What are Expected Goals (xG)?

What are Expected Goals (xG)?2020-05-25T12:49:28+00:00

What are Expected Goals (xG)?

Expected goals calculates how many goals a team should have scored based on the quality of the chances created.
By comparing actual goals to expected goals we can see whether a team has been lucky or unlucky in any given match. For example, the champions league final between Liverpool and Spurs had xG of 1.3 for Liverpool and 1.2 for Spurs. As such, a 1-1 draw would have been a fair outcome.  It can be concluded that Liverpool were lucky to win so comfortably.

The process

The process for calculating an expected goal from any given chance was done by Opta.  They reviewed hundreds of thousands of historical shots to then work out the percentage chance of a shot being scored from any particular situation.

Close range vs long range shots

In simple terms, shots from close range and in front of the goal tend to have the highest percentage chance of being scored. The best example is a penalty which is on average scored 76% of the time. That means that if a team were awarded one penalty and no other shots, their xG would be 0.76.

Long shots from outside the area have a lot smaller percentage chance of conversion and usually fall in the 1% chance of a goal bucket.

Using the same example, a team with one penalty versus a team who has taken 20 long range shots should on average win over the long term i.e. xG of 0.76 versus 0.2 (roughly 1-0). This is a good example of how shots can provide a misleading impression on how a game is going.

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Expected Goals (xG) to predict future matches

The main strength of Expected Goals (xG) over shots is there is a lot lower variability (small standard deviation) in the xG results.  Teams are fairly consistent in the xG created and conceded while shots can vary significantly game to game. Due to being more consistent, xG are a more useful tool in predicting future results.

Predicting match results

This site uses Expected Goals and multiple other factors (142 in total!) to predict the results of future matches.

The predictions are then converted to a percentage chance of winning.  These percentages are then converted into odds to see if there is value in the available match odds.

Reliable predictor

Lower standard deviation

History does not always predict the future

As with everything, Expected Goals comes with a number of limitations.

Expected versus actual

While Expected Goals are a good predictor of a result, actual goals do not always align to the expected goals. The Champions League final being a prime example. However, over the long run, the number of goals a team scores tends to revert to the Expected Goals. If a team is scoring a lot more than the Expected Goals, there is a good chance they are overachieving.  Therefore they could be a good option to lay in future games, especially if they come up against the opposite situation – a team who is creating lots of good chances but not putting them away.  For example, laying Man U towards the end of their good run of form (after OGS was appointed).

Player skill levels

The other main limitation is Expected Goals does not take account of the player taking a shot.  As any football fan will know, all players are not created equal.  For example, Vincent Kompany’s goal against Leicester which turned out to be crucial in the 2019 EPL title race only had a 1% chance of going in.  As this % chance is an average of all types of players, the true odds were probably a lot lower. If Kevin De Bruyne was taking the shot it would still be classed as 1% but the true chance would probably be closer to 5%.  Again, over the long run Expected Goals should revert to actual goals but there can be short term fluctuations.

Watch the match

Trying to predict the future purely based off history is not going to provide perfect results every time.  While xG provides a good indication of which team should be on top, I advise people to watch the match before putting on a trade. This provides crucial insights into whether the team we expect to win are actually playing like their form suggests.
The below sections provides some further analysis and answers a number of common questions.

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Football Expected Goals: Frequently Asked Questions

Why don’t you just use shots on goal?2019-06-28T16:46:30+00:00

Team A have taken 10 long shots. Team B have had 4 shots from close range 1-on-1 situations with the keeper. In pure shots terms the stats suggest that 10 shots are better than 4 (net +6) but in reality team B had a lot higher percentage chance of winning the game.

An analysis of 5469 games highlighted that shots only predict the home result correctly 54% of the time and away result 41%. This is significantly lower than Shots on Target and Expected Goals.

Is shots on target a fairer prediction?2019-06-28T16:53:23+00:00

Yes it is. Shots on target gives a better prediction than total shots.
Shots on target predicts the home team winner correctly 68% of time and the away team 53%.

So how much better is expected goals?2019-08-11T05:20:23+00:00

Expected goals predicts the correct home team result 66% of the time and away results 58% of the time. This is slightly better than shots on target on the away results and slightly worse on the home results.
For the astute among you, it can be seen that shots on target and expected goals provide similar results in predicting the outcome of a match.

Standard Deviation of Results

This is correct in a sense, but the above calculations are based on the shots and expected goals that occurred within the individual match i.e. predicting the outcome after the number of shots and expected goals are known. The biggest difference is that teams have a lot higher standard deviation in shots on target versus what they do in expected goals (3.6 vs 1.2). For anyone unfamiliar with standard deviation, it just means that the lower it is the smaller the spread of outcomes from the average. For example, a team over 5 games may have shots on target of 2,6,7,3,2. This shows a large variability in outcomes away from the average of 4. Conversely, expected goals will often look like 1.5, 2.8, 2.5, 2.2, 1.4 which shows a lot more consistency and translates to the real life amount of goals that should have been scored.
Due to the lower variability in outcomes, expected goals is a far superior tool for predicting the expected goals a team will score and concede in any future match.

Do shots and expected goals predict draws?2019-06-28T16:54:30+00:00

Using shots on target and expected goals has a very low success rate in predicting draws. This is largely due to the low scoring nature of football, and one lucky goal quickly turns a draw into a win or defeat. For anyone who claims they can predict draws, I will be very skeptical as I have not come across any good models or statistical analysis that provides long term profitability.

How does ‘Football Xg’ predict the outcome of a match?2019-06-28T16:51:42+00:00

By looking at expected goals of historical matches, current form and strength of opposition and running a number of statistical scenarios, the expected net goals is calculated which is then converted into a percentage chance of winning. Once we have the percentage chance of winning, we can easily convert that to odds to see if there is value in the price. A prediction can then be made to either back or lay any given team.

The bookies have hundreds of people doing this, how can your Xg predictions be any better?2019-06-28T16:55:13+00:00

You are correct that the bookies can predict the expected outcome of any match with a great amount of accuracy. The main difference is that the average punter is not always rational, so the bookies will adjust their book to where the demand is coming from rather than the actual chances of a team winning the match. That often means there is value in going against the crowd if the odds are an unfair reflection of their actual chance of winning. For example, last season Man U went on a good run when Solskjaer was appointed. The stats for their games showed they were far overachieving based on their actual chances. There was great value in laying Man U and as most people are aware, the wheels soon feel off that bandwagon.

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