Overround & book percentage

The most quietly useful calculation in the shop

Every bookmaker's book sums to more than 100%. It has to — the difference is their margin, the theoretical edge built into the prices. A race in which the implied probabilities add up to 115% is quoted to a 15% overround. The bookmaker, if they have priced every horse correctly and lay the right proportion of each, is guaranteed that 15% on total stakes whatever happens. Your job, if you are serious about this, is to find races and selections where that margin has been misallocated.

A low-overround race — typically the big Saturday handicaps, or a high-profile Group 1 — is one where the bookmakers are competing hard for money and have squeezed their own margins down to 105% or even less. A tight book is telling you that the professional money has already gone in and the prices are close to fair. A fat book — anything above 120%, which is almost all small-field mid-week handicaps and most jumping cards outside the Festivals — is telling you the firm has drawn prices conservatively because the market is thin.

Where the calculator earns its keep is the value-ratio column. For each runner, it shows the quoted price divided by the fair price once the overround has been stripped out. A ratio above 1.0 means the bookmaker has priced that horse more generously than the rest of the book implies; a ratio below 1.0 means the horse is being priced short relative to its notional share of the book. This does not, of course, mean the short one will not win — only that you are being asked to accept less than a fair share of the margin for backing it. Over time, consistently backing ratios above 1.0 at a price you have independently judged to be fair is very close to the entirety of what "betting for value" actually means.

Worked example

Take an eight-runner handicap chase at Cheltenham on a Saturday. The bookmaker's prices might be 5/2, 7/2, 5/1, 8/1, 10/1, 14/1, 20/1, 33/1. Convert each to an implied probability (100 ÷ decimal odds): 28.57%, 22.22%, 16.67%, 11.11%, 9.09%, 6.67%, 4.76%, 2.94%. Add them: 102.02%. The book is running at 102%, an overround of just 2%. That is exceptionally tight — the result of multiple major firms competing on a marquee race — and puts the bookmaker's theoretical edge at about 2p in the pound. A comparable-sized field on a Monday afternoon at Wolverhampton might run at 118%, an 18p edge. Same sport, same calculation; very different economics.

Common mistakes

The headline confusion is to mistake the overround for the bookmaker's actual margin in the sense of expected profit per pound. The 2% overround on our Cheltenham book does not mean the bookmaker earns 2p on every pound staked; it means the collective prices are 2% "short" of fair. Actual margin depends on how the money is distributed across the book and on how sharp the prices are. The second mistake is to treat overround as a pure quality measure — a tighter book is "better" from a value perspective, but if all firms are pricing the same horses the same way, the shape of the book matters more than its width. The third is to compare overrounds across fields of different sizes directly — an eight-runner book at 110% and a twenty-runner book at 118% are roughly equivalent in per-runner terms.

What to watch for

Ante-post books can run at extraordinary overrounds, sometimes 200% or more, because they carry the option value of non-runners. Arbitrage situations, where one bookmaker is noticeably out of line with the market, can produce theoretical overrounds below 100% across firms — a "Dutch" opportunity, though rare and usually vanishingly small once bet limits and commission are accounted for. The longer piece on overrounds walks through how to read a book for value rather than simply to measure the margin. The odds converter will do the implied-probability arithmetic on any individual price you want to check.

Frequently asked questions

What is an overround?

The overround is the sum of the implied probabilities of every runner in a bookmaker's market, expressed as a percentage. In a fair market, it would be exactly 100%. In practice, it is always more than 100% — the difference is the bookmaker's theoretical margin on the race.

What's a typical overround for a UK horse race?

Saturday ITV handicaps typically run at 105% to 112%. Midweek cards at lesser tracks run at 115% to 130%. Big field, low-grade handicaps can touch 135%. Ante-post books can run at 200% or more because of the option value of non-runners.

Why does overround matter?

Because it tells you, before you bet, how heavily the market is stacked against you on average. Choosing which races to bet on based partly on overround is one of the most fundamental forms of bet selection. The longer piece on overrounds walks through how to read a book for value.

Can an overround ever be below 100%?

Only in arbitrage situations where you can bet different outcomes with different bookmakers whose prices are inconsistent. A single bookmaker's book will never price below 100% deliberately.