Margin Calculator
Work out how big a margin (overround) a bookmaker is taking on a market, and see what the fair odds would be without it.
Enter odds for every outcome to see the margin.
This calculation is a guide only, based on the odds you enter. Odds move constantly at bookmakers — always check the current odds before you bet.
What is a bookmaker's margin?
The margin (also called overround or "the vig") is the bookmaker's built-in edge on a market. It appears when the implied probabilities of every outcome add up to more than 100%, which guarantees the bookmaker a profit regardless of the result if they get balanced action on all sides. The margin calculator shows you exactly how big that margin is — and what the odds would look like without it.
How we calculate the margin
Every odds figure corresponds to an implied probability: 1 divided by the odds. Adding up the implied probabilities for every outcome in a full market gives a number that's normally greater than 1 (100%) — the difference is the bookmaker's margin.
Margin = (Σ 1/odds − 1) × 100%
The fair odds are worked out by first normalising each implied probability so they add up to exactly 100% (fair probability = implied probability / sum of all implied probabilities), then inverting the fair probability (fair odds = 1 / fair probability).
Worked example
Take a two-way market where both outcomes are priced at 1.90. The implied probabilities are 1/1.90 = 52.63% each, so 105.26% in total.
The margin is therefore 5.26% (105.26% − 100%), and the fair odds without the margin would have been 2.00 on both outcomes (50% each).
Frequently asked questions
What's a good margin for a bookmaker to have?
The lower the margin, the better it is for you as a bettor — more of your stake comes back as winnings over time. Major markets like the Premier League or NHL often carry margins under 3–5% at competitive bookmakers, while niche markets, specials and live betting usually carry much higher margins.
Why does the margin differ between bookmakers on the same match?
Each bookmaker sets its own odds based on its own analysis, risk management, and how much action it wants on a given side. That means the margin can vary noticeably between bookmakers on the exact same match — which is exactly why it pays to compare odds before you bet.
Can the margin be negative?
Yes, in theory, if the odds (whether pooled from several bookmakers or, by mistake, from just one) add up to less than 100% implied probability. That signals an arbitrage opportunity where you could lock in a profit regardless of the result — but it's extremely rare at a single bookmaker and usually only shows up when combining odds across several different ones.
