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 overperformed or underperformed in any given match. For example, a match with xG of 1.3 for the home team and 1.2 for the away team would suggest a 1-1 result is fair outcome.  A 2-0 result would suggest outperformance by the home team and underperformance of the away team.

The process

The process for calculating an expected goal from any given chance was originally created 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.  There are now a range of different models that are getting more and more advanced (taking into account the location of the shot, the position of defenders and goalkeeper, height of shot).

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 have an xG close to 0.01 i.e. 1% chance of a goal.

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.

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 (visit the xG League tables section to see how xG compares to actuals). 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.  

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, a defender , who has never scored, taking a shot from the edge of the area will be classed as having 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, for example, Kevin De Bruyne was taking the shot it would still be classed as 1% but the true chance would probably be likely a few % higher.  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, we strongly 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.