Market percentage is the total of every runner's implied probability in a race. Turn each price into a percentage (100 divided by the decimal odds), add the whole field together, and anything over 100% is the bookmaker's margin. A race framed at 116% means the book is pricing the field as if the runners had a combined 116% chance of winning, when the real total can only be 100%. The lower the figure, the closer the prices sit to fair.
That's the short answer. The rest of this guide works through an invented eight-runner race: how to add it up, how to strip the margin out to get fair odds, why your best price across bookmakers matters more than any single book, and the two places punters most often get it wrong (place markets and exchange commission). Every number is an example, not a selection.
Market percentage, overround and book percentage
These are three names for the same idea. Australian racing tends to say market percentage, UK sources say overround or book percentage, and US sports betting says vig or juice. Wikipedia's mathematics of bookmaking entry defines the overround as the amount by which the book exceeds 100%, and treats it as the bookmaker's expected margin.
It's the same maths you'd use on a two-way market, which our guide to vig covers. Racing just has more runners to add up, and that changes how the margin behaves.
How to calculate a race's market percentage
- Convert every price. Implied probability = 100 / decimal odds. $4.00 is 25%.
- Add the whole field. Include every runner still in the race, not just the ones you like.
- Read the total. 100% is a book with no margin. The amount above 100% is the margin.
Here's an invented eight-runner race at one bookmaker:
| Runner | Price | Implied probability |
|---|---|---|
| 1 | $2.60 | 38.5% |
| 2 | $4.00 | 25.0% |
| 3 | $6.00 | 16.7% |
| 4 | $8.00 | 12.5% |
| 5 | $11.00 | 9.1% |
| 6 | $15.00 | 6.7% |
| 7 | $21.00 | 4.8% |
| 8 | $31.00 | 3.2% |
| Market | 116.4% |
This book is framed at 116.4%. If you backed every runner in proportion to its price so you'd collect $100 whichever one won, you would have to stake about $116.40 to do it. That gap is the margin. Sporting Life's overround guide works the same method through a football market, and makes the same point: the closer to 100%, the better the odds for the bettor.
One caution from the same Wikipedia entry: the overround is the bookmaker's built-in edge on the prices, not its actual profit on the race, because punters never stake neatly across the whole field.
How to turn market percentage into fair odds
Fair odds are what each price would be with the margin taken out. The simplest method is proportional: scale every runner's implied probability down so the field adds to exactly 100%.
- Fair probability = implied probability / market percentage
- Fair price = offered price × market percentage (as a decimal)
For runner 2 at $4.00 in a 116.4% market: 25.0% / 1.164 = 21.5%, and $4.00 × 1.164 = $4.65.
| Runner | Price | Fair probability | Fair price |
|---|---|---|---|
| 1 | $2.60 | 33.1% | $3.03 |
| 2 | $4.00 | 21.5% | $4.65 |
| 3 | $6.00 | 14.3% | $6.98 |
| 4 | $8.00 | 10.7% | $9.31 |
| 5 | $11.00 | 7.8% | $12.80 |
| 6 | $15.00 | 5.7% | $17.46 |
| 7 | $21.00 | 4.1% | $24.44 |
| 8 | $31.00 | 2.8% | $36.08 |
The fair column now adds to 100%. A calculator does this in seconds, and BetBank's free calculators are there if you'd rather not do eight divisions by hand.
Why proportional fair odds are only an estimate
Proportional scaling assumes the bookmaker spread its margin evenly across the field. Often it didn't. The favourite-longshot bias is the long-observed pattern where bettors overvalue longshots, and prices on outsiders tend to carry more of the margin as a result. Sporting Life notes it isn't unusual to see a 33/1 shot with bookmakers trading at more than double that price on the exchange.
If that's the case in a given race, a proportional method takes too much margin off the favourite and not enough off the roughies. Other methods (power, Shin) try to correct for it, and our implied probability guide compares them on two-way and three-way markets. Treat any fair price as an estimate with error around it, not a fact.
Best-of-market: why one book's percentage isn't the whole story
You don't have to bet every runner with the same bookmaker. If you take the best available price on each runner across several books, the combined market percentage drops, sometimes a lot. Using invented best prices for the same race:
| Runner | One book | Best of market | Best-of-market implied |
|---|---|---|---|
| 1 | $2.60 | $2.80 | 35.7% |
| 2 | $4.00 | $4.20 | 23.8% |
| 3 | $6.00 | $6.50 | 15.4% |
| 4 | $8.00 | $8.50 | 11.8% |
| 5 | $11.00 | $12.00 | 8.3% |
| 6 | $15.00 | $17.00 | 5.9% |
| 7 | $21.00 | $23.00 | 4.3% |
| 8 | $31.00 | $34.00 | 2.9% |
| Market | 116.4% | 108.2% |
Same race, same runners, and the margin you pay roughly halves, from 16.4% to 8.2%, just by taking the top price on whichever runner you back. Notice too that the fair-price estimates barely move between the two books (runner 1 works out at about $3.03 either way). The fair price is a property of the race. What changes is how close your price gets to it.
This is the whole case for line shopping, and in racing it means checking every runner, not just one price.
Place markets add up to about 300%, not 100%
This is the most common mistake. In a race paying three places, three runners place, so the true place chances across the field sum to 300%. A place market that adds to 341% isn't 241% over the odds. Its margin is the part above 300%.
| Runner | Place price | Implied |
|---|---|---|
| 1 | $1.25 | 80.0% |
| 2 | $1.60 | 62.5% |
| 3 | $1.90 | 52.6% |
| 4 | $2.30 | 43.5% |
| 5 | $2.80 | 35.7% |
| 6 | $3.50 | 28.6% |
| 7 | $4.60 | 21.7% |
| 8 | $6.00 | 16.7% |
| Total | 341.3% |
341.3% against a fair 300% is about 13.8% over (341.3 / 300 = 1.138). Scale runner 1 down the same way and its fair place chance is about 70.3%, a fair place price of roughly $1.42 against the $1.25 offered. Place markets often look generous at a glance and turn out to carry a margin as big as or bigger than the win market once you divide by the right number. If a race pays two places, the fair total is 200%.
Exchange markets and commission
On a betting exchange, punters set the prices for each other, so the back side of a busy market usually sits close to 100%. Betfair shows the figure beside the back prices on each market, and Sporting Life reports average UK exchange overrounds of 101% to 102% close to race time.
That isn't the whole cost. The exchange charges commission on your winnings when the market settles. Betfair's commissions page lists the Market Base Rate as the maximum share of winnings you'll pay: 8% or 10% on Australian racing depending on state and code, and 8% on Victorian thoroughbreds, the Melbourne Cup market included.
To compare an exchange price with a bookmaker's, convert it to an effective price:
- Effective price = 1 + (exchange price minus 1) × (1 minus commission rate)
Say runner 1 is 3.15 to back on the exchange:
| Commission rate | Effective price |
|---|---|
| 8% | $2.98 |
| 10% | $2.94 |
So the exchange's 3.15 beats the best bookmaker price of $2.80 in this example even after commission, but it's still a touch under the $3.03 fair estimate. Commission is charged on your net winnings in each market, so the effective price is an approximation if you hold more than one position in the same race. Our explainer on what evens means in betting runs the same commission maths on a $2.00 price.
A quick market-percentage checklist for a race
- Add the field at the bookmaker you're looking at. Note the win market percentage.
- Build the best-of-market book from the top price on each runner. Compare the two totals.
- Estimate fair prices with the proportional method, and remember longshots may carry more margin than it assumes.
- Divide place markets by the places paid: 300% for three places, 200% for two.
- Convert exchange prices to effective prices at the right commission rate before comparing.
- Treat the result as information, not a tip. A price above your fair estimate is still a bet that can lose, and one race tells you almost nothing. Our expected value guide explains why the sample matters.
See every bookmaker's price on a race
Building a best-of-market book by hand means opening a tab for every bookmaker and racing the jump. The BetBank racing odds screen lines up live fixed-odds win and place prices from the major Australian bookmakers runner by runner and highlights the top price on each, so you can see how far each book sits from the best of market before you decide anything.
Betting is for adults 18+ only. Every bet can lose, so set a limit before you bet and gamble responsibly. If you want a break, BetStop lets you block yourself from all licensed Australian online and phone betting providers for free.