Convert each odd into an implied probability (1 ÷ odd), add them up to get the book percentage, then subtract 100%: that is the overround. Example: odds 1.80 and 2.00 → 55.56% + 50.00% = 105.56%, an overround of 5.56 points. The overround shows the excess built into the prices, not the bookmaker's actual profit.
How do you calculate the margin in a few seconds?
To calculate the apparent margin of a market, turn each decimal odd into an implied probability using the formula 1 ÷ odd, add up all the probabilities obtained, then subtract 100%. The result is the market's overround, that is, the excess probability built into the odds.
In three steps: implied probability = 1 ÷ odd; book percentage = sum of the implied probabilities; overround = book percentage − 100%.
Quick example, with odds 1.80 and 2.00:
- 1 ÷ 1.80 = 55.56%
- 1 ÷ 2.00 = 50.00%
- Total = 105.56%
- Overround = 5.56 points
Key point. 105.56% is the book percentage. 5.56 points is the overround. These two values must not be confused with the bookmaker's actual profit.
What is a bookmaker's margin?
A bookmaker's margin refers to the edge built into the odds it offers. It usually shows up as a sum of implied probabilities greater than 100%. The observed excess is called the overround, book margin or vigorish depending on the market and usage.
In a perfectly fair market, the probabilities of all possible outcomes would total exactly 100%. For a two-outcome event estimated at 50% each, the theoretical odds would be 2.00 for each outcome. A bookmaker may, however, offer 1.91 and 1.91: each odd then corresponds to an implied probability of about 52.36%, and their sum reaches 104.71%.
| Term | Definition |
|---|---|
| Implied probability | Probability obtained directly from an odd |
| Book percentage | Sum of the implied probabilities of all outcomes |
| Overround | Part of the book percentage that sits above 100% |
| Underround | Sum below 100%, possible by combining the best odds from several sources |
| Realised hold | Share of the stakes actually kept after settlement |
| Margin-free probability | Estimate obtained after redistributing the overround |
In everyday language, "margin" and "overround" are often used as synonyms. They nonetheless describe two slightly different measures when you want to be mathematically precise, as the section on the overround and the theoretical margin shows.
How do you convert an odd into an implied probability?
With decimal odds, the implied probability is calculated by dividing 1 by the odd, then multiplying the result by 100. An odd of 2.00 therefore corresponds to 50%, while an odd of 4.00 corresponds to 25%.
Implied probability (%) = (1 ÷ decimal odd) × 100
| Odd | Calculation | Implied probability |
|---|---|---|
| 1.25 | 1 ÷ 1.25 | 80.00% |
| 1.50 | 1 ÷ 1.50 | 66.67% |
| 1.80 | 1 ÷ 1.80 | 55.56% |
| 2.00 | 1 ÷ 2.00 | 50.00% |
| 3.00 | 1 ÷ 3.00 | 33.33% |
| 4.00 | 1 ÷ 4.00 | 25.00% |
This probability is called implied because it is contained in the displayed price. It is not directly a margin-free estimate, and even less a certain, true probability.
How do you calculate the book percentage and the overround?
You add up the implied probabilities of all the outcomes offered on the same market. The total obtained is called the book percentage. The part that sits above 100% is the overround.
For a market with n outcomes and odds O₁, O₂, … Oₙ:
- Book percentage = (1 ÷ O₁) + (1 ÷ O₂) + … + (1 ÷ Oₙ)
- Book percentage (%) = sum of the implied probabilities × 100
- Overround (%) = book percentage (%) − 100%
Three-outcome example, with a 1X2 market priced at 2.10 / 3.40 / 3.60:
- 1 ÷ 2.10 = 47.62%
- 1 ÷ 3.40 = 29.41%
- 1 ÷ 3.60 = 27.78%
- Book percentage = 104.81%
- Overround = 4.81 points
Key point. The overround is calculated market by market. There is no single, permanent margin for an entire bookmaker.
The same operator can apply very different levels depending on the sport, the competition, the type of bet and the moment at which the odd is observed.
Full example on a tennis match
Starting point: Player 1 priced at 1.80, Player 2 priced at 2.00.
Step 1 — implied probabilities: Player 1 = 1 ÷ 1.80 = 55.56%; Player 2 = 1 ÷ 2.00 = 50.00%.
Step 2 — book percentage: 55.56% + 50.00% = 105.56%.
Step 3 — overround: 105.56% − 100% = 5.56 points.
Step 4 — normalised probabilities (implied probability ÷ book percentage): Player 1 = 55.56 ÷ 105.56 = 52.63%; Player 2 = 50.00 ÷ 105.56 = 47.37%.
Step 5 — theoretical margin-free odds: Player 1 = 1 ÷ 52.63% ≈ 1.90; Player 2 = 1 ÷ 47.37% ≈ 2.11.
| Outcome | Displayed odd | Implied probability | Normalised probability | Estimated margin-free odd |
|---|---|---|---|---|
| Player 1 | 1.80 | 55.56% | 52.63% | 1.90 |
| Player 2 | 2.00 | 50.00% | 47.37% | 2.11 |
The normalised probabilities do total 100%.
What is the difference between the overround and the theoretical margin?
The overround measures the part of the book percentage that sits above 100%. To convert this excess into a theoretical rate applied to the total amount, you have to take the full size of the book into account.
If B represents the book percentage in decimal form: overround = B − 1; theoretical return rate = 1 ÷ B; theoretical margin = 1 − (1 ÷ B).
Applied to the previous example, with B = 1.0556:
- Overround = 1.0556 − 1 = 5.56%
- Theoretical return rate = 1 ÷ 1.0556 ≈ 94.74%
- Theoretical margin = 100% − 94.74% = 5.26%
| Indicator | Result |
|---|---|
| Book percentage | 105.56% |
| Overround | 5.56 points |
| Theoretical return rate | 94.74% |
| Corresponding theoretical margin | 5.26% |
Saying that the market has an overround of 5.56% is correct. Saying that the bookmaker will necessarily keep 5.56% of the stakes is not. The amount actually kept depends on the stakes received, how they are distributed, the results and how the market is managed. Economic research also points out that the observable overround does not always exactly reflect the profit rate really produced by the odds.
How do you remove the margin from the odds?
The simplest method is to divide each implied probability by the sum of all the probabilities in the market. This normalisation brings the total back to 100% and produces an estimate of the margin-free market probabilities.
Normalised probability of outcome i = implied probability of outcome i ÷ sum of the implied probabilities. Estimated margin-free odd = 1 ÷ normalised probability.
Proportional normalisation assumes that the margin is spread proportionally across all outcomes. This assumption is simple and instructive, but it is not always perfectly realistic. Other methods can spread the margin differently, in particular when a market has a very strong favourite or several longshots: logarithmic or power methods, the Shin method, or models that account for the favourite-longshot bias.
Removing the margin always involves a spreading method. The result obtained is an estimate, not the revelation of an objective probability.
It is better to talk about a normalised probability, a margin-free market probability or a theoretical margin-free odd, rather than a "true probability" or "real probability".
Why does the margin vary?
The margin varies mainly according to market liquidity, competition between bookmakers, the available uncertainty, staking limits and the cost of managing the bet. Popular, highly competitive markets generally display tighter prices than secondary markets or ones that are hard to assess.
- Expected liquidity and volume — a heavily played market generally allows more volume to be accepted with a lower unit margin.
- Competition between operators — on the most closely followed matches, punters easily compare prices, which can push margins down.
- Difficulty of assessment — a secondary market or a very specific bet carries more uncertainty and may be given a higher margin.
- Staking limits — markets accepting large stakes often need more accurate and more competitive prices.
- Moment of observation — the margin can evolve between the opening of the market and the start of the match.
- Commercial policy — an operator may deliberately reduce its margin on certain popular events. This orientation is what separates sharp and soft profiles.
A three-outcome market does not automatically have a higher margin than a two-outcome market. The number of outcomes makes the calculation different, but it is not enough to explain the final level of the overround: a very liquid 1X2 market can have a lower margin than a much less competitive two-outcome market.
A betting exchange does not read the same way: it matches bettors with one another and earns a commission on winnings rather than a margin built into the odds.
Understand how bookmakers make money →
How do you compare margins correctly?
To compare several bookmakers correctly, you have to use the same event, the same market, the same settlement rules and an observation moment that is as close as possible. Comparing different markets produces a misleading conclusion.
A valid comparison covers: the same match, the same type of bet, the same line (for example over or under 2.5 goals), the same settlement rules, odds taken at the same moment, all the outcomes of the market and the same odds format.
Comparing the margin of the 1X2 market of a football match with that of the winner of a tennis match makes no sense. Comparing the 1X2 market of the same match, at the same moment, across three different bookmakers is, on the other hand, relevant.
When you select the best odd for each outcome across several bookmakers, the calculation obtained no longer represents the margin of one specific bookmaker: it represents a composite market built from several sources. This selection can even produce a total below 100% — this does not mean that a bookmaker offers a negative margin, but that the best odds come from several different operators.
What the margin tells you and what it does not
The overround indicates the level of excess built into the odds of a market, the apparent competitiveness of the prices, the gap between the raw probabilities and a market totalling 100%, a basis of comparison between several similar markets, and the theoretical return level associated with the odds.
It does not indicate the bookmaker's guaranteed profit, the future result of the match, the exact true probability of each outcome, the sporting quality of the bookmaker's model, the actual distribution of the stakes, the future profitability of a bet, nor the precise reason why an odd changed.
A low margin means that the prices offered are generally closer to a market totalling 100%. It does not mean that a bet automatically becomes worthwhile, nor that the estimated probabilities are correct.
How does OddScore remove the margin?
OddScore removes the margin in order to compare probabilities rather than raw commercial prices. Without this step, a movement can come from a change in probability, a change in margin or a combination of the two.
Two bookmakers can assess a match in a similar way while displaying different odds because they do not apply the same margin. OddScore turns the odds into comparable probabilities and removes the margin built into the prices before analysing the direction of the movement, its size, its speed, the number of operators concerned and the overall consistency of the market.
How OddScore analyses odds movements →
Dig into the market
Odds movements are only part of the story. Here are the next topics to read.
How do bookmakers make money?
Stakes, payouts, margin and overall risk management.
Understand the modelHow does a bookmaker work?
How odds are built, margin, risk and operator categories.
Read the guideHow do bookmakers adjust their odds?
Models, information, exposure and market movement.
Understand the adjustmentsHow do you read an odds movement?
Direction, size, timing and consistency across bookmakers.
Understand the movementsFrequently asked questions
What is a bookmaker's margin?
The margin is the edge built into the odds a bookmaker offers. It usually appears when the sum of the implied probabilities of all outcomes exceeds 100%.
What is the overround?
The overround is the part of the book percentage that sits above 100%. A market totalling 105% therefore has an overround of 5 points.
How do you convert an odd into a probability?
With a decimal odd, divide 1 by the odd and then multiply the result by 100. An odd of 2.50 therefore corresponds to an implied probability of 40%.
How do you calculate the overround?
Add up the implied probabilities of all the outcomes in the market, then subtract 100%. With a total of 106%, the overround is 6 points.
Does the overround equal the bookmaker's profit?
No. The overround measures the excess contained in the prices. The profit actually made depends on the stakes received, how they are distributed, the results and the operator's costs.
How do you remove the margin from the odds?
The simplest method is to divide each implied probability by the sum of all the probabilities in the market. The total obtained after normalisation is then equal to 100%.
Does a margin-free odd give the true probability?
No. It provides a normalised estimate based on the available odds. The result depends on the market prices and on the method used to spread the margin.
Why does the margin vary from one market to another?
It depends in particular on liquidity, competition, staking limits, how hard the market is to assess and the bookmaker's commercial policy.
Does a low margin mean a bet is worthwhile?
No. A low margin indicates prices that are generally tighter, but it does not tell you whether the market's sporting estimate is correct.
Does OddScore provide tips?
No. OddScore transforms and compares odds to make market movements easier to read. The platform does not provide betting advice.
Sources & methodology
This page draws on the standard formulas for converting decimal odds, on the economic research devoted to the overround and to betting-market prices, on the normalisation methods used to estimate margin-free probabilities, and on the odds-analysis methodology developed by OddScore.
- Convert each decimal odd into an implied probability (1 ÷ odd).
- Add the probabilities to get the book percentage, then subtract 100% for the overround.
- Normalise the probabilities to estimate a margin-free market, keeping in mind that the result depends on the chosen spreading method.