Price Elasticity of Demand

Elasticity measures how strongly quantity demanded reacts to a price change - and that reaction varies enormously from good to good.

The law of demand tells you the direction quantity demanded moves when price changes - down when price rises. It doesn’t tell you how much. That’s where price elasticity of demand comes in, and it turns out the size of that reaction varies enormously depending on what’s being sold.

Measuring the size of the reaction

Price elasticity of demand measures how responsive quantity demanded is to a change in price, calculated as the percentage change in quantity demanded divided by the percentage change in price. When that measure is large - meaning quantity demanded changes a lot in response to even a small price change - demand is called elastic. When it’s small - meaning quantity demanded barely budges even with a large price change - demand is called inelastic.

Insulin versus a specific brand of soda

Consider insulin for someone with diabetes. If the price rises, quantity demanded barely changes - people need it regardless of cost, and there's no good substitute, so demand is highly inelastic. Now consider one specific brand of soda among many available brands. If its price rises even a little while competitors' prices stay the same, many buyers switch brands easily, so quantity demanded for that specific brand can fall sharply - demand is elastic. Same basic law of demand, very different sizes of response.

What makes demand more or less elastic

Several factors reliably predict how elastic demand for a good will be. Availability of substitutes matters most: goods with many close substitutes tend to have more elastic demand, since buyers can easily switch away when price rises. Necessity matters too: necessities tend to have more inelastic demand than luxuries, since people need them regardless of price. The share of a buyer’s budget a good takes up matters as well: a small everyday purchase, like a pack of gum, tends to have inelastic demand simply because a price change barely affects a buyer’s overall budget, while a major purchase, like a car, tends to have more elastic demand. Time also matters - demand for most goods becomes more elastic over a longer time horizon, since buyers have more opportunity to find substitutes or adjust habits.

Why elasticity matters for pricing and policy

Assuming raising price always raises total revenue

A common mistake is assuming a higher price always means more total revenue for a seller. Whether that's true depends entirely on elasticity. If demand is elastic, raising price causes quantity demanded to fall by a larger percentage than the price rose, so total revenue actually falls. If demand is inelastic, raising price causes quantity demanded to fall by a smaller percentage than the price rose, so total revenue rises. This is exactly why businesses study elasticity carefully before changing prices, and why governments consider elasticity when deciding what to tax, covered in this module's lesson on taxes and subsidies - taxing an inelastic good raises more predictable revenue with less change in behavior.

A concept that reappears constantly

Elasticity reasoning shows up throughout this curriculum - in why cigarette and alcohol taxes are often targeted at goods with relatively inelastic demand, in why airlines and hotels adjust prices constantly based on how elastic demand is at different times, and in why essential goods like water or basic medicine, covered in other modules, raise such difficult policy questions when prices rise sharply. Understanding elasticity turns the simple law of demand into a genuinely predictive tool.

Key takeaways

Price elasticity of demand measures how much quantity demanded changes in response to a price change.
Elastic demand means quantity demanded is highly responsive to price; inelastic means it's not.
Availability of substitutes, necessity, budget share, and time horizon all affect elasticity.
Whether a price increase raises or lowers total revenue depends on whether demand is elastic or inelastic.
Elasticity helps explain pricing strategy and why certain goods are common targets for taxation.