Every bookmaker builds a profit into the odds they offer. This built-in profit is known as the overround, although it is also commonly called the vig, juice, or simply the bookmaker's margin.
The overround is one of the most important concepts in sports betting because it represents the hidden cost of placing a bet. The larger the overround, the more difficult it becomes to make a long-term profit.
In a perfectly fair betting market, the implied probabilities of all possible outcomes would add up to exactly 100%.
Bookmakers deliberately price markets so that the combined implied probabilities exceed 100%. The percentage above 100% is their profit margin.
This margin allows bookmakers to make money over the long run regardless of which outcome wins.
Calculating the overround is straightforward.
First, convert every outcome into its implied probability.
Implied Probability = 1 ÷ Decimal Odds
Then add all of the implied probabilities together.
Finally, subtract 100%.
Overround = Total Implied Probability − 100%
Suppose a football match has the following implied probabilities:
The total implied probability is:
47.6% + 29.4% + 27.8% = 104.8%
Subtracting 100% gives:
104.8% − 100% = 4.8%
The bookmaker's overround is therefore 4.8%.
Many new bettors assume that correctly predicting winners is enough to make money.
Unfortunately, the bookmaker's margin makes betting more difficult than that.
Even if your selections win at exactly their true probabilities, a market with a 5% overround means you can expect to lose approximately 5% of your total turnover over a very large number of bets.
For example, if you stake £10,000 over the course of a season, a 5% margin represents an expected loss of around £500 before considering the quality of your selections.
This hidden cost is one of the main reasons why consistently profitable betting is challenging.
Bookmaker margins vary depending on both the bookmaker and the type of market.
Typical examples include:
Lower margins mean you keep more of the value from your winning bets, making long-term profitability easier to achieve.
Professional bettors do not only search for better predictions—they also search for better prices.
Two identical bets placed at different bookmakers can produce different long-term results simply because one bookmaker charges a smaller margin.
Reducing the overround you pay is one of the easiest ways to improve your expected returns without changing your betting strategy.
The overround is the bookmaker's built-in profit margin and is calculated by adding together the implied probabilities of every possible outcome and measuring how far the total exceeds 100%. Higher margins make long-term profitability more difficult, while lower-margin bookmakers and markets allow bettors to retain more value from every wager. Understanding the overround helps you choose better markets, compare bookmakers more effectively, and reduce the hidden cost of betting.
