The margin (also called overround, vig, or juice) is the bookmaker's built-in profit mechanism. It works by setting odds so that the implied probabilities of all outcomes add up to more than 100%. This excess above 100% represents the bookmaker's guaranteed long-term return on every market they price.
In a perfectly fair coin-flip market, each side would be priced at 2.0 (50% probability each, summing to 100%). A bookmaker adds a margin by pricing both sides at 1.90 (52.6% each). The combined implied probability is 105.3% — the extra 5.3% is the margin. No matter which side wins, the bookmaker collects more from losers than they pay to winners over time.
Margin varies by market type and bookmaker. Major football 1X2 markets at premium bookmakers carry 3-6% margins. Smaller leagues, niche sports, and less competitive markets carry higher margins of 8-15%. Asian handicap and over/under markets generally have the lowest margins in football, which is why professional bettors gravitate towards these markets.
The practical impact is that you are always betting at a slight disadvantage unless you have an edge. At a 5% margin, a random bettor loses approximately 5% of their total turnover. This compounds across millions of bets, which is why bookmakers are consistently profitable even when they pay out on large winners.
Example
Premier League 1X2 market: Home Win 2.10, Draw 3.40, Away Win 3.70. Implied probabilities: 47.6% + 29.4% + 27.0% = 104.0%. Margin = 4.0%. For every £100 you stake at random in this market, you expect to lose £4 on average over the long run.