Expected goals (xG) explained

Goals in football are rare and shaped by chance. A deflected shot, a strike off the post, a great save from the goalkeeper – and the result looks completely different. Expected goals try to strip out this element of chance. Instead of counting goals, they rate every chance.

How xG is calculated

Every shot is given a value between 0 and 1. It indicates how often a shot from that situation leads to a goal on average. The basis is hundreds of thousands of shots from many leagues. Key factors are the distance from goal, the angle, the body part (foot or head), the type of assist and whether it was a counter-attack or a set piece.

Depending on the model, a penalty is worth around 0.76 to 0.79 xG. A shot from 25 metres often yields only 0.03 to 0.05 xG. A big chance from six metres, unmarked in front of goal, can be worth 0.5 xG or more. The sum of all shots gives a team's xG for the match.

What xG is useful for in betting

xG is a good gauge of a team's actual strength, better than the table after just a few matchdays. Using expected goals for and against, you can build predictions, for example via the Poisson distribution for correct scores, over/under or both teams to score.

Limits

• Different providers use different models. The figures can differ by 10 to 20 per cent.
• xG only rates shots. Dangerous situations without a shot are missing.
• The score matters: a team that leads early sits back and allows more chances – its xG record then looks worse than the team really is.
• Bookmakers know xG just as well. The fact that a team is "underperforming its xG" is usually long since priced in in the major leagues.