Engel's law is an economic relationship proposed by the statistician Ernst Engel in 1857. Even though Engel's law was proposed roughly 160 years ago, it holds relevance today in the context of poverty, especially the reduction of poverty. For instance, the lines and rates for national poverty are often determined by the food share of household expenditure.

One definition of Engel's law by the Merriam-Webster dictionary is as follows: "a generalization in economics: such as family income increases, the percentage spent for food decreases, that spent for clothing, rent, heat, and light remains the same, while that spent for education, health, and recreation increases"

A quotation of Engel himself from 1932 reveals the same relationship between income and percentage of income spent on food, but also indicates the application of Engel's Law in measuring standard of living: "The poorer is a family, the greater is the proportion of the total outgo [family expenditures] which must be used for food. ...The proportion of the outgo used for food, other things being equal is the best measure of the material standard of living of a population."