Bookmaker Margin
The bookmaker's built-in edge, seen when the implied probabilities of all outcomes in a market sum to more than 100%.
Definition
The bookmaker's margin is the amount by which a market's odds are shortened from their fair value so the book profits over time. Convert every outcome's odds to an implied probability (1 divided by the decimal odds) and add them: a fair market would total 100%, but real markets total more, and the excess is the margin. This means the odds are systematically priced against the bettor, so on average bettors lose the margin over many bets and consistent long-term profit is rare. Margins vary by sport, market, and book, and are wider on parlays, props, and outrights.
Worked example
A two-way market prices both sides at 1.91. Each implies 1 / 1.91 = 52.36%, summing to 104.71%. The 4.71% above 100% is the margin; the fair, no-margin price would be 2.00 on each side.
Why it matters
The margin is the single most important reason casual betting loses money over time, and every serious bettor measures it to know how big an edge they need just to break even.