WhichBookmakerEvery price a bookmaker quotes contains a fee. It is not itemised, it does not appear on your statement, and most bettors never calculate it. It is called the margin, or the overround, and over a season it takes more from a regular bettor than any welcome offer gives back.
This guide shows you how to find it in any market, in about thirty seconds, with a calculator.
Odds are probabilities in disguise
Decimal odds convert to an implied probability with one division:
implied probability = 1 ÷ decimal odds
A price of 4.00 implies 1 ÷ 4.00 = 0.25, or a 25% chance. A price of 1.50 implies 1 ÷ 1.50 = 0.667, or 66.7%.
That is the whole tool. Everything below is applying it.
A two-way market, priced two ways
Take a tennis match the bookmaker considers a coin flip. In a world with no fee, both players would be priced at 2.00 — each implying a 50% chance, together adding to exactly 100%.
No bookmaker quotes that. Here is the same match at two real operators:
| Sharp book | Mainstream book | |
|---|---|---|
| Player A | 1.95 | 1.90 |
| Player B | 1.95 | 1.90 |
| Implied probability A | 51.28% | 52.63% |
| Implied probability B | 51.28% | 52.63% |
| Total | 102.56% | 105.26% |
Both books have squeezed the probabilities above 100%. That excess is the margin.
To turn the total into the margin figure the industry quotes:
margin = 1 − (1 ÷ total)
- Sharp book: 1 − (1 ÷ 1.0256) = 1 − 0.975 = 2.5%
- Mainstream book: 1 − (1 ÷ 1.0526) = 1 − 0.95 = 5.0%
You will also see the raw excess quoted — 2.56% and 5.26% here — described as the overround. Both describe the same thing from slightly different angles. We use the margin convention on this site, and so do most operators when they discuss pricing.
The practical reading: on this market, one book keeps about 2.5% of everything staked and the other keeps about 5%. Same match, same probabilities, double the fee.
A three-way market
Football’s 1X2 market works identically, with three prices instead of two. Take a match where a fair assessment is home 42%, draw 28%, away 30%.
| Sharp book | Mainstream book | |
|---|---|---|
| Home | 2.32 | 2.26 |
| Draw | 3.48 | 3.39 |
| Away | 3.25 | 3.17 |
| Implied home | 43.10% | 44.25% |
| Implied draw | 28.74% | 29.50% |
| Implied away | 30.77% | 31.55% |
| Total | 102.61% | 105.30% |
| Margin | 2.5% | 5.0% |
Look at the home price: 2.32 against 2.26. A difference of six pence in the pound. It looks like nothing. That is precisely why it works.
What the difference actually costs
Margin is charged on turnover, not on losses. It applies whether the bet wins or loses, because it was priced into the odds before the event started.
So the expected cost of betting is approximately:
turnover × margin
Consider a bettor staking £50 a time, four times a week, across a forty-week season. That is 160 bets and £8,000 of turnover — a level many recreational bettors would not describe as heavy.
| 2.5% book | 5% book | |
|---|---|---|
| Turnover | £8,000 | £8,000 |
| Expected cost of the margin | £200 | £400 |
The gap is £200 a season, from doing nothing differently except opening the account somewhere else.
Now compare that with the industry’s standard welcome offer: bet £10, get £30 in free bets. Because a free bet does not return its stake, £30 in free bets is worth around £20 in expectation when used at odds near 3.0 — the arithmetic is in our free bets guide.
So the welcome offer is worth roughly £20, once. The margin difference costs roughly £200, every season, for as long as you hold the account. The offer is worth about a tenth of the thing it distracts you from.
This is why our scoring methodology weights odds at 30%, more than any other pillar, and why the operator that wins on price on this site — Pinnacle, at roughly 2–2.5% — runs no promotions at all.
An important limit on all of this
A lower margin improves your expected return. It does not create one.
Betting remains an activity in which the great majority of participants lose money over time. A 2.5% book does not turn a losing bettor into a winning one; it means the same bettor loses more slowly. Nothing in this guide is a method for beating a bookmaker, and you should be sceptical of anyone who claims to have one.
What the margin figure genuinely gives you is a way to stop overpaying for the same bet — and a way to see through a headline offer to the price underneath it.
Where margins differ most
Margin is not uniform across a bookmaker’s own book. As a rule:
- Major markets are cheapest. Match odds on a big football league or an ATP tennis match attract competition, so operators price them tightly.
- Niche markets are dearest. Lower divisions, obscure competitions and novelty markets carry noticeably wider margins, because fewer customers compare prices there.
- Multiples compound. Each leg of an accumulator carries its own margin, so a five-fold does not cost you 5% — it costs roughly 5% compounded five times. This is why accumulators are the most profitable product a bookmaker sells and the most heavily promoted.
- In-play is usually wider than pre-match, because the operator is pricing under time pressure and carrying more risk.
If you want to compare two operators fairly, check the same market at the same moment. Comparing a Premier League match at one book against a third-tier fixture at another tells you nothing.
How to check any market in thirty seconds
- Write down the decimal odds for every outcome.
- Divide 1 by each and add the results.
- Subtract 100%. That excess is the overround.
- If you want the margin figure: 1 − (1 ÷ total).
Anything at or under about 3% is genuinely sharp. Around 5% is the mainstream norm. Above 6% you are paying a substantial premium, and it is worth knowing what you are getting for it — sometimes local licensing and local payment rails, which have real value, and sometimes nothing but a louder advertisement.
We publish the indicative margin on every operator’s card on this site, precisely because almost nobody else does.
Related reading
- How to read betting odds — decimal, fractional and American, and how to convert between them
- Betting exchange vs bookmaker — where commission replaces margin, and when that is cheaper
- Why bookmakers restrict winning accounts