Understanding how bookmakers set odds and make money gives you a real edge as a bettor. Most people think of bookmakers as operating like a casino - house wins everything. The reality is more nuanced, and knowing the mechanics helps you find better value and choose where to place your bets.
How Bookmakers Set Odds
Bookmakers employ odds compilers - specialists who set the initial odds for each market. They use statistical models (team rankings, recent form, historical head-to-head data, expected goals data) combined with market intelligence from other bookmakers and sharp bettors.
The initial odds are not necessarily the bookmaker's best guess at the true probability. They are designed to attract balanced betting across all outcomes, so the bookmaker profits regardless of which team wins.
The Margin (Overround)
The bookmaker's built-in profit is called the margin, overround or vigorish. It works like this: on a coin flip, a fair bookmaker would price both outcomes at 2.00 (50% implied probability each). A bookmaker with a 5% margin might price them at 1.91/1.91. The implied probability of each outcome is 1/1.91 = 52.4%, and 52.4% + 52.4% = 104.7% - the 4.7% above 100% is the margin.
On a football 1X2 market, margins typically run between 5% and 8%. On less popular markets (correct score, first goalscorer) the margin can be 10-15% or higher. Use our margin calculator to calculate the exact margin on any market you are considering.
How Markets Stay Balanced
Bookmakers ideally want roughly equal liability on all outcomes - so whoever wins, they pay out less than they took in overall. When too much money comes in on one outcome, they shorten the odds on that outcome (making it less attractive) and lengthen the odds on the others (making them more attractive).
This is why odds move before kick-off. It is not always because the bookmaker has new information - it is often because money is flowing in one direction and they are re-balancing the book.
Sharp vs Recreational Bettors
Bookmakers distinguish between sharp bettors (professional or semi-professional gamblers who bet large amounts based on genuine edge) and recreational bettors (most customers, who bet for fun). Sharp action moves odds. Recreational action is generally accepted at any price.
When a bookmaker restricts your account or reduces your maximum stake, it is usually because your betting pattern resembles a sharp bettor - you are consistently winning, or your bets correlate with sharp market movements. Unfortunately, being a winning bettor can get your account limited.
Pricing Power and Favourite-Longshot Bias
Bookmakers generally set their margins differently across odds ranges. Short-priced favourites (odds under 1.50) often carry higher margins than longer shots. This is partly due to favourite-longshot bias - recreational bettors systematically underestimate how often short-priced favourites win, so bookmakers can charge more margin on them without losing customers.
Promotions and Their True Cost
Acca insurance, free bets, enhanced odds and cashback offers all have costs built into them. A free bet worth 10 costs the bookmaker far less than 10 in cash - typically 30-50% of face value, accounting for the wagering requirement and the probability of that bet winning. Promotions are marketing tools, but they still represent genuine value for savvy bettors who use them correctly.
Exchanges vs Traditional Bookmakers
Betting exchanges (like Betfair) work differently. You bet against other customers, not against the exchange. The exchange takes a commission on winning bets (usually 2-5%) but does not set odds or build in a margin the same way. Exchanges often offer better odds than traditional bookmakers, particularly on popular markets, and they do not restrict winning accounts in the same way.