Bookmaker margin explained
The bookmaker margin, also called the overround or vig, is the built-in fee in a set of odds. It is how a sportsbook earns money from handling bets, and it is the main reason that betting costs more than it returns over time.
You can measure it yourself with simple arithmetic: convert each price to an implied probability and add them up. If the total is above 100%, the excess is the margin. This guide shows the method with a two-way and a three-way market, and what it means when you compare sportsbooks.
What the margin is
In a fair market on a coin toss, heads and tails would each pay decimal 2.00, because each has a 50% chance. The implied probabilities would add up to exactly 100%. Sportsbooks do not offer that. They shorten the prices a little, so the implied probabilities add up to more than 100%.
The amount above 100% is the margin. If the total is 104.7%, the margin is 4.7%. It is not a charge you see on your bet slip. It is hidden inside the prices, which is why it is easy to overlook.
How to calculate it
- Take the decimal odds of every outcome in the market.
- Turn each into an implied probability: 1 divided by the odds.
- Add the probabilities together.
- Subtract 1 (or 100%). The remainder is the margin.
If your odds are in another format, convert them to decimal first with the odds converter or the rules in our odds formats guide.
Worked example: a two-way market
Imagine a tennis match with one player at 1.91 and the other also at 1.91. The implied probability of each is 1 / 1.91 = 0.5236, or 52.36%.
Add the two: 52.36% + 52.36% = 104.71%. The margin is 4.71%. If the two prices were 2.00 and 2.00, the total would be exactly 100% and the margin would be 0%.
A different sportsbook offering 1.95 on both sides would have implied probabilities of 51.28% each, which total 102.56%, a margin of 2.56%. The lower total means better prices for the customer on this market.
Worked example: a three-way market
A football match result has three outcomes. Suppose the prices are home 2.50, draw 3.40 and away 2.90.
| Outcome | Decimal odds | Implied probability |
|---|---|---|
| Home | 2.50 | 40.00% |
| Draw | 3.40 | 29.41% |
| Away | 2.90 | 34.48% |
| Total | 103.89% |
The margin is 3.89%. To estimate the margin-free probabilities, divide each implied probability by the total. Home becomes 40.00 / 103.89 = 38.5%, the draw 28.3% and the away side 33.2%. This simple method treats the margin as spread evenly, which is only an approximation, because operators can load the margin unevenly.
Why a lower margin matters
Over many bets, the margin is roughly the share of your turnover that the sportsbook keeps. A market with a 2.5% margin costs less to bet on than one with 5%, for the same stake. It does not mean that the lower-margin bet will win. It means the price of playing is lower.
Margins vary by sport, league and market type. Popular markets on major events often have lower margins than niche markets. Outright winners and exotic props tend to carry bigger ones. Margins can also change as an event nears.
Comparing the same market at two sportsbooks, the one whose implied probabilities sum closer to 100% has the lower margin.
How the margin grows in combined bets
Margins compound in accumulators. Take three legs, each a two-way market priced at 1.91 on the side you choose. The combined odds are 1.91 x 1.91 x 1.91 = 6.97. If each leg had a fair 50% chance, the fair combined odds would be 8.00. The implied probability of 6.97 is 14.35%, compared with a fair 12.5%, so the cost has risen to about 14.8%.
With four legs at 1.91, the combined price is 13.31 against a fair 16.00, a cost of about 20%. Details on this are in the bet types guide. You can multiply any set of odds in the odds calculator.
Knowing the margin will not turn betting into a profitable activity. It helps you see the real cost. Set a budget that you can afford to lose, and see our responsible gambling page for tools.
Frequently asked questions
Is the overround the same as the vig?
Mostly yes. Overround is the amount by which implied probabilities exceed 100%. Vig or juice is the American term for the commission built into the price. In practice the words describe the same cost.
What is a good margin?
A lower one is better for the customer. A market whose implied probabilities add up to 102% to 103% is cheaper to bet on than one that adds up to 108%, though what is typical varies by sport and market.
Can the margin be 0%?
Not in a normal sportsbook offer, because the operator has to cover its costs. Prices like 2.00 and 2.00 on a two-way market would give 0%, but sportsbooks do not usually publish them.