To place the most effective bets possible, it’s vital to be able to evaluate the fairness and competitiveness of available markets. Understanding overround betting helps you do just that. This simple guide will teach you how to calculate the bookmaker’s margin on any set of odds, ensuring you’re getting the best value on every bet you make.

To understand overround sports betting, we first need to wrap our heads around the overround. This is a profit margin built into the odds. This can be calculated as a percentage above 100% when all implied probabilities are added together.

How it works is simple. Odds indicate the likelihood of a particular outcome occurring. Therefore, all betting odds can be converted into a percentage of probability.

For instance, -500 implies an 83.3% chance of that prediction being correct. +100 would be 50%, +200 is 33% and +100000 is 1%, as just a few examples.

When you consider all markets, it would be logical that the final percentage would be 100%, as one of the possibilities occurring is certain.

However, because of the overround in betting, the final percentage of all probabilities will be above that amount.

For instance, let’s say on a three-way moneyline Team A is given odds with a converted probability percentage of 60%, Team B of 35% and the possibility of a draw is given 11%. That leaves us with an overall percentage of 106%, and the 6% above 100% is the overround.

From a strategic point of view, you can think of overround betting as wagering with the bookmaker’s margin in mind. The lower the percentage, the bigger the potential returns.

Alternatives to the overround

There as some betting sites, such as betting exchanges and pool betting sites, which use a commission instead of an overround. The commission is added to the net winnings of the users who won their bets. This is typically around 2-5%. One easy way of thinking of the overround is a type of commission built into the odds themselves.

Since the betting odds overround is simply odds converted into percentages and added together, that’s all you need to do to calculate the overround.

Screenshot of BetMGM Phillies vs Giants Odds.
(Source: BetMGM)

As an example, let’s say in a two-way moneyline (meaning draws are excluded) between the Philadelphia Phillies and the San Francisco Giants, the latter come in as favorites at -120 and the former are slight underdogs with +100 odds.

Different odds formats

These may also come in fractional and decimal odds, for instance +100 would be 1/1 in fractional and 2.00 in decimal. -120 would be 5/6 in fractional and 1.83 in decimal. All of these represent the same implied probability and overround.

Regardless of the odds, it’s always easy to find the exact implied probability and overround, here’s an example of how to work out overround betting.

  1. Go to the odds converter calculator (which you can think of as a betting overround calculator) found on our US guide to betting odds

  2. Type the odds for the Philadelphia Phillies (any format) into the calculator. Make a note of that number (50%). Do the same for the San Francisco Giants (54.64%).

  3. Add these amounts together. So, 50% + 54.64% equals 104.64%. 4.64% is your overround. This works the same for all markets.

The overround is fundamental to how most online sports betting works. For the betting sites, the betting market overround is about maintaining profitability. For you, overround betting is a way of checking your betting site is providing you with a decent return on your bets.

With a few clicks, which with our betting overround calculator can take moments, you can simply determine the quality of your odds. That’s incredibly valuable to any strategic bettor.

Generally speaking, an overround of under 110% is reasonable, and under 105% is exceptional.

It can also allow you to compare and utilize multiple bookmakers for the best possible deal. On a basic level, if you check the same odds across several sportsbooks, and one offers a smaller overround, then you know you’re getting the better deal.

The vig and the overround are both ways of calculating the bookmaker’s margin and the quality of the odds you’re being offered.

However, the vig focuses on the profitability of individual bets, while the overround considers all possible markets.

Vig as Profit

To understand the vig, imagine a two-way moneyline bet on two evenly matched teams. True odds would be plus +100 (50%) for both teams, meaning if you bet $100, you get $100 either way.

But that’s without a profit margin. So, let’s say the bookmaker instead moves the odds of both teams to -110. In this scenario, if $110 is placed on both teams, the bookmaker collects $220 and pays $210 (original $110 bet plus $100 profit). The vig is the difference in odds which creates the $10 profit.

Vig as a Percentage

You can also understand the vig, just as you can the overround, as an implied percentage of probability.

Using the above example, let’s say you have been offered odds of -110, and the true odds for this event would be +100. The implied probability for true odds is 50%, and with vig is 52.38%. The difference between these numbers is 2.38%, and that’s your vig.

The overround for these markets would be 4.76%, and if we double the vig across both markets we also get 4.76%.

In other words, they are different ways of measuring value, and ultimately provide the same conclusions.

Vig vs Overround Conclusion – Latter More Practical for Most

The benefits of the vig are that it provides detailed information about the value of a single bet as opposed to all markets together.

However, working out the vig, which on more convoluted markets requires you to normalize the probabilities and then convert true probabilities back to odds, can be very complicated. The overround is much simpler, especially with a calculator and will be more suitable for most sports fans.

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Comparing the value of single bets using overround – You don’t need the vig to determine the quality of the odds on an individual market. You could simply find out the implied probability of that market and compare it to that of other top operators, the smaller the percentage, the better returns for you.

The basic principles of overround betting remain the same regardless of the type of bet.

What’s important in understanding how to work out overround betting for all markets is to determine which odds should count towards your calculations.

Essentially, the odds should cover all possibilities.

On a basic level if we were looking at spread betting on an American football game, it would work just the same as a two-way moneyline bet. These bets may be different, but you only have two sets of odds, one of which must come true.

In another example, a horse race can have several possible winners and would require you to add up the possibilities of all participants as any of them could win.

So, figure out which markets when put together will create a certain outcome (barring the total abandonment of the event), convert, and add them together. The overround betting meaning always remains the same.

Overround betting is an assurance that you never have to settle for anything less than excellent odds.

In just minutes if not moments, you can convert odds, add them up and understand their quality with unmatched clarity. It really puts into perspective whether your wager is a worthwhile one, and if so, where best to make it.

There are many elements to an effective betting strategy, but few are as important as ensuring maximum value out of each wager. Understanding the overround in betting is one of the most powerful ways of achieving just that.

The overround is the bookmaker’s built in profit margin, calculated as a percentage above 100% when all possible odds are converted into percentages and added together.

The sports betting overround for horse races is conceptually the same as any other sport. All the odds converted into percentages and added together to get a total above 100% representing the bookmaker’s margin. The only difference is there are often many more potential winners in a horse race than most other sporting events so there’s more to add up.

As an example, we’ve looked at the markets on a boxing event between Raymond Ford and Nick Ball. Ford is given odds of -175 and Ball odds of +138. Converted into percentages, the former represents 63.69% probability and the latter 42.02% probability. Added together it comes to 105.71%, so the overround is 5.71%.

The overround are included as part of the odds on virtually all sportsbooks. There’s nothing you can do to remove them in any practical sense. The only thing you can do is use your calculations of the overround to find the best value odds possible.

Yes. The addition of a bookmaker’s margin into the odds, in other words the overround, is both legal and standard practice from sportsbooks. Overround betting, where you convert odds and use your knowledge of the overround to assess and compare the quality of odds, is also entirely legal.

No. Betting exchanges instead make their profits by adding a commission onto net winnings, typically between 2-5%.

WRITTEN BY Ben Gibson View all posts by Ben Gibson

Ben has been writing professionally for over a decade. His articles are a great outlet for his inexhaustible supply of sporting facts and enthusiasm. He resides in Yorkshire, where his work is powered by the ever-present sound of a kettle.

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