Price Elasticity of Demand

Core Idea

Price elasticity of demand measures how responsively quantity demanded changes to a price change, expressed as the percentage change in quantity divided by the percentage change in price. Elastic demand (|ε| > 1) means consumers are very sensitive to price; inelastic demand (|ε| < 1) means quantity changes little when price changes. This concept is fundamental for understanding how firms set prices and how price changes affect total revenue.

How It's Best Learned

Start with the midpoint formula for calculating elasticity on specific demand curves. Then examine real-world goods (e.g., salt vs. restaurant meals) and predict elasticity before calculating it. Use graphs to visualize why steeper demand curves are more inelastic.

Common Misconceptions

• Confusing the slope of the demand curve with its elasticity—a flatter curve is not necessarily more elastic.
• Assuming elasticity is constant along an entire demand curve—elasticity varies at each point.
• Forgetting that elasticity is the responsiveness of quantity, not just whether it changes.

Explainer

Supply and demand tells you that quantity demanded falls when price rises — but it doesn't tell you by *how much*. Price elasticity of demand fills that gap. It measures the percentage change in quantity demanded for a one-percent change in price: ε = (%ΔQ) / (%ΔP). Because price and quantity move in opposite directions along a demand curve, this number is always negative — but economists typically report its absolute value. An elasticity of 2 means a 1% price increase causes a 2% drop in quantity demanded. An elasticity of 0.3 means quantity barely budges.

The key threshold is |ε| = 1. When |ε| > 1, demand is elastic — consumers are very responsive, which usually happens for goods with many substitutes, luxury items, or goods that represent a large share of the budget. When |ε| < 1, demand is inelastic — consumers are relatively unresponsive, typical of necessities like insulin, gasoline, or salt. The words "elastic" and "inelastic" are not vague descriptions; they have precise meaning in relation to this threshold.

The most important application is the relationship between elasticity and total revenue (price × quantity). If demand is elastic, a price increase reduces quantity so sharply that revenue falls — the quantity effect dominates. If demand is inelastic, a price increase causes only a small drop in quantity, so revenue rises — the price effect dominates. This is why drug companies can charge high prices for patented medicines (inelastic demand) but airlines hold sales aggressively (elastic demand in leisure markets). Elasticity translates the abstract demand curve into a practical pricing decision.

Elasticity varies along a linear demand curve, which is the most common student pitfall. At the high-price, low-quantity end of a linear demand curve, the percentage change in quantity for a given absolute change is large, so demand is elastic there. At the low-price, high-quantity end, the same absolute change represents a small percentage shift, so demand is inelastic. The slope of the curve is constant, but the elasticity is not — they are related but distinct measures. Always calculate elasticity at a specific point; don't assume it describes the whole curve. Determinants that make demand more elastic include: availability of substitutes, longer time horizons, higher budget share, and the narrowness with which you define the market.