Elasticity
Summary :What Price Elasticity of Demand Measures
Price elasticity of demand measures how sensitive the quantity consumers purchase is to a change in price, expressed as the percentage change in quantity demanded divided by the percentage change in price. This single number lets economists compare responsiveness across completely different goods on a common scale, something raw price and quantity figures cannot do on their own. A grocery chain raising the price of a store-brand cereal by ten percent and watching sales fall by thirty percent is dealing with a very different situation than a utility company raising electricity rates by ten percent and watching usage fall by only two percent, and elasticity is the tool that quantifies exactly how different those two situations are. Because demand curves slope downward, a rise in price almost always produces a fall in quantity demanded, so elasticity of demand is technically a negative number, though economists commonly report its absolute value for simplicity.
Calculating Elasticity: The Midpoint Method
Economists generally calculate percentage changes for elasticity using the midpoint method, dividing the change in a value by the average of the starting and ending values rather than by the starting value alone. This avoids a problem that would otherwise arise: calculating the elasticity of a price increase from ten dollars to twelve dollars would normally give a different percentage than calculating the elasticity of a price decrease from twelve dollars back to ten dollars, even though it is the same two points on the same curve. Suppose a bookstore lowers the price of a bestselling novel from twenty dollars to sixteen dollars, and weekly sales rise from two hundred copies to three hundred copies. Using the midpoint method, the percentage change in quantity is the change of one hundred divided by the average quantity of two hundred fifty, or forty percent, and the percentage change in price is the change of four dollars divided by the average price of eighteen dollars, or about twenty-two percent. Dividing forty by twenty-two gives an elasticity of roughly 1.8, meaning demand for the novel is elastic at that price range.
Elastic, Inelastic, and Unitary Demand
Demand is classified as elastic when the elasticity value is greater than one, meaning quantity demanded changes proportionally more than price does. Demand is inelastic when the value is less than one, meaning quantity demanded changes proportionally less than price. Demand is unitary when the value equals exactly one, meaning quantity and price change by the same percentage. Two extreme cases bound this spectrum: perfectly inelastic demand, where quantity demanded does not change at all regardless of price, produces a vertical demand curve, while perfectly elastic demand, where any price increase causes quantity demanded to fall to zero, produces a horizontal demand curve. Very few real goods sit at these extremes, but insulin for a diabetic patient who has no substitute comes close to perfectly inelastic, while a single farmer's wheat sold into a large global commodity market, where raising the price even slightly would send every buyer elsewhere, comes close to perfectly elastic.
What Determines Whether Demand Is Elastic
Several factors consistently predict whether a good's demand will be elastic or inelastic. The availability of close substitutes matters most: a good with many substitutes, such as one particular brand of laundry detergent among a dozen similar options on the shelf, tends to have elastic demand, because consumers can easily switch away if its price rises. Whether a good is a necessity or a luxury also matters; insulin and basic prescription eyewear tend toward inelastic demand because consumers need them regardless of cost, while a vacation cruise tends toward elastic demand because consumers can simply postpone or skip it. The share of a consumer's budget a good represents plays a role too, since a large purchase like a car draws far more attention to a price change than a small one like a pack of gum. Finally, time horizon matters: demand for gasoline is relatively inelastic in the short run, since drivers cannot immediately change their commute, but becomes considerably more elastic over several years as people can choose more fuel-efficient vehicles or relocate closer to work.
Elasticity and Total Revenue
The relationship between elasticity and total revenue, price multiplied by quantity sold, gives businesses a practical reason to care about elasticity when setting prices. When demand is elastic, raising price reduces total revenue, because the resulting drop in quantity outweighs the higher price per unit; lowering price in this range increases total revenue instead. When demand is inelastic, the opposite holds: raising price increases total revenue, because quantity falls by proportionally less than the price rises. Consider a parking garage near a stadium that currently charges fifteen dollars per game and fills every space; if demand for that parking is inelastic because nearby alternatives are scarce on game day, raising the price to twenty dollars would likely increase total revenue even if a handful of drivers park farther away instead. A movie theater deciding whether to raise ticket prices faces the opposite risk if moviegoers have easy access to streaming alternatives, since even a modest price increase in a market with elastic demand could reduce overall revenue rather than grow it.
Elasticity Beyond Price: Income and Cross-Price Elasticity
Elasticity is not limited to measuring responsiveness to a good's own price. Income elasticity of demand measures how quantity demanded responds to a change in consumer income, calculated as the percentage change in quantity divided by the percentage change in income. A positive income elasticity marks a normal good, one people buy more of as income rises, such as dining out at restaurants, while a negative income elasticity marks an inferior good, one people buy less of as income rises, such as instant ramen noodles, which many consumers replace with pricier alternatives once they can afford to. Cross-price elasticity of demand measures how the quantity demanded of one good responds to a change in the price of a different good. A positive cross-price elasticity indicates substitutes, such as butter and margarine, where a price increase in one raises demand for the other, while a negative cross-price elasticity indicates complements, such as printers and ink cartridges, where a price increase in one reduces demand for the other.
Quick Revision Summary
Price elasticity of demand measures the percentage change in quantity demanded relative to the percentage change in price, typically calculated using the midpoint method to avoid inconsistent results between price increases and decreases. Demand is elastic when this value exceeds one, inelastic when it is below one, and unitary when it equals one, with perfectly inelastic and perfectly elastic demand marking the two theoretical extremes. Elasticity depends on the availability of substitutes, whether a good is a necessity or a luxury, the share of a budget it represents, and the time horizon available for consumers to adjust. The relationship between elasticity and total revenue guides pricing decisions: raising price increases revenue when demand is inelastic but decreases revenue when demand is elastic. Finally, income elasticity distinguishes normal goods from inferior goods, while cross-price elasticity distinguishes substitutes from complements, extending the same responsiveness logic beyond a good's own price.