Footiehound

Bookmaker Odds Explained: Why They’re Not What You Think

• Ken Anucha
• July 12, 2025

It was the 89th minute of a Premier League game between Arsenal and Brighton. Arsenal were leading 2-1, but Brighton had just won a corner. Kola, a long-time punter from Lagos, stared at his betting slip, sweating.

He had backed “No more goals.” The odds looked decent when he placed it. But as the ball floated in, chaos erupted in the box, and Brighton equalised. Game ended 2-2. Slip torn. Heart broken.

Kola wasn’t unlucky. He just misunderstood something critical. He thought the odds he bet on were a true reflection of what was likely to happen. They weren’t. Not exactly. And that’s where most bettors make the same mistake.

Understanding the difference between bookmaker odds and real probability is not just for academics or maths geeks. It’s for every single person who wants to win more and lose less. Let’s break it down in a way that makes real sense.

SEE: Odd 4 Sure Wins

What are bookmaker odds really based on?

Bookmakers don’t offer odds to tell you the truth about an outcome’s chances. They offer odds to make profit. A bookmaker is a business. Its goal is not to predict the game correctly. Its goal is to balance the book—a term that means they want to collect more money than they pay out, no matter who wins.

Bookmaker odds are set using complex models that combine data from:

• Historical stats

• Market behavior

• Injuries and suspensions

• Weather

• Fan betting patterns

But even more crucially, bookmakers add a margin to ensure they stay profitable. This is sometimes called the overround.

For example, in a fair coin toss (50/50), the fair odds should be 2.00 on either side. But a bookmaker might offer 1.91 vs 1.91. That 0.09 on both sides is the profit baked in.

Now stretch that concept over 10+ outcomes in a football match: win, draw, over/under goals, corners, cards, HT/FT, etc. That’s how they build their margin.

How is real probability different from bookmaker odds?

Real probability is the actual chance that an outcome will occur, free from bookmaker profit motives. In statistics, it’s calculated as:

Probability (%) = 1 / Decimal Odds × 100

But if we flip this around, we can find implied probability from bookmaker odds. Let’s take a La Liga match: Real Madrid vs Sevilla.

SEE: Free 10 Odds For Today

Say Real Madrid’s win odds are 1.50

Implied Probability = (1 / 1.50) × 100 = 66.67%

But wait. Does that mean Real Madrid has a 66.67% chance of winning?

Not quite.

This is bookmaker probability, not real probability. To find real probability, you would need advanced statistical modelling, unbiased match simulations, or historical win percentages under similar conditions. And even then, you’d still only get an estimate.

So why do bookmakers’ odds differ from reality?

There are four major reasons:

1. Profit Margins (Overrounds):
   Bookies build a buffer into odds. So the sum of implied probabilities from all outcomes is often above 100%. This ensures they profit regardless of the outcome.

2. Market Movements:
   Odds are adjusted based on betting volumes. If more people bet on one side (even irrationally), bookies shorten those odds to limit exposure.

3. Public Bias:
   Popular teams like Manchester United or Barcelona often have odds lower than their real chances, simply because people love betting on them. The bookie takes advantage of the emotional crowd.

4. Limited Information Models:
   Bookies also rely on models. And those models can be wrong. Especially when surprise injuries, dressing room unrest, or unusual weather disrupt predictions.

Can you calculate real probabilities yourself?

Yes, but it’s not easy. Some bettors use:

• Expected Goals (xG) data from platforms like Understat or FBRef

• Team Form metrics over past 5–10 games

• Head-to-head history

• Injury/line-up analytics

• Game state trends (e.g., how often a team scores first and wins)

SEE: Big Odd Prediction

Let’s say you find that Brentford wins at home against teams below the top 10 about 70% of the time in the last 3 seasons. If a bookmaker gives you odds of 2.00 (50%), there’s a potential value bet. Because you believe the real probability is 70%.

But beware. Sample size, recency, and context all matter. A win streak during COVID-19 lockdown is not the same as current form with full fans and travel.

What’s an example of value vs trap?

Example 1 (Value Bet):

Arsenal vs Burnley
Bookmaker Odds: Arsenal Win 1.40
Implied Probability: (1 / 1.40) × 100 = 71.43%

But based on your own model, you calculate Arsenal has an 80% chance of winning due to:

• No injuries

• Top form

• Burnley away record of 3% wins in 20 matches

There’s value here. You’re getting 1.40 for something you think should be 1.25. That’s an edge.

Example 2 (Trap):

Manchester United vs Fulham
Odds: United Win 1.60
Implied Probability: 62.5%

But United just played a midweek Champions League match, are tired, and have key defenders suspended. You calculate real chance of win at only 48%. But fans are still pouring money on United.

The odds are inflated because of public bias. There’s no value here, only risk.

SEE: Zero Risk Football Prediction

How can understanding this make you a better bettor?

Once you realise that bookmaker odds are not gospel truth but tools for business, your mindset shifts. You stop thinking like a customer and start thinking like a trader. This is the heart of value betting.

Pro Tip:
Look for matches where your estimation of real probability is higher than the implied bookmaker probability. That’s where consistent profit lies.

Just like traders find underpriced stocks, bettors find underpriced outcomes. You don’t have to win every bet. You just need to bet where the odds are in your favour long term.

FAQ SECTION

Why do bookmakers change odds even before a game starts?
Because odds reflect both probability and public betting behaviour. If too much money goes on one side, the bookie shifts the odds to balance potential losses.

Are decimal odds better for understanding probability?
Yes. Decimal odds directly show potential return per unit. They’re clearer when converting to implied probability compared to fractional or American odds.

What is overround in simple terms?
It’s the extra percentage above 100% that bookmakers build into their odds to guarantee profit. A fair market adds up to 100%. A bookie’s market might add up to 105–110%.

Can odds reflect real probability in some cases?
Sometimes yes, especially in low-volume markets where bookies have less information or interest. But in high-profile games, odds are shaped more by market demand than real stats.

What tools can help me find real probability?

• Expected Goals (xG) data

• Betting exchanges (like Betfair)

• Pinnacle’s closing lines

• Python or Excel models using team stats

• Past performance databases (e.g., WhoScored, SofaScore)

SEE: Over 1.5 Goals

Conclusion

Never take bookmaker odds at face value. Treat them like supermarket prices — sometimes inflated, sometimes discounted. But always set by sellers who want profit.

Your job as a punter isn’t just to pick winners. It’s to find underpriced outcomes based on your better understanding of real probability. That’s what separates casual bettors from long-term winners.

So next time you open your bet slip, ask yourself: “Do I really believe this outcome is more likely than the odds suggest?” If the answer is yes, that’s your edge. Use it wisely.