Bookmaker margin: does it make long-run winning almost impossible?
"The margin makes long-run winning almost impossible" is half confirmed. The arithmetic shows that a margin cuts the average return: a market priced at 1.91 on both sides carries 4.71%. The word "impossible" is not confirmed, because no primary source we have read measures customers' long-run results. Two sources are still to be logged.
How margin is calculated
For each outcome, the implied probability is 1 divided by the decimal price. Add them across the market. A fair book adds to 100%. Anything above is the margin, also called the overround.
This is a definition, not a claim about any one operator. Our term pages cover implied probability and margin and overround.
Worked examples
Illustrative examples, not prices. Each row shows prices we chose to show the method.
| Market | Decimal prices | Sum of implied probabilities | Margin |
|---|---|---|---|
| Two outcomes | 1.91 and 1.91 | 104.71% | 4.71% |
| Three outcomes | 2.60, 3.40 and 2.90 | 102.36% | 2.36% |
| Four-leg multi, 5% on each leg | Fair price divided by 1.05 per leg | 121.55% | Average return A$82.27 per A$100 |
At 1.91 a bettor needs to win 52.36% of bets to break even. A customer who thinks the chance is 50% faces an expected value of A$100 × (0.50 × 1.91 − 1) = −A$4.50 per A$100 staked.
Short answer: the margin lowers the average return and compounds across legs; that winning is almost impossible is not confirmed.
What operators and exchanges publish
Our plan records two kinds of published rules: how best odds guaranteed (BOG) works, and how a betting exchange charges commission. We have not read either yet, so we state no figure. See best odds guaranteed.
We also have two primary sources to read on how odds are built, from a regulator or a help service. Until they are logged, we describe the formula only.
When this is wrong
The page is wrong if a market is priced below 100% in total, which would show a negative margin. It would also be wrong if a primary source shows customers profiting at a stated rate.
The formulas are re-checked once a year. Your own markets can be run through the expected value calculator.
Check it yourself
- Write down every price in a market.
- Divide 1 by each price and add the results.
- Subtract 1. The result is the margin.
For a multi, see what is a multi bet and how to read decimal odds.
How the margin grows across legs
A multi bet applies the margin once per leg. The effect compounds, because each price is multiplied by the next.
Illustrative example, not a price. Assume each leg carries a 5% margin, so each price is the fair price divided by 1.05. Average return per A$100 staked then falls as legs are added:
- 2 legs: A$90.70, from 1 / 1.05 squared;
- 4 legs: A$82.27;
- 6 legs: A$74.62.
The larger payout on a longer multi is therefore paid for with a lower average return. One lost leg still loses the whole bet.
This is arithmetic on an assumed margin. We have not read an operator's margin on any real multi.
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FAQ
How do you calculate the bookmaker margin?
Add the implied probabilities of every outcome in a market (1 divided by each decimal price), then subtract 1. A total of 104.71% is a margin of 4.71%.
Does a margin make winning impossible?
Not confirmed. Arithmetic shows a margin lowers the average return. It does not prove that no customer can profit, and no primary source we have read measures that.
Does the margin grow on a multi bet?
Yes, because it applies to each leg. In the illustrative example below, four legs each carrying a 5% margin return on average A$82.27 per A$100 staked.
Do operators publish their margins?
Not confirmed on 13 November 2026. We have not found an operator page that states a margin, so we calculate it from published prices and label it as a calculation.