A bookmaker margin, also called the vig or overround, is the built-in percentage advantage that makes a book’s prices sum to more than 100 percent. This guide explains the margin, shows the arithmetic on a real example, and explains why margins differ between markets and operators.
What is a bookmaker margin?
A bookmaker margin is the amount by which the implied probabilities from all prices in an outcome set exceed 100 percent. In a fair market, those implied probabilities would total exactly 100 percent because one outcome must happen. The excess is the overround or margin. It is a property of the quoted market, not a claim about the true probabilities and not a fee charged separately to an individual customer.
The formula behind the margin takes one line:
Margin = the sum of every outcome’s implied probability, minus 100 percent.
The overround arithmetic, worked
Take a simple three-outcome football market priced at 2.00, 3.50, and 4.00. Implied probability is one divided by the decimal price:
- 1 divided by 2.00 = 50.0 percent
- 1 divided by 3.50 = 28.6 percent
- 1 divided by 4.00 = 25.0 percent
Summing them gives 50.0 + 28.6 + 25.0 = 103.6 percent. The extra 3.6 percent above 100 is the overround. Removing it proportionally produces a comparison point, not proof of the true probabilities.
Why margins differ between markets
Margins are not uniform. Major two-way markets can be priced more tightly than long-list outright markets, niche sports, or exotic props because competition and uncertainty differ. The number of outcomes matters too: each additional price adds another component to the total.
The practical consequence is that a quoted price should be read alongside the total margin of its market. Comparing the implied-probability sum is more informative than treating one price as an isolated measure of value.
Margins and the two-sided check
A useful check on a two-way line is to calculate the margin from both outcomes. If a match is priced 1.90 and 1.90, the implied probabilities are 52.6 and 52.6, giving a total of 105.3 percent. A lopsided 1.25 and 4.00 line gives 80.0 and 25.0, or 105.0 percent. Similar totals can distribute the margin differently between outcomes.
What the margin means for analysis
Removing the overround converts offered prices into a more comparable probability baseline. It is the first step in probability-based frameworks such as the implied probability guide and the expected value guide. The calculation does not remove variance, model error, changing prices, or incomplete information.
The Asian handicap guide and Poisson framework cover related analytical assumptions. This page is an educational reference, not betting advice, a recommendation, or a promise of profit. Readers should also review the Gambling Commission safer-gambling guidance.
