"Expected Goals" (xG) is a metric in soccer that indicates the probability of a particular shot resulting in a goal. This probability is calculated based on a variety of factors derived from historical data.
We took a closer look and provide here an easy-to-understand overview of the mysteries behind xG values.
xG values are typically calculated for individual shots, but also for players, teams, or entire matches. For the calculation to be as accurate as possible, matches must be thoroughly analyzed, and a wide range of data needs to be collected.
Key Factors in the Calculation of xG
- Shot Position
The closer the shot is to the goal, the higher the probability that it will result in a goal. Shots from inside the penalty area, for example, have a higher xG value than long-range shots.
- Angle to the Goal
Shots from tight angles are more difficult to convert and therefore have a lower xG value.
- Type of Shot / Shooting Technique
Headers, volleys, and shots with the foot all have different success probabilities. For example, headers are generally harder to place and often have a lower xG value.
- Shot Situation
A shot following a precise pass, such as a cross, typically has a higher xG value than a shot taken in a chaotic situation.
- Type of Assist
A pass splitting the defense (such as a through ball) often has a higher xG likelihood than a cross from the wing.
- Type of Attack
Whether the attack comes from a quick counter or after a set-piece like a corner or free-kick. Set-pieces often have lower xG values.
- Defensive Pressure
If a player is heavily pressured during the shot, the likelihood of scoring decreases, lowering the xG value.
- Goalkeeper Position
If the goalkeeper comes far off the line, the probability of a goal changes, depending on the match situation.
Probability Calculation
The exact weighting of these factors depends on the model used, as different analytics firms or soccer statistics platforms use different calculation methods. Ultimately, the combination of these factors generates a statistical probability, expressed as how likely it is that the shot will result in a goal (on a scale of 0 to 1). A value of 0.1 (or 10%) means, for instance, that, on average, 1 out of 10 similar shots resulted in a goal in the past.
This metric helps to assess the quality of goal-scoring opportunities and goes beyond merely counting shots on goal, as it considers the context and difficulty of the chances.
Further factors to consider when calculating the xG value include:
- Shots with negligible xG values: Shots that have a statistically low chance of becoming a goal, such as from extreme distances or unfavorable angles, may have very low xG values close to 0.
- Summation over the entire game: The xG value can be calculated for a single half, the entire game, or for specific phases of the match.
- Comparison between teams: By calculating the xG values for both teams, one can better assess which team had more high-quality chances, regardless of the actual outcome. This is often used to evaluate a team’s performance more objectively.
An xG value of 2.5, for example, means that, based on the quality of its chances, the team should have scored around 2 to 3 goals on average. If they only scored 1 goal, it could be said that they "underperformed," while the opposing team may have "overperformed" if they scored more goals than their xG value suggested. Put simply: a team might either capitalize on its chances effectively or require too many attempts from good positions to score.
Data Collected – Now Comes the xG Calculation
Summary: A team’s "Expected Goals" (xG) value is the sum of the expected goals values for each shot taken by the team during the match. Each individual shot is evaluated for its probability of resulting in a goal. The xG values for all shots are then added together to determine the team’s overall xG value.
Example Calculation of "Expected Goals"
- Shot 1
A shot from 10 meters out in a central position has an xG probability of 0.3 (i.e., a 30% chance of resulting in a goal).
- Shot 2
A header from a cross, near the edge of the penalty area, has an xG probability of 0.1 (i.e., 10%).
- Shot 3
A shot from 25 meters out, with no pressure, has an xG probability of 0.05 (i.e., 5%).
The team’s total xG value would be the sum of these individual probabilities:
0.3 (Shot 1) + 0.1 (Shot 2) + 0.05 (Shot 3) = 0.45 xG
This means that, based on the quality of the chances in this example, the team theoretically should have scored 0.45 goals. Another team that took 5 shots might have a higher or lower xG value, depending on the quality of their chances.