Every bookmaker and every casino builds an advantage into its prices. On a sports bet it is called the margin (or overround). In a casino it is called the house edge. They do the same job, but they work differently, and knowing how each one works tells you where your money is likely to last longer.
What is a bookmaker margin?
A bookmaker sets odds so that, if bets came in evenly on every outcome, it would pay out a little less than it took in. That gap is the margin.
You can see it by turning odds into implied probabilities. For decimal odds, the implied probability is 1 divided by the odds. Take a football match priced at:
- Home win 2.10, which is 1 ÷ 2.10 = 47.6%
- Draw 3.40, which is 1 ÷ 3.40 = 29.4%
- Away win 3.60, which is 1 ÷ 3.60 = 27.8%
The three outcomes add up to 104.8% rather than 100%. The extra 4.8% is the overround. In terms of money kept, it means the bookmaker expects to keep about 4.6% of everything staked on that market if the book is balanced.
Typical bookmaker margins
Margins vary by bookmaker, sport and market. Big markets, such as the match result in top-flight football, are usually the most competitive. Smaller leagues, player props, novelty markets and many in-play markets tend to carry bigger margins. Horse racing margins grow with the size of the field.
Accumulators multiply the margin. If each leg of a five-fold has a margin of around 5%, the combined price carries an effective margin of roughly 22%. That is why accas pay big when they land but cost more in the long run than the same selections as singles. Our sports betting basics guide explains the main bet types.
What is the house edge?
The house edge is the casino’s built-in advantage on a game. It comes from the rules: in European roulette, for example, there are 37 numbers but a winning straight bet pays 35 to 1, as if there were 36. That difference gives the casino an edge of 2.7% on every bet.
For slots, the edge is the opposite of the return to player (RTP). A slot with 96% RTP has a house edge of 4%.
Typical house edges
- Blackjack with basic strategy: often around 0.5%, depending on the table rules.
- Baccarat, banker bet: about 1.06% after the 5% commission.
- European roulette: 2.7%.
- Online slots: usually 3% to 6% (RTP of 94% to 97%).
- American roulette: about 5.3%, because of the extra double zero.
- Baccarat, tie bet: around 14% at the usual 8 to 1 payout.
Our online casino basics guide covers how these games work.
The key difference: fixed edge or a price you can shop around
A casino game’s house edge is fixed by its rules. Every European roulette table has the same 2.7% edge, and no system of bets changes it. The only choices that move it are which game you play and, in blackjack, how well you play your hand.
A bookmaker’s margin is different, because it is built into prices you can compare. If one bookmaker offers 2.10 and another offers 2.25 on the same outcome, taking the better price lowers the margin you pay. A bettor who knows a sport well can also find odds that are better than the true chance of the outcome, which is the idea behind value betting. That doesn’t make betting easy to win at: most bettors lose over time, and bookmakers can restrict accounts that win consistently.
Where bonuses fit in
Welcome offers change the maths for a single bet or session. A free bet or no-wagering free spins give you a chance to win without risking the full amount yourself. Since 19 January 2026, wagering on UK bonuses has been capped at 10x, which makes casino offers easier to value. Our how we rate offers page explains how we estimate what each offer is worth, and the offers page compares them.
Removing the bookmaker margin to find fair odds
Take the margin out and you have the fair odds. In the example above, the three outcomes add up to 104.8%, so you divide each implied probability by 1.048. That turns 2.10, 3.40 and 3.60 into fair odds of about 2.20, 3.56 and 3.77. Any price above the fair odds is worth a closer look; anything below it carries the bookmaker margin against you.
Use our fair odds calculator to do this for any market, and see value betting explained for how to use it.
What this means for you
- In the casino, the game you choose matters most. Blackjack with basic strategy and European roulette cost much less over time than American roulette or high-edge side bets.
- In sports betting, the price you take matters most. Compare odds, prefer singles to long accumulators, and be wary of markets where the overround is high.
- In both, the edge works over time. Short-term results swing a lot, so set a budget you are happy to lose and treat it as the cost of entertainment.
New to the terms used here? Our glossary of betting and gambling terms explains them in plain English.
18+ only. Please gamble responsibly. Set limits before you play and never chase losses. For free, confidential support, visit BeGambleAware or GamCare.