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Elasticity

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On this page 9 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Quick check
  8. Study tools
  9. Sources & references

In 30 seconds

measures how strongly one variable responds to another, expressed in percentages. The headline case is : the percentage change in quantity demanded divided by the percentage change in price. If that ratio (in absolute value) is above one, demand is elastic; below one, inelastic; exactly one, unit elastic. Because it uses percentages, elasticity is unit-free, so you can compare the price sensitivity of coffee, gasoline, and airline seats on the same scale.

Why this matters

Elasticity turns a vague hunch that 'people will buy less when prices rise' into a number you can act on. A business deciding whether a price increase will raise or shrink revenue needs the , which depends entirely on elasticity. Governments use elasticity to predict who really pays a tax, how much a cigarette or gas tax cuts consumption, and how a subsidy spreads through a market. Economists estimating the effect of a minimum wage, a tariff, or a drought all lean on elasticities. Learning to compute and interpret one is a core quantitative skill that reappears throughout micro and macro.

The college version

What elasticity measures

Elasticity is a ratio of percentage changes. It answers the question: when one variable moves by a certain percentage, by what percentage does another variable respond? The most-used case is the price elasticity of demand, defined as the percentage change in quantity demanded divided by the percentage change in price. Because both the top and bottom of the ratio are percentages, the units (gallons, cups, dollars) cancel out. That is the point: elasticity is a pure number, so you can compare how sensitive gasoline demand is to price against how sensitive movie tickets are, even though they are measured in completely different units. The law of demand tells us price and quantity demanded move in opposite directions, so the raw price elasticity of demand is negative; by convention economists usually report its absolute value and compare that to one.

Calculating price elasticity of demand: the midpoint method

A percentage change depends on which point you start from, so a naive calculation gives a different answer moving up a demand curve than moving down it. The midpoint (or arc) method removes that asymmetry by dividing each change by the average of the starting and ending values rather than by the starting value alone. The percentage change in quantity is (Q2 minus Q1) divided by the average of Q1 and Q2, times 100; the percentage change in price is (P2 minus P1) divided by the average of P1 and P2, times 100. The elasticity is the first result divided by the second. Using the same two points, you now get one consistent number whether you move from A to B or from B to A. This is the standard approach in an introductory course precisely because it is direction-independent.

Elastic, inelastic, and unit elastic

Compare the size of the two percentage changes. If quantity demanded changes by a larger percentage than price, the elasticity (in absolute value) exceeds one and demand is elastic: buyers are quite responsive. If quantity changes by a smaller percentage than price, elasticity is below one and demand is inelastic: buyers barely budge. If the two percentages are equal, elasticity equals one and demand is unit elastic. Two extremes anchor the scale. Perfectly has an elasticity of zero and a vertical demand curve, meaning quantity does not change at all when price changes. Perfectly has an elasticity approaching infinity and a horizontal curve, meaning even a tiny price rise sends quantity demanded to zero. Real goods almost always sit somewhere between these poles.

What determines elasticity

Four factors do most of the explaining. First, the availability of close substitutes: the more easily buyers can switch to something else, the more elastic demand is, which is why one brand of soda is far more elastic than soda as a whole. Second, necessity versus luxury: essentials people cannot easily forgo tend to be inelastic, while discretionary purchases are elastic. OpenStax cites illustrative long-run estimates of roughly 0.12 for housing (very inelastic) and about 2.3 for restaurant meals (elastic). Third, the share of the budget the good absorbs: a good that eats a large fraction of income invites more shopping around and is more elastic. Fourth, the time horizon: demand is usually more elastic in the long run than the short run, because people need time to find alternatives, change habits, or replace equipment after a lasting price change.

The total revenue test

Total revenue is price times quantity, and a price change pushes those two factors in opposite directions, so the net effect on revenue depends on elasticity. If demand is inelastic, quantity falls by a smaller percentage than the price rises, so raising the price increases total revenue. If demand is elastic, quantity falls by a larger percentage than the price rises, so raising the price reduces total revenue; here a price cut would raise revenue instead. If demand is unit elastic, the two effects cancel and revenue is unchanged. This is the total revenue test, and it is the practical reason a business, a museum, or a transit agency must know its elasticity before changing prices. It also explains why firms facing inelastic demand can pass cost increases on to customers more easily.

Other elasticities: supply, cross-price, and income

The same percentage-ratio logic extends beyond own-price demand. Price elasticity of supply is the percentage change in quantity supplied divided by the percentage change in price, capturing how responsive producers are; it too is more elastic given more time to adjust. is the percentage change in quantity demanded of one good divided by the percentage change in the price of another; its sign is the useful part, positive for substitutes (a pricier tea nudges people toward coffee) and negative for complements (a pricier printer depresses ink sales). is the percentage change in quantity demanded divided by the percentage change in income; it is positive for normal goods and negative for inferior goods, whose demand falls as people grow richer. Market equilibrium, the demand and supply curves themselves, and full consumer-choice theory are neighboring topics covered separately.

Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Elasticity is a stretchiness score for how much people change what they buy when a price changes. First figure the percentage the amount bought changed, then the percentage the price changed, then divide the first by the second. If the amount bought swings by a bigger percentage than the price did, the score is above one and we call it elastic, meaning shoppers are picky and quick to walk away. If the amount barely moves while the price jumps, the score is below one, or inelastic, meaning people keep buying no matter what. A score of exactly one is right in the middle. The neat trick is that using percentages makes the score a plain number, so you can compare gum and gasoline on the very same scale.

Picture it like this

Think of a rubber band. Pull it and it stretches a lot for a small tug: that is elastic, like fancy restaurant dinners people skip the moment prices climb. A thick, stiff band barely moves no matter how hard you pull: that is inelastic, like a medicine someone needs every day and will buy at almost any price.

Where the picture stops working

A rubber band always springs back to its original length, but demand does not automatically return when a price falls again, and habits, incomes, and substitutes can permanently reshape how buyers respond. The band also stretches the same amount each time you pull, while a good's elasticity can differ at high prices versus low prices and shifts as more time passes.

Worked example

A coffee shop currently sells 200 cups a day at $4.00 and is considering raising the price to $5.00, where it expects to sell 140 cups. Use the midpoint method. Percentage change in quantity: (140 minus 200) divided by the average of 200 and 140 (which is 170), times 100, equals about -35.3%. Percentage change in price: (5 minus 4) divided by the average of 4 and 5 (which is 4.5), times 100, equals about +22.2%. Price elasticity of demand is -35.3% / 22.2%, or about -1.59; in absolute value 1.59, so demand here is elastic. The total revenue test predicts the price increase should shrink revenue, and it does: revenue falls from $4.00 x 200 = $800 to $5.00 x 140 = $700. Raising the price cost the shop money because its customers were price-sensitive.

Key takeaway

Price elasticity of demand is the percentage change in quantity demanded over the percentage change in price; above one means elastic and a price rise shrinks revenue, below one means inelastic and a price rise grows it. Compute it with the midpoint method so the answer does not depend on direction.

Quick check

3 questions here, of 5 in this lesson’s practice set. Answers stay hidden until you check.

Question 1 of 3foundational

Price elasticity of demand measures which of the following?

Choose an answer, then check it.
Question 2 of 3intermediate

A firm sells a good whose demand is inelastic. If it raises the price, total revenue will most likely:

Choose an answer, then check it.
Question 3 of 3intermediate

Using the midpoint (arc) method, a coffee shop raises its price from $4 to $5 and daily sales fall from 200 to 140 cups. The price elasticity of demand is closest to:

Choose an answer, then check it.
Practice all 5

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Practice this lesson
Study tools & related lessonsYou’ll learn to · Common mistakes · Easily confused · Key vocabulary · Related

You’ll learn to

  • Define elasticity as a percentage-based measure of responsiveness and explain why it is unit-free
  • Compute price elasticity of demand using the midpoint (arc) formula
  • Distinguish elastic, inelastic, and unit-elastic demand and identify the perfectly elastic and perfectly inelastic extremes
  • Explain the determinants of price elasticity of demand: substitutes, necessity versus luxury, budget share, and time horizon
  • Apply the total revenue test to predict how a price change affects revenue
  • Distinguish price elasticity of demand from price elasticity of supply, cross-price elasticity, and income elasticity

Common mistakes

  • Treating elasticity as a raw slope, using change-in-quantity over change-in-price in units rather than percentages.

    Elasticity is a ratio of percentage changes, which is what makes it unit-free. A slope in cups-per-dollar changes if you switch to ounces or cents; elasticity does not.

  • Assuming a price increase always raises revenue.

    It raises revenue only when demand is inelastic. When demand is elastic, the drop in quantity outweighs the higher price and total revenue falls, so a price cut would raise revenue instead.

  • Getting two different elasticities for the same pair of points depending on direction.

    That happens with the simple percentage-change formula. The midpoint method divides by the average of the two values, giving one consistent elasticity whether price rises or falls.

  • Confusing an inelastic good with one nobody will stop buying entirely.

    Inelastic means quantity responds by a smaller percentage than price, not that it never responds. Only perfectly inelastic demand (elasticity of zero, a vertical curve) has quantity fixed regardless of price.

  • Reading a negative cross-price or income elasticity as an error.

    The sign carries meaning. Negative cross-price elasticity signals complements; negative income elasticity signals an inferior good. The minus sign is information, not a mistake.

Easily confused

Elastic demand vs. Inelastic demand

Elastic demand has elasticity above one, so quantity responds more than proportionally to price; inelastic demand has elasticity below one, so quantity responds less than proportionally.

Price elasticity of demand vs. Cross-price elasticity of demand

Price elasticity uses the good's own price; cross-price elasticity uses the price of a different good, and its sign reveals whether the two are substitutes or complements.

Simple percentage-change method vs. Midpoint (arc) method

The simple method divides by the starting value and gives different answers in each direction; the midpoint method divides by the average and gives one direction-independent answer.

Key vocabulary

Elasticity
A measure of how much one variable responds to a change in another, expressed as a ratio of percentage changes and therefore unit-free.
Price elasticity of demand
The percentage change in quantity demanded divided by the percentage change in the good's own price.
Midpoint (arc) method
A way of computing percentage change that divides each change by the average of the starting and ending values, giving the same elasticity in either direction.
Elastic demand
Demand for which the elasticity is greater than one in absolute value: quantity demanded changes by a larger percentage than price.
Inelastic demand
Demand for which the elasticity is less than one in absolute value: quantity demanded changes by a smaller percentage than price.
Unit elastic demand
Demand for which the elasticity equals one: quantity demanded and price change by the same percentage.
Total revenue test
The rule that a price increase raises total revenue when demand is inelastic, lowers it when demand is elastic, and leaves it unchanged when demand is unit elastic.
Cross-price elasticity of demand
The percentage change in quantity demanded of one good divided by the percentage change in the price of another; positive for substitutes, negative for complements.
Income elasticity of demand
The percentage change in quantity demanded divided by the percentage change in income; positive for normal goods, negative for inferior goods.

Sources & references

  1. Principles of Economics 2e, Section 5.1: Price Elasticity of Demand and Price Elasticity of Supply — OpenStax (Rice University)
  2. Principles of Economics 2e, Section 5.3: Elasticity and Pricing — OpenStax (Rice University)
  3. Principles of Economics 2e, Section 5.4: Elasticity in Areas Other Than Price — OpenStax (Rice University)

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Researched 2026-08-19

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