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If you’ve spent any time looking at betting odds, you’ll probably have a decent feel for what different prices mean. A 1.50 shot is expected to have a much better chance than something priced at 5.00.
What might be less obvious is that every set of odds can also be expressed as a percentage. This is known as implied probability, and it’s a useful way of looking at what the bookmaker’s price is actually suggesting.
At odds of 2.00, for example, the implied probability is 50%. At 4.00, it drops to 25%. The calculation itself is very easy, but understanding it becomes particularly useful when you start thinking about whether the odds on offer represent good value.
I’ll go through how it works, how to calculate implied probability from different odds formats, and why the percentages across a bookmaker’s market don’t usually add up to a nice, neat 100%.
What Is Implied Probability in Betting?
The easiest way to think about implied probability is as the percentage sitting behind the odds. It gives you another way of judging a price, particularly if percentages feel more intuitive than decimal or fractional odds.
Saying a team is 2.50 to win is one thing; saying the price represents a 40% chance of winning can make it much easier to put that price into context.
There is one important distinction though. Implied probability doesn’t tell you the actual chance of something happening. It only tells you the probability represented by the odds.
That difference becomes important later on when we look at whether a price might actually be worth backing.
How to Calculate Implied Probability From Decimal Odds
If you’re using decimal odds, working out the implied probability only takes one calculation:
Implied probability = (1 ÷ decimal odds) × 100
Let’s say the odds are 2.50.
1 ÷ 2.50 = 0.40
Multiply that by 100 and you get 40%.
So, odds of 2.50 are effectively saying that the outcome has a 40% chance of happening.
Here’s another example using odds of 1.80.
1 ÷ 1.80 = 0.5556
Multiply by 100 and the implied probability is 55.56%.
You don’t need to sit there doing the maths every time you place a bet, but it’s useful to understand where the percentage comes from. After a while, you’ll probably start to recognise the rough percentages behind common prices anyway.
Implied Probability Examples
To give you a better idea of how decimal odds translate into percentages, here are a few common prices:
| Decimal Odds | Implied Probability |
| 1.20 | 83.33% |
| 1.50 | 66.67% |
| 1.80 | 55.56% |
| 2.00 | 50% |
| 2.50 | 40% |
| 3.00 | 33.33% |
| 4.00 | 25% |
| 5.00 | 20% |
| 10.00 | 10% |
How to Convert Fractional Odds Into Implied Probability
Fractional odds need a slightly different calculation, although it’s still easy enough once you know the formula:
Implied probability = denominator ÷ (numerator + denominator) × 100
For example, let’s take odds of 2/1:
1 ÷ (2 + 1) = 0.3333
Multiply that by 100 and you get an implied probability of 33.33%.
At 4/1, the calculation would be:
1 ÷ (4 + 1) = 0.20
So the implied probability is 20%.
If you don’t fancy remembering another formula, you can always convert fractional odds into decimal odds first and use the calculation from earlier.
Just divide the first number by the second and add 1. For example, 5/2 becomes 3.50 in decimal odds, which gives an implied probability of 28.57%.
Why Implied Probability Matters When Betting
Knowing the implied probability behind the odds can help you look at a betting market in a slightly different way.
Let’s say Arsenal are priced at 1.50 to win a match. You might look at that price and think it seems short, but converting it into a percentage tells you something more useful: those odds represent a 66.67% chance of them winning.
Looking at the percentage can make a price easier to put into context. It can also make it easier to compare different prices and understand what a small movement in the odds actually means.
That relationship between odds and probability is useful on its own, but it becomes even more important when you start looking at whether the price available represents value.
Implied Probability and Value Betting
Implied probability becomes particularly useful when you’re trying to work out whether a bet offers value.
Imagine a bookmaker is offering odds of 2.50 on a team to win. As we’ve already seen, that works out at an implied probability of 40%.
After doing your own research, you might estimate that the team’s actual chance of winning is closer to 45%. If your estimate is accurate, odds of 2.50 would be bigger than the price you’d expect for a 45% chance, which suggests there could be some value in the bet.
The difficult part, of course, is deciding what the true probability should be. Anyone can convert a set of odds into a percentage. Working out whether that percentage is too high or too low takes a lot more work.
This is the basic idea behind value betting. You’re looking for occasions where you believe the odds available underestimate the chance of an outcome happening. Over the long term, consistently finding those discrepancies is far more important than whether any individual bet wins or loses.
Why Bookmaker Probabilities Add Up to More Than 100%
If you convert every set of odds in a betting market into implied probability and add them together, you might expect the total to come to 100%. With bookmaker odds, you’ll usually find it’s higher.
For example, imagine the odds on a football match are:
- Home win: 2.00 – 50%
- Draw: 3.50 – 28.57%
- Away win: 4.00 – 25%
Add those percentages together and you get 103.57%.
The extra 3.57% is known as the bookmaker’s overround. It’s the margin built into the market and is one of the ways bookmakers give themselves an advantage over bettors.
You’ll also find that the size of the overround varies. Some markets are priced much more tightly than others, while less competitive or more specialised markets can have a considerably bigger margin.
This is worth remembering when looking at implied probability, because the percentages you calculate directly from bookmaker odds aren’t the same as a set of fair probabilities that add up to exactly 100%.
How to Remove the Bookmaker Margin
If you want a better idea of the probabilities behind a market without the bookmaker’s margin included, you can remove the overround.
Using our example from above, the three implied probabilities came to 103.57%.
One straightforward way to remove the margin is to divide each percentage by the total of 103.57%.
For the home win, that would be:
50 ÷ 103.57 × 100 = 48.28%
Doing the same for all three outcomes gives us:
| Outcome | Implied Probability | Without Margin |
| Home | 50% | 48.28% |
| Draw | 28.57% | 27.59% |
| Away | 25% | 24.14% |
Allowing for rounding, those adjusted figures now add up to 100%.
This gives you a rough idea of the probabilities once the bookmaker’s margin has been stripped out. It’s useful for comparing prices and getting a clearer picture of how the market is priced, although it does assume the margin is spread proportionally across each outcome.
In reality, bookmakers don’t necessarily apply their margin evenly, so these adjusted percentages shouldn’t automatically be treated as the true chances of each result.
Implied Probability on Betting Exchanges
Implied probability works in exactly the same way when you’re using a betting exchange such as Betfair. The odds can be converted into percentages using the same calculation we’ve already covered.
The main difference is how the market is formed. On an exchange, you’re betting against other users rather than taking a price directly from a bookmaker, with exchanges typically making their money through commission or other charges rather than building a bookmaker margin directly into the odds.
You can still use implied probability to put exchange prices into context in exactly the same way. If you’re comparing prices with a bookmaker, though, remember to account for any commission you would pay on the exchange before deciding which offers better value.
For anyone trading on betting exchanges, percentages can also be a useful way of thinking about price movements. If the odds shorten from 3.00 to 2.50, for example, the implied probability has moved from 33.33% to 40%.
Final Thoughts
Implied probability sounds more complicated than it really is. Once you know the basic calculation, it’s just another way of looking at betting odds and understanding what a particular price represents.
Where it becomes more useful is when you start comparing the percentage behind the odds with your own assessment of an event. The maths is the easy part; deciding whether the market has priced that outcome accurately is where the real judgement comes in.
You certainly don’t need to convert every price you see into a percentage. Even having a rough understanding of the relationship between odds and probability can help you look at betting markets with a more informed eye.
