Bookmaker Margin (Overround): How to Calculate It and Why It Matters
Add up one divided by every price, and the number above 100% is the margin. Worked on two-way and three-way lines, compared across one coupon, and costed over a season of turnover.
Margin is the difference between what a market's prices imply and the 100% they would add up to if they were fair. Take a 1X2 line of 2.30 / 3.40 / 3.10: the implied chances come to 105.15%, so the book is 5.15 points over — and the operator keeps 4.90 kobo of every naira staked. Those two figures are both called "margin", they are not equal, and this page shows how to work out each of them on any coupon in about thirty seconds.
In short.
- Book percentage = sum of 1 ÷ each price. Hold = 1 − 1 ÷ book percentage. A 105.15% book is a 4.90% hold.
- Double chance books are compared against 200%, not 100%, because every outcome appears twice.
- On one sample coupon the cost ranged from 2.50% on an Asian handicap to 7.57% on corners.
- Margin is rarely spread evenly: on a lopsided line the outsider carried a 25% loading and the favourite 2%.
- ₦104,000 of yearly turnover costs ₦5,096 at 4.90% and ₦14,539 through three-folds at 13.99%.
Book percentage and hold are not the same number
Convert every price to a percentage and add them up. Anything above 100% is the book percentage, sometimes called the overround: it answers "by how much do these prices exaggerate the chances". A book of 105.15% exaggerates by 5.15 points.
Hold answers a different question: of the money staked across the market, what share never leaves. It is 1 − 1 ÷ 1.0515 = 4.90%. The gap looks small, but the first figure is always the larger, and a "5.15% margin" quoted in a comparison table is not what a punter pays. Going the other way, a stated hold converts back with book = 1 ÷ (1 − hold).
Use the book percentage to compare two versions of the same market. Use hold to work out what a season of betting will cost you, because it is a share of money rather than a share of probability. Where the mark-up comes from in the first place is covered in how bookmakers make money.
A two-way market, step by step
Over/under 2.5 goals in a Premier League fixture, priced 1.87 and 1.93.
Example. 1 ÷ 1.87 = 0.53476 and 1 ÷ 1.93 = 0.51813. Adding them: 1.05289, so the book is 105.29%. Hold = 1 − 1 ÷ 1.05289 = 5.02%. Every ₦500 through this market carries ₦25 of built-in cost, whichever side you take.
Two-way markets are the easiest to audit because there is nowhere to hide: if one side looks generous, the other must be carrying the load. A pair like 1.95 / 1.95 gives 1.02564, a 2.50% hold — half the cost of the line above, on the same match.
A three-way market, step by step
The same arithmetic with one more term. Prices 2.30 home, 3.40 draw, 3.10 away.
| Outcome | Price | 1 ÷ price | As a percentage |
|---|---|---|---|
| Home | 2.30 | 0.43478 | 43.48% |
| Draw | 3.40 | 0.29412 | 29.41% |
| Away | 3.10 | 0.32258 | 32.26% |
| Book | — | 1.05148 | 105.15% |
Hold = 1 − 1 ÷ 1.05148 = 4.90%. Notice that the three-way line here is cheaper than the two-way totals line above: the number of outcomes does not decide the cost, competition for the bet does.
Important. Double chance is the one place the method needs adjusting. Prices of 1.32, 1.33 and 1.62 sum to 2.12674, which looks catastrophic — until you notice each result appears in two of the three selections. Divide by two: 1.06337, a book of 106.34% and a hold of 5.96%.
What each market on one coupon costs
The prices below come from a single fixture, so the table compares markets rather than operators. Run the same arithmetic on your own coupon; the ranking is what matters, not these exact figures.
| Market | Prices | Book | Hold | Cost per ₦500 |
|---|---|---|---|---|
| Asian handicap −0.5 | 1.95 / 1.95 | 102.56% | 2.50% | ₦12.50 |
| Match result | 2.30 / 3.40 / 3.10 | 105.15% | 4.90% | ₦24.50 |
| Over/under 2.5 | 1.87 / 1.93 | 105.29% | 5.02% | ₦25.10 |
| Draw no bet | 1.55 / 2.45 | 105.33% | 5.06% | ₦25.30 |
| Double chance | 1.32 / 1.33 / 1.62 | 106.34% | 5.96% | ₦29.80 |
| GG/NG | 1.72 / 2.05 | 106.92% | 6.47% | ₦32.35 |
| Corners over/under 9.5 | 1.80 / 1.90 | 108.19% | 7.57% | ₦37.85 |
The spread is three-to-one from top to bottom. A punter who expresses the same opinion through a handicap rather than a corners line keeps ₦25 more per ₦500 slip before anything is decided on the pitch, and the opinion has not changed at all.
The pattern behind the ranking is attention. Markets that everyone checks, and that are easy to compare between operators, stay tight. Markets that few people price independently — corners, cards, bookings, novelty lines — carry more. That is also why the same market can be priced differently in two apps, so a habit of reading the whole line rather than one number pays for itself.
Where the margin actually sits
The arithmetic so far treats the mark-up as if it were spread evenly. It usually is not. Take a lopsided fixture where fair chances are 70% home, 20% draw, 10% away, and the offered prices are 1.40, 4.60 and 8.00.
| Outcome | Fair chance | Price offered | Implied chance | Loading |
|---|---|---|---|---|
| Home | 70.0% | 1.40 | 71.43% | +2.0% |
| Draw | 20.0% | 4.60 | 21.74% | +8.7% |
| Away | 10.0% | 8.00 | 12.50% | +25.0% |
The whole book is 105.67%, a 5.36% hold — unremarkable. Inside it, the favourite is priced almost fairly and the outsider is carrying four and a half times the average load. Backers of long shots pay far more than the headline figure suggests, and the effect grows as the price lengthens.
This has a practical consequence for anyone stripping margin out to recover "true" chances. Dividing each implied figure by the book total gives the away side 11.83%, when the honest answer is nearer 10%. Proportional removal systematically flatters outsiders, which is why a value calculation on a 8.00 shot needs a wider cushion than the same calculation at 1.40.
What margin costs across a season
Four ₦500 bets a week is 208 slips and ₦104,000 of turnover in a year — a realistic pace on a ₦12,000 monthly budget once returns are recycled. Hold is a share of that turnover, so the annual bill follows directly.
| Where the turnover goes | Hold | Cost over ₦104,000 |
|---|---|---|
| Asian handicaps only | 2.50% | ₦2,600 |
| Match result only | 4.90% | ₦5,096 |
| Double chance only | 5.96% | ₦6,198 |
| Corners only | 7.57% | ₦7,873 |
| Three-folds of match-result legs | 13.99% | ₦14,539 |
The last row is the one worth staring at. Combining three 105.15% markets gives 1.05153 = 1.1626, a book of 116.26% and a hold of 13.99%. The same money, the same opinions, and ₦9,443 more handed over across the year than betting those legs singly — the compounding is set out in singles versus accumulators.
FAQ
What does a 105% book mean in naira?
It means the prices imply 105 chances where only 100 exist, and that 1 − 1 ÷ 1.05 = 4.76% of the money staked across that market stays with the operator. On a ₦500 slip that is ₦23.80 of cost built into the price before the match starts, paid whether the bet wins or loses.
Is a lower book always the better bet?
It is the better market, not automatically the better bet. A 102.56% handicap line is cheaper to trade in than a 108.19% corners line, but if your view is about corners, expressing it through a handicap is not an option. The rule is to prefer the cheaper market whenever both can carry the same opinion.
Can margin be calculated on live odds?
Yes, with the same formula, but the reading is unstable: prices change while you add them and in-play books are generally wider to cover the risk of trading during a match. Take a snapshot of all outcomes at the same moment, or the total will be nonsense rather than merely approximate.
Why do two apps show different books on the same match?
Because each sets its own mark-up and each has a different position on the fixture. Since the cost is a share of turnover, a book of 104% instead of 107% is worth roughly ₦2,700 a year on ₦104,000 of turnover — more than most punters gain from any single change to how they pick.
Read next
- Odds into probability — the conversion this whole calculation rests on.
- The market index — every line you can run the numbers on.
- The match centre — current prices to test the method against.
This article is for information only and is not an inducement to gamble. Betting involves the risk of losing money — never stake more than you can afford to lose. 18+. If gambling stops being entertainment, read our responsible gambling guide and seek help.
