In economics, an Engel curve shows how the demand for a good or service changes as the consumers income level changes. It is named after the 19th century German statistician Ernst Engel.

Graphically, the Engel curve is represented in the first-quadrant of the cartesian coordinate system. Income is shown on the X-axis and the quantity demanded for the selected good or service is shown on the Y-axis.

For normal goods, the Engel curve has a positive slope. That is, as income increases, the quantity demanded increases. For inferior goods, the Engel curve has a negative slope. That means that as the consumer has more income, they will stop buying the inferior goods because they are able to purchase better goods.

zh:恩格尔系数