In economics, the demand curve for most products and services slopes downward, reflecting an increase in the quantity demanded as the price declines and a decrease as the price rises. The slope of the demand curve reflects how responsive consumer demand is to changes in price. A perfectly vertical demand curve means no change in quantity demanded, regardless of price level. Many economists believe that a perfectly vertical demand curve may exist only in theory.


The slope of a given demand curve reflects what economists call elasticity of demand. Elasticity is a measure of the responsiveness of demand to changes in price or other factors that affect demand. If the quantity demanded shows only slight change in response to a price increase, for example, demand is inelastic. This is often the case with products that are considered necessities, such as food and housing. A perfectly vertical demand curve means that demand is perfectly inelastic.


Perfect inelasticity, as illustrated by a demand curve that runs parallel to the vertical axis, which measures price, is an extreme example of inelastic demand, according to economist Gregory Mankiw of Harvard University. A vertical demand curve means that quantity demanded remains the same, regardless of price. Under perfectly inelastic demand, the quantity demanded would remain the same, even when the price increases by a large amount.

Theories and Speculation

Some economists believe perfectly inelastic demand may exist only in theory, and that the demand for goods and services exhibits varying levels of elasticity depending on the specific good. However, other economists suggest that there may be rare cases of perfectly inelastic demand. For example, consumers may have a perfectly inelastic demand for medicine and the quantity demanded may remain the same at any price, provided the price is not drastically higher than what consumers can afford to pay.


For a demand curve to be vertical, meaning perfectly inelastic demand, the product in question must be a necessity for which no substitute goods exist, according to the economics education website AmosWeb. Consumers must purchase the product regardless of its price because they cannot switch to a similar product.