Bookmaker's margin (overround)
A bookmaker’s margin is the amount by which the implied probabilities of every outcome in a market add up to more than 100%, and it is the share of stakes the bookmaker expects to keep over time. You will also see it called the overround, the vig or the juice. It is never printed beside the odds, yet it sits inside every price you are offered, so working it out yourself is the only way to know what a market is costing you.
How the bookmaker's margin is calculated
Every price carries an implied probability. With decimal odds it is 1 divided by the odds, and with fractional odds of a/b it is b divided by a + b, so 4/1 works out at 1 in 5, or 20%.
Take a football match priced at 6/5 the home side, 9/4 the draw and 5/2 the away side. Those prices imply 45.45%, 30.77% and 28.57%, which together make a book of 104.79%. A fair book would total exactly 100%, because one of the three results has to happen, so the extra 4.79 points are the bookmaker’s cut.
Two formulas, two answers
Here is where guides disagree, and it is worth knowing why. The simpler method subtracts 1 from the book and calls the result the margin, which gives 4.79% in the example above. The other method takes 1 minus 1 divided by the book, which gives 4.57%, and that second figure is the share of turnover the bookmaker keeps if money comes in proportion to its prices. Neither is wrong as long as the page tells you which one it used, so when two sites quote slightly different margins for the same market, the formula is the first thing I check.
The inverse of the book, 1 divided by 1.0479, is the theoretical payout: roughly 95.4 cent back for every euro staked.
Where you meet the margin as a punter
Margins are set by each bookmaker and differ by sport, by competition and by market, so there is no single standard figure to hold an operator to. Published industry guides put the match result on major football fixtures at roughly 2-3% with the sharpest firms and around 5-8% with most high-street brands, while niche lines such as correct score, player props and outrights commonly run to 10% or more.
The trend on the biggest leagues has been downward. One study of six large bookmakers found the average Premier League margin fell from about 9% in 2005/06 to about 4% by 2017/18, with the lower English divisions dropping from 11% to between 6% and 8%.
Accumulators are where the cost quietly compounds.
Each leg of an acca carries its own margin, and multiplying the legs multiplies those margins too, so a five-fold built from markets at 5% each costs you far more than 5% in total. The same logic applies to bet builders, which combine several markets from one match into a single price. When a GAA championship game, a midweek race card or a Premier League fixture is priced across several firms, comparing the book on each is a quick way to see who is giving you the keener price before any question of value comes into it.
What the margin is often confused with
The margin is not the bookmaker’s actual profit on any given match. It describes the expected take if stakes are spread in line with the prices, and real money rarely arrives that neatly, so a firm can lose heavily on a single result while its margin stays positive.
Nor is it quite the same as your expected loss. Research from University College Dublin found that bookmakers often load a bigger margin onto longer-priced outcomes, so the standard overround formula understates what punters lose on average: in that sample, football bettors lost about 7.8% against the 6.5% the formula predicted. That makes the calculated margin closer to a minimum cost than an exact one, particularly if you lean towards outsiders.
It also differs from RTP in casino games, although the two are mirror images of a sort. RTP is a figure the game provider sets and publishes, whereas a sportsbook margin is something you work out from the odds on screen, market by market.
One last point on promotions: a free bet does not change the margin on the market, and most free bets return winnings only, without the stake, so the same sum still applies to the price you take.
