are tighter.

The Favourite-Longshot Bias

Here’s something bookmakers know that casual punters often don’t: people love backing longshots.

There’s an academic theory called the “favourite-longshot bias” that’s been proven across decades of betting data. Punters systematically overbet outsiders and underbet favourites. We’re drawn to the dream of landing a 33/1 shot, even when the maths doesn’t support it.

Bookmakers exploit this ruthlessly. They load more margin onto the outsiders because punters are less price-sensitive at big odds. Think about it: would you really notice the difference between 33/1 and 40/1 on a horse you fancy at big odds? Probably not. But you’d definitely notice the difference between 2/1 and 5/2 on a favourite.

This means:

Favourites often represent better mathematical value (lower margin)
Outsiders often carry the heaviest overround
The “fun” bets tend to be the worst value bets

It doesn’t mean you should only back favourites. But it does mean you should be especially critical when assessing big-priced horses – the odds might look generous, but they’re often worse value than they appear.

Ante-Post Markets: Where Margins Balloon

Ante-post betting – placing bets weeks or months before a race – comes with significantly higher margins than day-of-race markets.

Why? Several reasons:

Uncertainty. Horses might not even run. Injuries, changes of plan, unsuitable ground – plenty can go wrong between placing your bet and race day.

Time value of money. The bookmaker holds your stake for months. That has value to them.

Less competitive pressure. Ante-post markets attract less betting volume, so bookmakers face less pressure to sharpen their prices.

A Cheltenham Festival market opened in October might carry a 140-150% overround. The same race on the day might be 105-110%. That’s a massive difference in value.

If you bet ante-post, you need to be confident you’re getting significantly better odds than you’ll see on race day – enough to compensate for both the higher margin and the risk of non-runners.

How Margins Compound on Accumulators

Accumulators are where the maths really turns against you. Each leg of an acca carries its own margin, and those margins compound.

Example:

You place a 4-fold accumulator. Each leg has a 5% margin built into the price.

Leg 1: 5% margin
Leg 2: 5% margin
Leg 3: 5% margin
Leg 4: 5% margin

You might think: “That’s 20% total margin.” But it’s actually worse. The margins multiply, not add.

The true combined margin: 1.05 × 1.05 × 1.05 × 1.05 = 1.216, or 21.6% margin

On a 10-fold acca with 5% margin per leg, the combined margin exceeds 60%. You’re giving away more than half your expected value before the first race even starts.

This is why bookmakers love promoting accumulators. They’re fun, they offer life-changing potential returns, and they’re mathematically terrible for punters. The occasional big winner makes headlines; the structural edge grinding down millions of losing accas doesn’t.

Why Margins Matter to You

Every percentage point of overround comes directly out of potential returns. Over time, betting into high-margin markets is like running uphill – you’re fighting the maths from the start.

Example:

You’re betting £10 on a horse you believe has a 25% chance of winning.

Fair odds for 25% probability: 4.00 (3/1) → Returns £40
Bookmaker odds with margin: 3.50 (5/2) → Returns £35

If your assessment is correct and this horse wins 25% of the time over 100 bets:

At fair odds: 25 × £40 = £1,000 returned on £1,000 staked (break even)
At bookmaker odds: 25 × £35 = £875 returned on £1,000 staked (£125 loss)

That £125 difference is the overround doing its job. You were right about the horse’s chances, but the margin still ground you down.

How Bookmakers Balance Their Books

Contrary to popular belief, bookmakers don’t always try to “balance their book” so they profit regardless of the outcome. Modern bookmakers often take positions – they’re happy to have liability on certain horses if they believe the odds are in their favour.

However, they do manage risk through several mechanisms:

Adjusting odds based on money flow. If too much money comes in for one horse, they’ll shorten that horse’s price (making it less attractive) and push out the others. This is why odds move throughout the day.

Limiting successful punters. If you consistently beat the market, bookmakers will restrict your stakes or close your account. They’re under no obligation to accept your bets.

Laying off risk. Bookmakers can place bets with other bookmakers or on betting exchanges to reduce their exposure on any single outcome.

Volume. The real money is in turnover. A bookmaker with millions of customers betting small amounts will profit from the overround even if individual results go against them short-term.

Finding Better Value

You can’t eliminate the overround, but you can minimise its impact:

Shop around. Different bookmakers price the same race differently. The horse you fancy might be 5/1 at one firm and 11/2 at another. Those small differences compound over time. Use odds comparison sites before placing any bet.

Stick to competitive markets. Major races and popular meetings attract more betting volume, which generally means tighter margins. The 3:30 at Cheltenham will usually have a lower overround than the 2:15 at Plumpton.

Consider betting exchanges. Exchanges like Betfair don’t build in an overround – they charge commission on winning bets instead. For odds above about 3.0, exchanges often offer better value than traditional bookmakers.

Focus on win markets. Place and each-way markets typically carry higher margins. If you’re confident a horse will win, a straight win bet into a competitive market usually offers better mathematical value.

Avoid exotic multiples. The overround on accumulators compounds with each leg. A four-fold acca might look like fun, but the built-in margin can exceed 20% by the time all legs are factored in.

Be sceptical of longshots. Remember the favourite-longshot bias. Those big odds often carry the biggest margins.

Key Takeaways

The overround isn’t a conspiracy – it’s simply how bookmakers stay in business. Every price you see has been adjusted to ensure the house wins over time.

Understanding this helps you:

Recognise which markets offer tighter margins
See why odds comparison matters
Appreciate why exchanges can offer better value
Understand why accumulators and ante-post bets are worse value
Accept that long-term profit requires beating the margin, not just picking winners

The bookmaker’s edge is real, but it’s not insurmountable. Educated punters who shop around, stick to value markets, and maintain discipline can absolutely compete. But you have to know what you’re up against.