# How To Convert Odds Ratio To Probability

When a betting site offers odds about a selection, it is offering its subjective viewpoint about what it thinks is the probability of that selection winning.

Professional gamblers make a living by disagreeing with the opinions of betting sites and bet the odds when they think those odds underestimate the probability of a selection winning.

This concept is often referred to as finding the value and, for most professional gamblers, their ability to find the value is the cornerstone on which their success is built.

There are two main types of odds, decimal and fractional. Decimal odds are much easier to use when converting odds to probability. But it’s only slightly more complicated to convert fractional odds to probability.

## Converting Decimal Odds To Probability

If you use decimal odds, you’ll be familiar with odds such as 5.00 (which is 4/1 if you’re more used to using fractional odds).

If you place a bet of £20 at 5.00, you will get a total return of £100 (£80 winnings + £20 stake).

The calculation for converting decimal odds into probability is as follows:

1 ÷ by the decimal odds x 100 = probability
Using our decimal odds as an example:
1 ÷ 5.00 x 100 = 20%

This means that a betting site that offers odds of 5.00 about a selection thinks it has a 20% chance of winning. However, you must remember that betting sites rarely bet to 100%, so they, actually, think the chances of the selection winning are slightly less.

## Converting Fractional Odds To Probability

If you use fractional odds, you’ll be familiar with odds such as 4/1.
(which is as you now know is 5.00 if you’re more used to using decimal odds)

If you place a bet of £20 at 4/1, you will get a total return of £100 (£80 winnings + £20 stake).

The calculation for converting decimal odds into probability is as follows:

1 ÷ by (the decimal odds + 1) x 100 = probability
Using our decimal odds as an example:
1 ÷ (4 + 1) x 100 = 20%

As you can see, this slight adjustment in the calculation allows you to easily convert both fractional odds and decimal odds into probabilities.

## Using Probability To Find Value

Understanding probability and odds is one of the skills you need to find value selections in whatever sports you want to bet on.

Understanding probability will help you price-up matches, and you can then compare your probability calculations against the odds that are available at any betting sites you use. This comparison will instantly highlight any value in the market.

Let’s use a hypothetical tennis match between Dominic Thiem and Alexander Zverev as an example of how you can use probability to your advantage.

The match is taking place on hard court, and you check out the past head-to-heads between the two players and discover that Zverev leads Thiem 3-2 on this playing surface.

Based solely on those figures, Zverev has a 3 in 5 chance of winning = 60% and Thiem has a 2 in 5 chance of winning = 40%.

You look at how the bookies have priced this match up and see that the bookies make Thiem the 1.80 favourite, while you can get 2.00 about Zverev winning.

Odds of 1.80 = a probability of (1 ÷ 1.80 x 100) = 55.55%
Odds of 2.00 = a probability of (1 ÷ 2.00 x 100) = 50.00%

You’ll notice that the probability of Thiem winning + the probability of Zverev winning does not add up to 100% using the bookmaker’s odds.

This is because bookmakers do not bet to 100%. Instead, they create books that are bigger than 100%. This overround book as it is often referred to allows for market fluctuations due to demand and supply and helps bookmakers maximize profits.

So, when you’re betting, remember that you are betting against the odds to begin with.

Luckily, bookmakers make errors when pricing up plenty of markets and its these errors that professional gamblers exploit.

Using our tennis match between Thiem and Zverev, you can see that head-to-heads suggest Thiem only has a 40% chance of winning, yet the bookmaker’s odds suggest he has a 55% chance of winning.

However, the head-to-heads suggest that Zverev has a 60% chance of winning, yet the bookmaker’s odds suggest he only has a 50% chance of winning.

This means that you’re getting 10% better odds about Zverev winning than you should be, and most professional gamblers would bet on Zverev in this situation.

## How Do You Work Out The Probability Of A Selection Winning?

Our hypothetical tennis match between Thiem and Zverev highlights how you can use probability to find value. But how you arrive at these assumptions is up to you. The probability of the tennis match was based solely on head-to-heads, but you may want to consider other criteria.

You’ll also need to consider current form, player motivations, and be an excellent judge of when players are improving or regressing. How much you factor these criteria into your probability calculations is what will make the difference between you being a successful or unsuccessful gambler.

Of course, probability applies to all sports, including football, golf, cricket, basketball, ice hockey, American football, and many more.

Horse racing probabilities can be even more complex. Ground, track, distance, jockeys, trainers, and draw are all factors that can change the probability of a horse winning a race.

While the idea of using probability to find the value in betting markets is a simple approach, it can take a lot of hard work to work out the correct probability. If you don’t think you have the time or skill set to do it, don’t worry, there are many professional gamblers that are willing to share their advice and tips for an affordable monthly subscription.