Elasticity

The law of demand tells us that a higher price reduces quantity demanded, and the law of supply tells us that a higher price raises quantity supplied. But by how much? The answer matters enormously to businesses setting prices and to governments designing taxes and policies. Elasticity is the tool economists use to measure responsiveness, and this chapter develops its main forms.

Price Elasticity of Demand

The price elasticity of demand measures how responsive the quantity demanded is to a change in price. It is calculated as the percentage change in quantity demanded divided by the percentage change in price:

E_d = (% change in quantity demanded) / (% change in price)

Because the two move in opposite directions, the result is technically negative, but economists conventionally use its absolute value. Interpreting that number:

• Elastic demand (E_d > 1) — Quantity is very responsive; the percentage change in quantity exceeds the percentage change in price. Typical of luxuries and goods with many substitutes.
• Inelastic demand (E_d < 1) — Quantity is relatively unresponsive. Typical of necessities and goods with few substitutes.
• Unit elastic (E_d = 1) — The two percentage changes are equal.

Determinants of Price Elasticity

Demand tends to be more elastic when a good has many close substitutes, when it takes a large share of the buyer's budget, when it is a luxury rather than a necessity, and when buyers have more time to adjust. Each of these gives buyers more room to respond to a price change.

Elasticity and Total Revenue

The most important business application is the total-revenue test. Total revenue is price times quantity, and how it responds to a price change depends on elasticity:

• If demand is elastic, price and total revenue move in opposite directions: a price cut raises revenue, because the gain in quantity outweighs the lower price.
• If demand is inelastic, price and total revenue move in the same direction: a price increase raises revenue, because quantity falls only modestly.
• If demand is unit elastic, total revenue is unchanged.

This relationship explains why a bumper harvest can actually lower farmers' incomes (food demand is inelastic) and why firms with pricing power study elasticity so carefully.

Price Elasticity of Supply

The price elasticity of supply measures how responsive quantity supplied is to a change in price, calculated the same way. Its single most important determinant is time. In the immediate market period, output is essentially fixed and supply is highly inelastic. In the short run, firms can vary some inputs, making supply more elastic. In the long run, firms can fully adjust capacity and new firms can enter, making supply the most elastic of all. The more time producers have to respond, the more elastic supply becomes.

Other Elasticity Measures

Two further measures extend the idea to factors other than a good's own price.

The income elasticity of demand measures how quantity demanded responds to a change in consumer income. It is positive for normal goods and negative for inferior goods, and its size distinguishes necessities (small positive values) from luxuries (large positive values).

The cross-price elasticity of demand measures how the quantity demanded of one good responds to a change in the price of another. A positive value indicates substitutes (when the price of one rises, demand for the other rises); a negative value indicates complements (when the price of one rises, demand for the other falls). A value near zero indicates unrelated goods.

Why Elasticity Matters

Elasticity turns the qualitative laws of demand and supply into quantitative predictions. It tells a business whether a price cut will raise or lower revenue, tells a government who really bears the burden of a tax (the more inelastic side of the market), and tells a policymaker how a price control or subsidy will play out. Wherever we need to know not just the direction but the magnitude of a market response, elasticity is the essential tool.

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Next chapter: having measured how buyers respond to prices, we look behind the demand curve itself to the consumer choices that generate it — utility maximization.

Further Listening & Reading

• 🎥 Elasticity of Demand (MRU) — what elasticity means and what determines it.
• 🎥 Calculating the Elasticity of Demand (MRU) — working through the midpoint formula.
• 🎥 Elasticity of Supply: Why Housing Is Unaffordable (MRU) — supply elasticity applied to a real policy problem.