Horse Racing Odds
The overround on the Grand National ante-post market can reach as high as 180 per cent — a level that would be considered predatory in any raceday market. At the upper end, you find big-field handicaps with 20 or more runners, where each horse price can be inflated by a few ticks without any individual punter noticing the difference. If you bet at a bookmaker with a consistently higher overround, you need a higher strike rate just to break even.
For any given horse, you can see the best available price highlighted, and the difference between the best and worst price is often two or three points of odds — the difference between 7/1 and 5/1 on the same horse in the same race. Oddschecker is the most widely used in the UK and shows prices from all major bookmakers side by side, updated in near-real time. Despite these caps, BOG remains the single most consistently valuable promotion in horse racing.
Before placing multiple bets, check the specific bookmaker’s terms—assuming BOG applies universally is a common and costly mistake. BOG is a tool for capturing extra edge when your original selection was sound; it does not substitute for sound selection. Identify horses where you see value relative to implied probability. Alternatively, market support for another runner draws money away from your selection, and it drifts to 12/1 by the off. Throughout the day, money comes for the horse, and by race time it is 6/1 favourite. Lower overrounds indicate more competitive pricing; higher overrounds mean you need more accurate selections to overcome the margin.
Small-field races with well-fancied favourites tend to carry lower overrounds (closer to 110 per cent) because the market is more efficient and bookmakers compete harder on the headline price of the favourite. The overround also tells you something about the race itself. The overround is 5.9 per cent, meaning the bookmaker has built a theoretical edge of just under 6 per cent into the market. It is the bookmaker’s theoretical profit margin before a single bet is settled. In practice, they always sum to more — typically between 110 and 130 per cent for horse racing — and that excess is the overround. In a perfectly fair market with no margin, the implied probabilities of all runners in a race would sum to exactly 100 per cent.
But without calculating implied probability, you cannot begin to answer that question systematically. For odds comparison across multiple bookmakers, decimal format saves time and reduces errors. This format dominates betting exchanges and has become standard on many online platforms, particularly for European customers. At 4/6 (sometimes written 4-6), you need to stake £6 to win £4 profit, returning £10 total from a £6 stake.
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If you are betting early in the day and BOG is on offer, take the best available bookmaker price. If the exchange is at 7.0 and the bookmaker betting apps not on gamstop is at 6/1, the prices are closer, and after exchange commission the bookmaker might even be marginally better. If the exchange back price is 8.0 and the best bookmaker is offering 6/1, the bookmaker is taking a significantly wider margin on that horse. Open Betfair or Smarkets and look at the back price on your selection.
That difference — roughly 10 per cent on the individual price — is the cost of the bookmaker margin. The exchange takes a commission on the net winnings of the winning side, typically between 2 and 5 per cent depending on the user’s activity level. Instead of setting odds and accepting bets against them, the exchange matches punters on opposite sides of a bet. The betting exchange — Betfair being the dominant platform — operates on a fundamentally different model to a traditional bookmaker. That does not mean extra places are bad value — the additional place coverage can still be worth the wider margin, depending on the race — but it does mean you should not assume that an extra-places promotion is automatically a gift. Analysis from MatchedBettingBlog suggests that margins on extra-places markets can reach as high as 50 per cent.
Ante-post markets on the Cheltenham Gold Cup or Champion Hurdle typically run between 140 and 160 per cent, which is lower than the National but still significantly wider than raceday. The aggregate effect across 20 runners adds up to a much wider margin than in a six-runner race. At the lower end, you find small-field Flat races — Group 1 contests with five or six runners where the form is well established and bookmakers compete aggressively on the headline price of the favourite. Across UK and Irish horse racing, typical bookmaker overrounds range from about 110 to 130 per cent. It varies by operator, by race type, and by the time of day. The margin a bookmaker builds into horse racing odds is not fixed.
