Equivalence scales allow for the comparison of expenditure (or income) between households of different sizes and compositions, based on theories of the existence of economies of scale and equivalent units of consumption.

According to the theory of economies of scale, an increase in the number of members in a household is not usually accompanied by a proportional increase in spending to maintain the same consumption pattern, since there are shared expenses that are not proportional to the number of members (such as those related to housing, for example).

On the other hand, the theory of equivalent consumption units in households states that the consumption patterns of children are different from those of adults.