Every bookmaker needs to make a profit, regardless of which team wins. They achieve this through a mechanism known as the overround.
The overround is often called the vig, juice, or simply the bookmaker's margin. It is effectively a hidden cost built into every betting market.
If you want to become a profitable bettor, understanding the overround is essential because it determines how much value is lost before the match even begins.
In a perfectly fair market, the implied probabilities of all possible outcomes would add up to exactly 100%.
Bookmakers deliberately price markets so that the total implied probability exceeds 100%.
The amount above 100% is the bookmaker's built-in profit margin.
This means that every time you place a bet, you are effectively paying a small hidden tax for participating in the market.
Imagine a perfectly fair coin toss.
There are only two possible outcomes:
Each outcome has a true probability of 50%.
Fair decimal odds would therefore be:
The implied probabilities are:
Total:
50% + 50% = 100%
This is a perfectly fair market with no bookmaker profit.
Instead of offering fair odds, a bookmaker might price the market as:
Using the implied probability formula:
Implied Probability = 1 ÷ Decimal Odds
We calculate:
Total implied probability:
52.4% + 52.4% = 104.8%
The market now totals 104.8% instead of 100%.
The extra 4.8% is the bookmaker's overround.
The overround means you are betting into a market where the odds are slightly worse than the true probabilities.
Even if you could predict outcomes perfectly according to their real probabilities, the bookmaker's margin would gradually reduce your returns over time.
For example, in a market with a 5% overround, a bettor can expect to receive approximately:
£0.95 returned for every £1.00 staked
Over hundreds or thousands of bets, this small disadvantage compounds into a significant long-term loss.
This is why consistently beating the bookmaker requires finding bets where the available odds are better than the true probability suggests.
The overround is calculated by adding together the implied probabilities of every possible outcome.
The formula is:
Overround = (Sum of Implied Probabilities) − 1
Or expressed as a percentage:
(Sum of Implied Probabilities − 1) × 100
For example:
Total:
45% + 30% + 30% = 105%
The bookmaker's overround is therefore 5%.
Bookmaker margins vary considerably depending on where and what you are betting.
Several factors influence the size of the overround.
Understanding these differences helps bettors choose markets where less value is lost to bookmaker margins.
The overround affects every bet you place, whether you realise it or not.
Choosing bookmakers with lower margins and comparing prices through line shopping reduces this hidden cost and improves your long-term expected returns.
While you cannot eliminate the bookmaker's margin entirely, you can minimise its impact by consistently seeking the best available odds.
The overround is the bookmaker's built-in profit margin, created by pricing the combined implied probabilities above 100%. This hidden margin means bettors receive slightly worse odds than the true probabilities would justify, making it difficult to profit over the long run without identifying value bets. Understanding how the overround works allows you to compare bookmakers, select lower-margin markets, and make more informed betting decisions.
