How the calculator rates a shot
Every shot gets the probability of a goal from a logistic model of its distance to the centre of the goal and the angle between the posts as the shooter sees them. The coefficients are fitted to public average xG values by zone: tap-ins, the centre and the sides of the six-yard box, the penalty spot, the edge of the box, long range and tight angles by the goal line.
logit(xG) = -2.785 + 1.479 × angle − 0.096 × distance + 0.374 × ln(distance)
The angle is in radians and the distance in metres. A penalty gets a fixed xG of 0.79, the value Opta's model gives every penalty. A team's xG is the sum of its shots.
For the result, every shot is treated as a separate attempt that goes in with the probability of its xG. The calculator adds up every combination of goals exactly, without simulation, and compares the two teams. Expected points are 3 × the chance to win plus the chance of a draw.
What xG values look like
- Tap-in from a metre out: 0.73
- Central shot from the edge of the six-yard box: 0.28
- Central shot from the penalty spot in open play: 0.12
- Central shot from the edge of the penalty area: 0.06
- Shot from the corner of the penalty area: 0.02
- Central shot from 30 metres: 0.02
- Shot from a tight angle by the goal line: 0.07
- Penalty: 0.79
Limits of the model
The xG models of data providers also know the body part, the type of pass before the shot, the pressure from defenders and the goalkeeper's position, and they are trained on large sets of past shots. This calculator uses only the location, so its numbers are a rough guide and will differ from the xG in match statistics. Use it to see how the location changes a chance, not to re-rate a real match to the hundredth.
The real xG of every finished match is in its match statistics, and each league has a season xG table: all competitions. To turn a team's average xG into the probability of every score, use the Football Poisson calculator.
Frequently asked questions
How is xG calculated for a shot?
A model gives each shot the probability that a shot from that situation is scored, learned from past shots. This calculator uses the distance and angle to goal; providers' models add the body part, the type of pass and the position of defenders.
How do you calculate a team's xG for a match?
Add up the xG of all its shots. Five shots of 0.1 and one penalty give 0.5 + 0.79 = 1.29 xG.
Why is the xG here different from match statistics?
The calculator knows only where a shot was taken. A provider's model also accounts for headers, the pass before the shot and defenders, so the same shot can get a different number.
How do you get a win probability from xG?
Treat each shot as an attempt that goes in with the probability of its xG, add up every combination of goals for both teams and compare them. The calculator does this exactly and also gives each team's expected points.
More on the blog
The xG.Football model calculates result, totals and both-teams-to-score probabilities for the day's matches — handy to compare with the odds.
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