The Business of Setting Odds
Bookmakers are businesses, and like all businesses, they need to make a profit. They achieve this not by predicting winners, but by pricing their markets in a way that guarantees a return regardless of the outcome. Understanding how this works gives you a crucial advantage as a bettor.
Implied Probability and the Overround
Each set of odds implies a probability. If decimal odds are 2.00, the implied probability is 50% (1 ÷ 2.00 = 0.50). In a perfectly fair market, the implied probabilities of all possible outcomes would add up to exactly 100%.
In reality, they always add up to more than 100%. This excess is called the overround (also known as the "vig", "juice", or "margin") — and it represents the bookmaker's built-in profit.
A Simple Example: Coin Flip
A fair coin flip has two outcomes with a 50% probability each. A fair bet would offer odds of 2.00 on both heads and tails. Now consider a bookmaker offering:
- Heads: 1.91
- Tails: 1.91
Implied probability of Heads: 1 ÷ 1.91 = 52.4%
Implied probability of Tails: 1 ÷ 1.91 = 52.4%
Total: 104.8%
That extra 4.8% is the overround. No matter which side wins, the bookmaker profits over a large volume of bets.
How to Calculate the Overround
Formula: Overround = (Sum of all implied probabilities − 1) × 100
In a football match with three outcomes (Home Win, Draw, Away Win) priced at 2.10, 3.40, and 3.60 respectively:
- Home Win: 1 ÷ 2.10 = 47.6%
- Draw: 1 ÷ 3.40 = 29.4%
- Away Win: 1 ÷ 3.60 = 27.8%
- Total = 104.8% → Overround of 4.8%
Typical Margin Ranges by Market Type
| Market Type | Typical Overround |
|---|---|
| Major football match winner | 4–6% |
| Tennis match winner | 3–5% |
| Horse racing (each-way markets) | 10–20% |
| Accumulator/parlay bets | Compounds per leg |
| Casino games (e.g., roulette) | 2.7–5.26% (fixed) |
Why This Matters for Your Betting
The overround is the structural headwind every bettor faces. You are not just trying to pick winners — you are trying to pick winners at a rate that overcomes the margin. This is why:
- Fewer, selective bets are generally smarter than high-volume betting across many markets.
- Shopping for the best odds across multiple bookmakers directly reduces the overround you face on each bet.
- Accumulators compound the margin — each leg adds to the bookmaker's edge, which is why parlays are profitable for bookmakers despite their appeal to bettors.
Using This Knowledge Practically
Before placing any bet, calculate the implied probability and ask yourself: do I genuinely believe the true probability of this outcome is higher than what the odds suggest? If the answer is yes — and your reasoning is sound — you may have found value. If you're just picking a favourite because it feels likely, the margin is quietly working against you.