How expected goals are calculated
Expected goals, written xG, assign every shot a probability of scoring between 0 and 1 based on the historical conversion rate of similar chances. Models train on large datasets of shots and weigh features such as distance from goal, angle to the posts, body part used, whether the chance came from open play or a set piece, and the type of assist.
A penalty carries an xG around 0.76 because penalties are converted roughly three times in four. A speculative shot from 30 yards might be worth 0.03. Summing the xG of all a team's shots estimates how many goals an average team would have scored from those chances.
Why shot location dominates the heatmap
Plotting xG across the pitch produces a heatmap that peaks sharply inside the six-yard box and the central penalty area, where the angle to goal is widest and the keeper has the least time to react. Value falls off steeply with distance and toward the touchlines, where the scoring angle narrows.
This is why analysts prize chances created in the central zone and treat long-range shots as low-value, even though they look spectacular when they occasionally go in.