A good's price elasticity of demand (${\displaystyle E_{d}}$, PED) is a measure of how sensitive the quantity demanded is to its price. When the price rises, quantity demanded falls for almost any good (law of demand), but it falls more for some than for others. The price elasticity gives the percentage change in quantity demanded when there is a one percent increase in price, holding everything else constant. If the elasticity is −2, that means a one percent price rise leads to a two percent decline in quantity demanded. Other elasticities measure how the quantity demanded changes with other variables (e.g. the income elasticity of demand for consumer income changes).

Price elasticities are negative except in special cases. If a good is said to have an elasticity of 2, it almost always means that the good has an elasticity of −2 according to the formal definition. The phrase "more elastic" means that a good's elasticity has greater magnitude, ignoring the sign. Veblen and Giffen goods are two classes of goods which have positive elasticity, rare exceptions to the law of demand. Demand for a good is said to be inelastic when the elasticity is less than one in absolute value: that is, changes in price have a relatively small effect on the quantity demanded. Demand for a good is said to be elastic when the elasticity is greater than one. A good with an elasticity of −2 has elastic demand because quantity demanded falls twice as much as the price increase; an elasticity of −0.5 has inelastic demand because the change in quantity demanded change is half of the price increase.

At an elasticity of 0 consumption would not change at all, in spite of any price increases.

Revenue is maximized when price is set so that the elasticity is exactly one. The good's elasticity can be used to predict the incidence (or "burden") of a tax on that good. Various research methods are used to determine price elasticity, including test markets, analysis of historical sales data and conjoint analysis.

The variation in demand in response to a variation in price is called price elasticity of demand. It may also be defined as the ratio of the percentage change in quantity demanded to the percentage change in price of particular commodity. The formula for the coefficient of price elasticity of demand for a good is:

where ${\displaystyle P}$ is the initial price of the good demanded, ${\displaystyle \Delta P}$ is how much it changed, ${\displaystyle Q}$ is the initial quantity of the good demanded, and ${\displaystyle \Delta Q}$ is how much it changed. In other words, we can say that the price elasticity of demand is the percentage change in demand for a commodity due to a given percentage change in the price. If the quantity demanded falls 20 tons from an initial 200 tons after the price rises $5 from an initial price of $100, then the quantity demanded has fallen 10% and the price has risen 5%, so the elasticity is (−10%)/(+5%) = −2.

The price elasticity of demand is ordinarily negative because quantity demanded falls when price rises, as described by the "law of demand". Two rare classes of goods which have elasticity greater than 0 (consumers buy more if the price is higher) are Veblen and Giffen goods. Since the price elasticity of demand is negative for the vast majority of goods and services (unlike most other elasticities, which take both positive and negative values depending on the good), economists often leave off the word "negative" or the minus sign and refer to the price elasticity of demand as a positive value (i.e., in absolute value terms). They will say "Yachts have an elasticity of two" meaning the elasticity is −2. This is a common source of confusion for students.

Depending on its elasticity, a good is said to have elastic demand (> 1), inelastic demand (< 1), or unitary elastic demand (= 1). If demand is elastic, the quantity demanded is very sensitive to price, e.g. when a 1% rise in price generates a 10% decrease in quantity. If demand is inelastic, the good's demand is relatively insensitive to price, with quantity changing less than price. If demand is unitary elastic, the quantity falls by exactly the percentage that the price rises. Two important special cases are perfectly elastic demand (= ∞), where even a small rise in price reduces the quantity demanded to zero; and perfectly inelastic demand (= 0), where a rise in price leaves the quantity unchanged. The above measure of elasticity is sometimes referred to as the own-price elasticity of demand for a good, i.e., the elasticity of demand with respect to the good's own price, in order to distinguish it from the elasticity of demand for that good with respect to the change in the price of some other good, i.e., an independent, complementary, or substitute good. That two-good type of elasticity is called a cross-price elasticity of demand. If a 1% rise in the price of gasoline causes a 0.5% fall in the quantity of cars demanded, the cross-price elasticity is ${\displaystyle E_{cg}^{d}=(-0.5\%)/(+1\%)=-0.5.}$