Open Betfair on any UK race and look at the top of the runners list. There is a small percentage figure above the back column and another above the lay column. Most people skip past them.
Those figures are the market percentage. They show how much the prices on offer add up to, and the amount over 100% is a rough measure of how much room the market leaves for itself. Once you know how that number is calculated, you can see why some prices are worth taking and others are not.
Quick answer: how is market percentage calculated?
Market percentage is the sum of every runner’s implied win probability. Take each horse’s decimal odds, divide one by that number to get its implied probability (the chance the price is suggesting), then add them all up. Anything above 100% is the over-round, the extra built into the prices.
The quick formula: turning decimal odds into a win percentage
Start with one runner. Take its decimal odds, divide one by that number, then multiply by a hundred. That gives the chance of winning that the price implies.
1 / decimal odds × 100 = implied win percentage
- A horse at 2.44 on the exchange: 1 divided by 2.44 is 0.41, so the implied chance is about 41%. The price is suggesting it wins roughly 41 times in 100 similar runs.
- A horse at 7.6: 1 divided by 7.6 is 0.1316, so about 13.2%.
- A horse at 30: 1 divided by 30 is 0.033, so about 3.3%.
The arithmetic is the same every time.
Adding the runners up
The market percentage is every runner’s implied percentage added together. There is no second formula and no weighting.
In a fair market with no margin, the total would be exactly 100%. One horse has to win, so the chances of all the runners must add up to one.
Real markets rarely land on exactly 100%, because bookmakers price in a cushion for themselves. The amount above 100% is the over-round, and it is a rough guide to the cost of betting in that market.
Why bookmaker markets sit near 115% and exchanges near 100%
A bookmaker sets its own prices and takes the other side of your bet, so it builds a cushion into those prices. On a betting exchange, another customer takes the opposite side of your bet, and the exchange earns a small commission on winnings whichever customer wins. The prices come from customers backing and laying each other, which is why the whole book tends to sit closer to 100%.
(Two terms used below: SP means starting price, the price the race starts at. Betfair SP is the exchange’s version of it.)
What 13 years of UK racing showed about over-round
In an earlier Race Advisor analysis of 81,003 UK races between July 2013 and March 2026, the bookmaker SP market sat well above 100% in every field-size band we looked at, while Betfair SP stayed close to 100%. These figures describe that stretch of racing and are not a promise that future races will look the same.
| Field size | Races | Bookmaker SP median | Betfair SP median |
|---|---|---|---|
| 3 to 6 runners | 19,250 | 110.4% | 100.12% |
| 7 to 10 runners | 37,911 | 115.6% | 100.20% |
| 11 to 14 runners | 19,647 | 121.0% | 100.19% |
| 15 or more | 4,195 | 129.2% | 100.28% |
Overall, the median bookmaker SP race came out at about 116%, against about 100.2% for Betfair SP. That is a difference of roughly 16 percentage points between the median bookmaker book and the median exchange book in this sample. It is a comparison of whole markets, and what any one bet costs you depends on the price you actually take and, on the exchange, the commission.
The exchange total hardly moves whether the race has four runners or eighteen. The bookmaker over-round nearly triples between the smallest and largest fields, from about 10 points to about 29.
The split by race type, from the same earlier analysis, points the same way:
- Chase on turf (13,006 races): bookmaker SP median 113.0%, Betfair SP median 100.1%.
- Flat on turf (26,577 races): 115.8% against 100.2%.
- National Hunt Flat (2,774 races): 118.2% against 100.2%. This is the highest bookmaker over-round of the three.
For that reason, Race Advisor uses Betfair SP as the default pricing basis for its A/E ratio analysis. A/E means actual against expected: how many winners a group of horses produced compared with how many the prices suggested. Betfair SP, with a 2% commission applied, gives a consistent exchange-based price series to measure against, which is the narrower reason we use it.
Why this number matters when you bet
When the prices you take carry a bigger over-round, your own judgement has more to beat before you are level. Here is an illustration, not a real race. A horse at 5.0 on the exchange implies 20%. If the bookmaker’s price for the same horse were 4.0 to 4.3, it would imply 23% to 25% for the same outcome. A good part of any edge can disappear before the stalls open.
This connects directly to finding value bets. Value is your own estimate of a runner’s chance set against the chance the price implies. Suppose your estimate says a horse should be 4.5 and the exchange offers 5.0. That is value on your numbers. If the same horse is 4.0 with a bookmaker, the bookmaker’s over-round may have eaten it. This is an idea to think with, and your estimate still has to be right.
It also helps explain a pattern many bettors never account for. In the sample above, larger fields carried bigger bookmaker margins, which fits with big fields being harder to price. That is a reading of the figures, not a proven cause. Backing blind at SP in a field of fifteen or more meant a median bookmaker over-round near 29% in that sample, which leaves a lot to overcome.
You do not need special tools to check the cost of a market. Note the runners’ decimal odds, divide one by each, and add the percentages. Anything over 100 is the cost. If the cost looks high, the bet may not be value even if you like the horse.
If you want to try the sum on a live market, open today’s race cards and add up the implied chances for one race before you decide.
