Elasticity: How Sensitive Are Buyers and Sellers?
Elasticity tells you whether a price change will make buyers flee or barely flinch — and it determines who really pays a tax.
Price Elasticity of Demand
Price elasticity of demand (PED) measures how responsive the quantity demanded of a good is to a change in its price. The formula is: PED = (% change in quantity demanded) ÷ (% change in price). Because demand curves slope downward, PED is always negative — economists typically report the absolute value.
When |PED| > 1, demand is elastic: a 1% price rise causes more than a 1% fall in quantity demanded. Buyers are sensitive to price. Airline tickets are a good example — a 10% fare increase on a competitive route can cause a large drop in bookings as travelers switch airlines, choose trains, or cancel trips. When |PED| < 1, demand is inelastic: buyers barely respond to price changes. Insulin is the textbook case — diabetics need it regardless of price, so a doubling of the price causes only a small reduction in quantity demanded. When |PED| = 1, demand is unit elastic: the percentage changes are equal.
Price elasticity of demand = |% ΔQd / % ΔP|. Values above 1 indicate elastic demand; values below 1 indicate inelastic demand; a value of exactly 1 is unit elastic.
Key takeaway: Elasticity is not a fixed property of a good — it depends on the price range, the time horizon, and the availability of substitutes. The same good can be elastic in one context and inelastic in another.
What Determines Elasticity
Four main factors shape how elastic demand is for any given good. First, the availability of substitutes: the more close substitutes a good has, the more elastic its demand. Petrol from one station is nearly identical to petrol from the next, so demand at any single station is highly elastic. But petrol as a category has few substitutes in the short run, making overall petrol demand inelastic.
Second, whether the good is a necessity or a luxury. Necessities like bread, water, and basic medicines tend to have inelastic demand — people buy them regardless of price. Luxury goods like sports cars or designer handbags have elastic demand because consumers can easily forgo them if prices rise.
Third, the time horizon. Demand is almost always more elastic in the long run than in the short run. When petrol prices rise sharply, drivers can't immediately switch to electric vehicles — but over several years, they can. Short-run demand for petrol is inelastic; long-run demand is considerably more elastic.
Fourth, the share of income spent on the good. Goods that take up a large fraction of a consumer's budget (rent, a car) tend to have more elastic demand than goods that cost very little (salt, matches). A 20% rise in the price of salt is barely noticeable; a 20% rise in rent is a crisis.
Key takeaway: When analyzing elasticity, always consider substitutes, necessity, time, and budget share together. No single factor tells the whole story.
Elasticity and Total Revenue
One of the most powerful applications of elasticity is predicting what happens to a firm's total revenue (TR = Price × Quantity) when it changes its price. The relationship depends entirely on whether demand is elastic or inelastic.
If demand is elastic, a price increase causes a proportionally larger fall in quantity — total revenue falls. Conversely, a price cut causes a proportionally larger rise in quantity — total revenue rises. Budget airlines understand this well: cutting fares on price-sensitive routes fills planes and boosts revenue.
If demand is inelastic, a price increase causes a proportionally smaller fall in quantity — total revenue rises. A price cut causes a proportionally smaller rise in quantity — total revenue falls. This is why pharmaceutical companies can charge high prices for patented drugs with no substitutes: demand is inelastic, and higher prices mean higher revenue.
At unit elasticity, price changes leave total revenue unchanged — the percentage changes in price and quantity exactly cancel out.
Key takeaway: Elastic demand → price and revenue move in opposite directions. Inelastic demand → price and revenue move in the same direction. This rule is essential for pricing strategy and tax incidence analysis.
Income Elasticity of Demand
Income elasticity of demand (YED) measures how quantity demanded responds to a change in consumer income: YED = (% change in quantity demanded) ÷ (% change in income). The sign of YED tells you what kind of good you're dealing with.
For normal goods, YED is positive — as income rises, people buy more. Most goods fall into this category. Within normal goods, luxury goods have YED > 1 (demand rises faster than income — think foreign holidays, fine dining, or premium cars), while necessities have 0 < YED < 1 (demand rises, but more slowly than income).
For inferior goods, YED is negative — as income rises, people buy less of them, switching to higher-quality alternatives. Bus travel, instant noodles, and second-hand clothing are classic examples. As incomes in developing economies rise, demand for these goods often falls as consumers upgrade to cars, fresh food, and new clothing.
Key takeaway: YED reveals how a good fits into consumers' lives. Positive YED means the good is normal; negative YED means it's inferior. Luxury goods have YED above 1 — their demand is especially sensitive to economic booms and recessions.
Cross-Price Elasticity of Demand
Cross-price elasticity of demand (XED) measures how the quantity demanded of one good responds to a change in the price of another: XED = (% change in Qd of good A) ÷ (% change in price of good B). The sign tells you the relationship between the two goods.
A positive XED indicates that the goods are substitutes: when the price of good B rises, consumers switch to good A, increasing its demand. Butter and margarine, Pepsi and Coca-Cola, or two competing streaming services are substitutes. The higher the positive XED, the closer the substitutes.
A negative XED indicates that the goods are complements: when the price of good B rises, demand for good A falls because the two are consumed together. Cars and petrol, printers and ink cartridges, and coffee and milk are complements. If petrol prices spike, fewer people buy large cars — the negative XED captures this relationship.
A XED of zero (or close to it) indicates that the goods are unrelated — changes in the price of one have no effect on demand for the other.
Key takeaway: XED is a powerful tool for market analysis. Firms use it to understand competitive threats (high positive XED means a close rival), and regulators use it to define market boundaries in antitrust cases.