Overround explained: how to measure what a bookmaker charges
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Bookmakers do not charge a fee at the till. Their price is built into the odds themselves. The measure of that price is the overround, also called the margin or the vig. Knowing how to calculate it lets you compare bookmakers fairly, spot expensive markets and see roughly what the true chances might be.
If you are new to turning odds into percentages, start with our article on implied probability. This guide builds on it and focuses on the margin itself.
The basic idea
In a fair market, the chances of all possible outcomes add up to exactly 100%. Bookmakers shade their prices so that the implied probabilities add up to more than 100%. The amount above 100% is the overround.
Implied probability for each outcome is 1 / decimal odds. Add them all, then subtract 100%.
A two-way market
Take a tennis match with no draw. Player A is 1.80 and Player B is 2.05.
- Player A: 1 / 1.80 = 55.6%.
- Player B: 1 / 2.05 = 48.8%.
- Total: 104.3% (55.56% + 48.78% before rounding).
The overround is 4.3%. If the two prices were fair, the total would be 100%. The bookmaker has priced the market as if the combined chance were 104.3%, and the extra 4.3 points are its built-in margin.
A three-way market
Football's 1X2 market has three outcomes: home win, draw and away win. Suppose the prices are 2.10, 3.40 and 3.60.
- Home: 1 / 2.10 = 47.6%.
- Draw: 1 / 3.40 = 29.4%.
- Away: 1 / 3.60 = 27.8%.
- Total: 104.8%.
The margin is 4.8%. The method is identical whatever the number of outcomes, which is why it also works for outright markets with dozens of runners. Those can carry margins of 20% or more, because there are so many prices to shade. Our guide to reading 1X2 odds explains the market in more detail.
What the margin costs you
A rough way to see the cost: the margin is approximately the percentage you lose on average per unit staked if you bet on outcomes at random. A market with a 4.3% overround costs a bettor about 4% of every stake over the long run, before any skill or luck. A market at 10% costs more than double that. This is why the same pick can be a decent bet at one bookmaker and a poor one at another.
Comparing bookmakers
Suppose two bookmakers price the same two-way market.
- Bookmaker X: 1.91 and 1.91. Implied total: 52.4% + 52.4% = 104.7%.
- Bookmaker Y: 1.95 and 1.95. Implied total: 51.3% + 51.3% = 102.6%.
Bookmaker Y charges a margin of 2.6% against 4.7% at X. On a 100 stake at an even-money pick, Y pays 95 profit instead of 91. Over hundreds of bets, that difference is significant. Favouring lower-margin markets is one of the few things a bettor controls with certainty. Popular leagues and main markets usually have lower margins; niche competitions and props are often higher.
Removing the margin
Once you know the total, you can estimate the "fair" probabilities by scaling each implied probability down so they add to 100%. This is often called removing the vig, or a no-vig price.
Using the football example, with implied probabilities of 47.6%, 29.4% and 27.8% and a total of 104.8%:
- Home: 47.6 / 104.8 = 45.4%.
- Draw: 29.4 / 104.8 = 28.1%.
- Away: 27.8 / 104.8 = 26.5%.
These now sum to 100%, and the fair odds are 1 / 0.454 = 2.20, 3.56 and 3.77. The no-vig calculator does this instantly for any number of outcomes.
A caution: proportional scaling is a simplification. Bookmakers often add more margin to favourites or to long shots, so the true split of the margin is not always even. Treat no-vig figures as a sensible baseline, not as the exact chance.
Why this matters for value
You cannot judge a price without knowing how much of it is margin. If you think the home win in the example has a 48% chance, the offered 2.10 gives an expected return of 0.48 x 2.10 - 1 = +0.8%, a thin edge against a no-vig estimate of 45.4%, and one that vanishes if your estimate is 46%. For the full idea of comparing your estimate to the price, see value betting. To go the other way, check how a bookmaker builds its numbers in our guide to how bookmakers set odds.
Margin and accumulators
The margin compounds across a multiple. Three legs each carrying a 5% margin do not cost 5% in total; they cost closer to 14% on average. This is one reason accumulators are usually worse value than singles, even though the potential payout looks exciting.
Common mistakes
- Comparing only the headline price and ignoring the other side of the market.
- Treating no-vig probabilities as the truth. They are an estimate.
- Forgetting that the margin is a long-run average and says nothing about the next result.
Key takeaways
To find the overround, add the implied probabilities of all outcomes and subtract 100%. Compare it across bookmakers and markets, prefer lower margins, and remove it to see a clearer picture of the underlying chances. A low margin helps, but it never guarantees a profit.
Betting is for adults only (18+). Only stake money you can afford to lose, set limits before you start, and stop if it stops being fun. If gambling is causing you problems, contact a local support organisation.