The bookmaker's margin is the extra percentage built into every market so the implied probabilities add up to more than 100%. To measure it, convert each price to an implied probability (1 ÷ odds), add them up and subtract 100%: a 2-way market priced 1.85 / 1.95 sums to 105.34%, so the margin is 5.34%.
That margin, also called the overround or vig, is the bookmaker's built-in edge. This guide shows how to calculate it on a 2-way and a 3-way market, how to remove it to estimate fair odds, and why it matters more than most bettors think.
Every market below is fictional. Prices are invented to make the arithmetic clear and were checked in Python.
Why implied probabilities add up to more than 100%
If a coin flip were priced fairly, both sides would be 2.00. Each implies 50%, and 50% + 50% = 100%. A bookmaker offering 2.00 on both sides would expect to break even in the long run.
Instead, the bookmaker shades both prices down a little — say to 1.90 each. Now each implies 52.63%, and the market adds up to 105.26%. If equal money comes in on both sides, the bookmaker pays out 95 for every 100 staked and keeps 5.
| Market (fictional) | Prices | Book total | Margin | Payout ratio |
|---|---|---|---|---|
| Fair coin | 2.00 / 2.00 | 100.00% | 0.00% | 100.00% |
| Shaded coin | 1.90 / 1.90 | 105.26% | 5.26% | 95.00% |
The payout ratio is 100 ÷ book total. It's the share of stakes the market returns on average if money is balanced. It's the same information as the margin, seen from your side.
Worked example 1: a 2-way esports market
A fictional best-of-three match between two invented teams:
Step by step:
Halcyon 1 / 1.85 = 0.5405 → 54.05%
Ironveil 1 / 1.95 = 0.5128 → 51.28%
Book total = 105.34%
Margin = 105.34% − 100% = 5.34%
Payout ratio = 100 / 105.34 = 94.93%So on average this market returns about 94.93 for every 100 staked across both sides.
How to remove the margin
To estimate what the bookmaker "really" thinks, you remove the margin and rescale the probabilities back to 100%. The simplest approach is the proportional method: divide each implied probability by the book total.
fair probability = implied probability ÷ book total
fair odds = 1 ÷ fair probabilityApplied to the 2-way market:
| Team (fictional) | Price | Implied | Fair probability | Fair odds |
|---|---|---|---|---|
| Halcyon | 1.85 | 54.05% | 51.32% | 1.95 |
| Ironveil | 1.95 | 51.28% | 48.68% | 2.05 |
| Total | 105.34% | 100.00% |
The fair odds (1.95 and 2.05) are what the market would pay with no margin. The gap between 1.85 and 1.95, and between 1.95 and 2.05, is the price you pay to bet.
Other methods exist. Some assume more margin sits on the outsider than on the favourite (often called "favourite–longshot bias"), and spread it unevenly. They give slightly different fair odds, especially for long shots. The proportional method is the easiest to compute by hand and is a sensible starting point.
Worked example 2: a 3-way market
Some esports markets have three outcomes. A common one is a fictional two-map series where the result can be a 2–0 for either side or a 1–1 draw.
The calculation is the same, just with three lines:
| Outcome (fictional) | Price | Implied | Fair probability | Fair odds |
|---|---|---|---|---|
| Kestrel 2–0 | 2.40 | 41.67% | 38.61% | 2.59 |
| Draw 1–1 | 3.60 | 27.78% | 25.74% | 3.88 |
| Lumen 2–0 | 2.60 | 38.46% | 35.64% | 2.81 |
| Total | 107.91% | 100.00% |
The margin here is 7.91%, noticeably higher than the 2-way example. That's typical: markets with more outcomes tend to carry more margin, and outright markets with many teams often carry more still. A fictional eight-team outright in our market types guide shows the same calculation across eight prices.
Margin checklist
Run through this before comparing prices:
- Write down every outcome in the market, including the draw if there is one.
- Convert each price to implied probability with 1 ÷ odds.
- Add them up. Anything above 100% is margin.
- Divide each implied probability by the total to get fair probabilities.
- Take 1 ÷ fair probability to get fair odds.
- Compare the margin, not just the headline price, when you look at the same market in different places.
Why the margin matters over many bets
A 5% margin sounds small on a single bet. But it applies to every bet you place. If your bets are, on average, priced exactly as fairly as the market, the margin is the long-run cost of betting: roughly 5 units lost for every 100 staked at that margin.
The margin is also why "picking more winners" isn't enough. At 1.90 / 1.90, you'd need to win 52.63% of your bets just to break even, not 50%. Our bankroll guide simulates what that gap does over 1,000 bets.
What removing the margin does not do
Calculating fair odds is a measurement, not a strategy.
- It doesn't tell you who will win. The fair probabilities are still the market's estimate, just without the margin.
- It doesn't make any bet profitable. You still get paid at the bookmaker's price, not the fair one.
- It doesn't account for settlement rules, voided maps or price movement.
What it does do is make the cost of a bet visible. If you understand how odds are written and how much margin sits in each market, you'll know exactly what you're paying for the entertainment, and you can set your budget accordingly.
FAQ
What is the overround?
It's the amount by which the implied probabilities of every outcome in a market add up to more than 100%. A market that sums to 105.34% has an overround, or margin, of 5.34%.
How do I calculate the margin on a market?
Turn each decimal price into an implied probability with 1 / odds, add them up, then subtract 100%. The result is the margin.
What are fair odds?
Fair odds are the prices with the margin removed. With the proportional method, divide each implied probability by the total book, then take 1 over the result.
Does a lower margin mean I will win?
No. A lower margin means you give up less on average, but the expected result of a bet is still negative unless your probability estimate is better than the market's, which is rare and hard to prove.