Every betting market has a hidden ingredient that works quietly against the bettor: the bookmaker's margin, also called the overround or the vig. It is the reason the odds in any market add up to more than a fair 100%, and understanding it is essential to judging whether a price is worth taking.
Margin is not a scandal; it is simply how bookmakers make money. But knowing how to measure it turns you from a passive price-taker into a bettor who can tell a sharp price from a poor one.
What the Overround Is
If you convert every outcome in a market to its implied probability and add them, a fair market would total exactly 100%. In reality the total is higher, and that excess is the overround. A market totalling 106% carries a 6% margin.
How to Measure Margin
Take each price, convert it to a percentage, and sum them. For a two-way market priced 1.90 and 1.90, each implies 52.6%, totalling 105.2%. The 5.2% above 100% is the bookmaker's edge on that market.
Why Lower Margin Is Better for You
The smaller the overround, the closer the odds are to a fair reflection of probability, and the more of any value you find stays in your pocket. Comparing margins across bookmakers is a simple way to make sure you are taking competitive prices.
Margin and Value
Because margin is baked into every price, a bet only offers value if your edge is large enough to overcome it. This is why shopping around for the best available odds matters so much; a smaller margin directly improves the value of your bets.
Margin Varies by Market
Overrounds tend to be tighter on high-profile markets with lots of competition and wider on obscure ones. Being aware of this helps you focus your betting where the maths is least stacked against you.
See Margin in Action
Convert a few markets to percentages yourself, then use the Bet Calculator Betmentor to compare the returns different prices deliver. Once you can see the overround, you read every market with sharper eyes, which is the foundation of understanding odds.