Elasticity

Definition

Elasticity is the measurement of how sensitively one economic variable responds to changes in another. The most important form is price elasticity of demand: the percentage change in quantity demanded divided by the percentage change in price that caused it. A good is elastic if demand is highly sensitive (|E| > 1) — a 10% price rise leads to more than a 10% drop in quantity. It is inelastic if demand barely changes (|E| < 1) — a 10% price rise leads to less than a 10% drop in quantity.

Elasticity is expressed as a ratio of percentage changes rather than absolute changes, making it unit-free and comparable across goods with very different price levels and quantities. Whether insulin and luxury yachts are elastic or inelastic can be compared on the same scale.

Why it matters

Key takeaways

Elastic demand (|E| > 1): a price increase reduces total revenue, because quantity falls proportionally more than price rises. Sellers should cut prices, not raise them.
Inelastic demand (|E| < 1): a price increase raises total revenue, because quantity falls proportionally less than price rises. Sellers can raise prices without losing proportional revenue.
Tax incidence — who actually bears the cost of a tax — is determined by relative elasticities: the less elastic side pays more, regardless of who writes the check.
Four determinants of demand elasticity: availability of substitutes (most important), necessity vs. luxury status, share of income spent on it, and time horizon.
Demand is more elastic in the long run than the short run — consumers need time to adjust habits, find substitutes, or change capital equipment.
Price elasticity of supply measures producer responsiveness — constrained by production capacity and input availability in the short run, more flexible in the long run.

Elastic versus inelastic — the revenue implications

Read it as: A price increase has two opposing effects on revenue — the price is higher (raises revenue) but quantity sold falls (reduces revenue). Which effect dominates depends on elasticity. For elastic goods (luxury cars, designer clothes), the quantity effect dominates and revenue falls. For inelastic goods (insulin, gasoline, cigarettes), the price effect dominates and revenue rises.

Determinants of elasticity

The four key factors

Availability of substitutes is the most important determinant. Goods with many close substitutes are elastic — if your brand of yogurt raises its price, you switch. Goods with no substitutes are inelastic — if insulin prices rise, diabetics still need insulin.

Necessity vs. luxury: necessities (food, basic transport, medication) are inelastic — people continue buying even at higher prices. Luxuries are elastic — people reduce or eliminate purchases when prices rise.

Budget share: goods that take a large share of income are more elastic — a 10% rise in housing costs forces adjustment. Goods that take a tiny share (salt, toothpicks) are inelastic — the absolute dollar impact is too small to bother substituting.

Time horizon: demand is always more elastic in the long run. When gasoline prices rise, consumers cannot immediately buy a new car — but over several years, they shift toward fuel-efficient vehicles. Short-run elasticity is almost always lower than long-run elasticity.

Tax incidence and elasticity

The burden of a tax is shared between buyers and sellers, and the split is determined by relative elasticities — not by who legally pays the tax. When demand is inelastic and supply is elastic (as with cigarettes), sellers can pass most of the tax to buyers, who cannot easily substitute. When demand is elastic and supply is inelastic (as with luxury yachts under the 1990 US luxury tax), sellers bear most of the burden — consumers switch to alternatives, and sellers’ revenue falls.