Bookmakers are businesses, and like any business they need a way to make money. Their method is called the overround — sometimes the “vig”, “juice” or simply the margin. It is built into every set of odds a bookmaker publishes, and once you understand it, you can see at a glance which bookmakers are pricing more competitively and why, over the long run, the house is expected to come out ahead.
The fair-odds baseline
Start with a coin toss. Heads and tails each have a 50% chance, so a fair price on either would be evens — 2.00 in decimal odds. Stake £10, win £10. If a bookmaker offered evens on both sides and took equal money on each, they would pay out exactly what they took in and make nothing.
Now suppose the bookmaker offers 10/11 (1.91) on each side instead. Convert 1.91 into an implied probability by dividing 100 by the decimal odds: 52.4%. Do that for both outcomes and add them together:
| Outcome | Odds offered | Implied probability |
| Heads | 10/11 (1.91) | 52.4% |
| Tails | 10/11 (1.91) | 52.4% |
| Total | 104.8% |
The total exceeds 100% by 4.8 percentage points. That is the overround. If the bookmaker takes £100 on each side, it holds £200 and pays out £191 whichever way the coin lands, keeping £9 — about 4.5% of turnover.
A real market
Football match odds are a three-way market: home win, draw, away win. A typical Premier League price might look like this:
| Selection | Decimal odds | Implied probability |
| Home win | 2.10 | 47.6% |
| Draw | 3.40 | 29.4% |
| Away win | 3.60 | 27.8% |
| Total | 104.8% |
Again, roughly 105%. Competitive bookmakers on major football markets tend to sit between 103% and 107%. Less competitive ones, or smaller leagues, can be 110% or more.
Why margins vary between markets
The overround is not uniform across a bookmaker’s offering. Several factors push it up or down:
| Market type | Typical overround | Reason |
| Premier League match odds | 103–106% | High volume, intense competition, easy to price |
| Lower-league football | 108–115% | Less information, less competition |
| 8-runner horse race | 115–125% | More outcomes, more room for margin |
| 20-runner handicap | 125–140% | Many outcomes; each carries its own margin |
| Novelty/political markets | 120%+ | Low liquidity, hard to price |
| Betting exchange (after commission) | 101–103% | No built-in margin; commission on winnings instead |
The general rule is that the more selections a market has, the higher the overround, because each selection’s price is shaded slightly and the shading accumulates. This is one reason multi-runner races and multi-leg accumulators are expensive over time: the margin compounds.
How to calculate it yourself
- Convert each selection’s odds into a percentage (100 ÷ decimal odds).
- Add all the percentages together.
- Subtract 100. The result is the overround.
For fractional odds, first convert to decimal by dividing the first number by the second and adding one: 5/2 becomes 2.5 + 1 = 3.50.
Several free tools do this automatically, but doing it by hand a few times makes the concept stick.
What the overround tells you
About the bookmaker. Comparing overrounds on the same market across several bookmakers shows who is offering the best value in general. A bookmaker at 104% is giving away less than one at 108%, regardless of which selection you fancy.
About the true probability. To estimate what the bookmaker actually thinks, divide each implied probability by the market total. In the football example above, the home win at 47.6% ÷ 104.8% ≈ 45.4%. This “de-vigged” figure is closer to the bookmaker’s real opinion.
About the long run. Every bet you place carries the margin. Over hundreds of bets, if your selections are no better than random, you should expect to lose roughly the overround percentage of your total stakes. A 5% margin on £1,000 of turnover is about £50. That is the cost of the entertainment, and it is worth knowing in advance.
Accumulators and the compounding margin
An accumulator multiplies the odds of each leg together — and multiplies the margins too. Four legs at a 5% margin each produce a combined margin of roughly 20%. The headline odds look attractive, but the bookmaker’s edge on the bet is far larger than on any single leg.
Where the overround doesn’t show
Bookmakers rarely display the market percentage. Price comparison sites sometimes do. Betting exchanges show a “book percentage” on both the back and lay side, which makes the concept visible.
Related reading on site: “How to Read Betting Odds Like a Pro”; Bookie Guide; “What is an Accumulator Bet?”
Betting carries risk. Set a budget and stick to it. Help is available at GambleAware and GamCare.
