If you add up the chances implied by a bookmaker's odds on a football match, you will notice something odd. They come to more than 100%. That extra slice is the bookmaker's margin, and once you understand it you will look at every price a little differently.
This guide explains what the margin is, how to calculate it on any market in a couple of minutes, and why it matters so much if you bet.
What odds really say
Odds are a price, but they are also a statement about probability. Decimal odds of 2.00 imply a 50% chance, because 1 divided by 2.00 is 0.50. Odds of 4.00 imply 25%. The formula is always the same:
Implied probability = 1 ÷ decimal odds
If a bookmaker priced every outcome at its true chance, the implied probabilities across a market would add up to exactly 100%. Those would be fair odds. In practice, they never do.
The overround
The amount by which the implied probabilities go over 100% is called the overround. It is the bookmaker's built-in edge, and it is how they make money over time regardless of which outcome happens.
Think of a coin toss. The true chance of heads is 50%, so the fair price is 2.00. A bookmaker might instead offer 1.91 on heads and 1.91 on tails.
- Heads: 1 ÷ 1.91 = 52.36%
- Tails: 1 ÷ 1.91 = 52.36%
- Total: 104.71%
That market has an overround of about 4.7%. Nothing about the coin has changed. The bookmaker has simply shaved a little off the price of both sides.
A worked example on a football match
Here is a made-up match result market to show the method. These are illustrative numbers, not real prices.
| Outcome | Odds | Implied probability | |---|---|---| | Home win | 2.10 | 47.62% | | Draw | 3.40 | 29.41% | | Away win | 3.60 | 27.78% | | Total | | 104.81% |
The overround here is 4.81%. To work it out yourself:
- Turn each price into an implied probability (1 ÷ odds).
- Add them up.
- Subtract 100%. What is left is the overround.
Finding the fair odds
You can strip the margin out to get a rough idea of what the bookmaker thinks the true chances are. The simplest way is to divide each implied probability by the total.
- Home win: 47.62 ÷ 1.0481 = 45.43%
- Draw: 29.41 ÷ 1.0481 = 28.06%
- Away win: 27.78 ÷ 1.0481 = 26.50%
These now add up to 100%. Turn them back into odds (1 ÷ probability) and you get fair prices of roughly 2.20, 3.56 and 3.77. Compare those with the offered prices of 2.10, 3.40 and 3.60, and you can see exactly where the margin has gone: a little has been taken off every outcome.
This proportional method is not the only one. Some people believe bookmakers put more margin on longer odds than on favourites, and use other methods to account for that. For a beginner, the simple version is good enough to understand what is going on.
Margin as a payout percentage
Another way to describe the same thing is the payout percentage, sometimes called the return to player. It is 100 divided by the total implied probability.
In our match example, 100 ÷ 104.81 = 95.4%. Put simply, if the prices reflected the true chances perfectly, you would expect to get back about 95.4p for every £1 staked over a very large number of bets. The missing 4.6p is the bookmaker's cut.
Why margins differ between markets
Margins are not the same everywhere. As a general pattern:
- Popular markets on big matches, like the match result in a top league, tend to have lower margins because bookmakers compete hard for that business.
- Niche markets such as exact scores, player props or lower league matches often carry bigger margins.
- Markets with lots of outcomes usually have more margin spread across them, because each outcome can carry a small slice.
It is worth checking the margin yourself rather than assuming. The method above works on any market where you can see every outcome.
Why accumulators make it bigger
When you combine selections in an accumulator, you multiply the odds together. You also multiply the margins. A small edge on each leg compounds into a much bigger edge on the whole slip.
As a rough illustration, if each leg carries about a 5% overround, a four leg acca built from those legs has a payout percentage of around 0.954 multiplied by itself four times, which is about 83%. That is why long accumulators look exciting but are very hard to profit from over time.
Why beating the margin is the real challenge
Here is the key point. To make money over the long run, it is not enough to pick winners. You need to find prices that are higher than the true chance of the outcome, by more than the margin the bookmaker has added.
If you think a home win has a 50% chance and the bookmaker offers 2.10, that price implies 47.6%. On your numbers, that is value. But your estimate has to be better than the bookmaker's, and bookmakers employ a lot of people and data to set prices. Most bettors do not manage this consistently.
That is why it helps to treat margin as a cost. Knowing roughly how much you are paying makes it easier to compare bookmakers, avoid the most expensive markets and keep your expectations realistic.
Quick summary
- Implied probability is 1 ÷ decimal odds.
- Add the implied probabilities across a market. Anything over 100% is the overround.
- Divide each implied probability by the total to estimate fair odds.
- Margins tend to be higher in niche markets and grow quickly in accumulators.
- Long term profit only comes from finding prices better than the true chance plus the margin, which is hard.
Bet responsibly
Betting is for adults aged 18 and over. Only stake what you can afford to lose, and treat it as entertainment rather than a way to make money. If gambling stops being fun, free and confidential support is available at BeGambleAware.org.