Elastic vs Inelastic Demand: Formula, Examples, and Exam Traps

Demand is elastic when buyers respond a lot to a price change and inelastic when they barely respond at all. Both words are measured by the same tool, the price elasticity of demand, and the dividing line is simple: if the percentage change in quantity is larger than the percentage change in price, demand is elastic; if it is smaller, demand is inelastic. This guide gives the exact definitions, works the midpoint formula in full, runs the total revenue test as a table, covers the perfectly elastic and perfectly inelastic limiting cases, and walks through the exam mistakes that cost the most marks.

What elastic and inelastic mean

The price elasticity of demand (Ed) measures how much the quantity demanded of a good responds to a change in its price. It is the ratio of two percentage changes:

Ed = percentage change in quantity demanded / percentage change in price

Because demand slopes downward, price and quantity move in opposite directions and this ratio is technically negative, so economists compare its absolute value to 1. That gives three cases:

So if a 10 percent price rise cuts quantity by 25 percent, Ed = 25 / 10 = 2.5, which is elastic: buyers are sensitive. If the same 10 percent rise cuts quantity by only 4 percent, Ed = 4 / 10 = 0.4, which is inelastic: buyers are insensitive. Elastic means stretchy and responsive; inelastic means rigid and unresponsive.

The midpoint formula, worked in full

The percentage changes above have to be computed consistently, and the standard tool is the midpoint method, which divides each change by the average of the start and end values so you get the same elasticity whether the price rose or fell. The formula is:

Ed = [(Q2 − Q1) / ((Q1 + Q2) / 2)] / [(P2 − P1) / ((P1 + P2) / 2)]

Work a full example. The price of a good falls from $10 to $8, and quantity demanded rises from 40 units to 60 units.

Step 1, the quantity change. The change is 60 − 40 = 20. The average quantity is (40 + 60) / 2 = 50. So the percentage change in quantity is 20 / 50 = 0.40, or 40 percent.

Step 2, the price change. The change is 8 − 10 = −2. The average price is (10 + 8) / 2 = 9. So the percentage change in price is −2 / 9 = −0.222, or about −22.2 percent.

Step 3, the ratio. Ed = 0.40 / (−0.222) = −1.8. Taking the absolute value, Ed = 1.8.

Because 1.8 is greater than 1, demand over this price range is elastic: the 40 percent jump in quantity outran the roughly 22 percent fall in price. You can drill this calculation in the price elasticity of demand calculator, and practice the percentage-change step on its own in the percentage change calculator.

What makes demand elastic or inelastic

Whether a good comes out elastic or inelastic depends on the buyer's real-world options. Four determinants do most of the work.

Availability of substitutes. The more close substitute goods a good has, the more elastic its demand, because buyers can easily switch away when the price rises. A single brand of cola is elastic; salt, with no real substitute, is inelastic.

Necessity versus luxury. Necessities tend to be inelastic because people buy them almost regardless of price, while luxuries are elastic because they are easy to postpone or skip. Insulin is inelastic; a cruise is elastic.

Share of the budget. Goods that eat a large share of income tend to be more elastic, because a price change is felt sharply, while cheap items whose cost is trivial, like a pack of gum, are inelastic.

Time horizon. Demand is more elastic over longer periods, because buyers need time to find alternatives, change habits, or replace equipment. Gasoline is inelastic this week but more elastic over several years as people buy more efficient cars.

The total revenue test

Elasticity has a direct payoff for a seller through total revenue (TR), which is price times quantity. The total revenue test says that whether a price cut raises or lowers total revenue tells you the elasticity, and vice versa. The rule: when demand is elastic, price and total revenue move in opposite directions, so cutting the price raises revenue; when demand is inelastic, price and total revenue move in the same direction, so cutting the price lowers revenue.

See it in two tables. First, an elastic good, using the numbers from the worked example (Ed = 1.8):

The price cut from $10 to $8 raised revenue from $400 to $480, because the big quantity gain outweighed the smaller price drop. That is the elastic signature.

Now an inelastic good, such as a necessity where buyers barely change quantity:

Here the same price cut lowered revenue from $1,000 to $880, because the tiny quantity gain could not make up for the lost price. Check the elasticity: quantity rose 10 / 105 ≈ 9.5 percent while price fell about 22.2 percent, so Ed ≈ 0.43, inelastic, exactly as the revenue drop predicted. The full mechanics live in the total revenue test guide.

The two limiting cases

The extremes of the elasticity scale are worth drawing because exams love them. Perfectly elastic demand has Ed = infinity and appears as a horizontal line: at one price buyers will take any quantity, and the smallest price increase drops quantity demanded to zero. It describes the demand facing a single seller in a perfectly competitive market. See perfectly elastic demand for the graph.

Perfectly inelastic demand has Ed = 0 and appears as a vertical line: quantity demanded is the same no matter the price, because buyers must have a fixed amount whatever it costs. A life-saving drug taken in a fixed dose approximates this. See perfectly inelastic demand. Read the two together with the diagonal cases and the whole scale, from vertical to horizontal, makes visual sense.

Common exam mistakes

Confusing slope with elasticity. This is the big one. Slope is the steepness of the demand curve in price-per-unit terms, and it is constant along a straight line. Elasticity is a ratio of percentage changes, and it is not the same thing as slope. A steep curve is not automatically inelastic and a flat one is not automatically elastic, because elasticity depends on the price and quantity where you measure it, not just the tilt of the line.

Forgetting that elasticity varies along a linear demand curve. On a single straight-line demand curve, elasticity is not one number. It is elastic at high prices (near the top, where quantity is small so a given quantity change is a large percentage) and inelastic at low prices (near the bottom, where quantity is large so the same change is a small percentage). The curve moves smoothly from elastic to inelastic as you slide down it, even though its slope never changes. This is the cleanest proof that slope and elasticity are different.

Missing the unit-elastic midpoint and the revenue peak. Exactly halfway down a linear demand curve, elasticity equals 1, the unit elastic point, and this is precisely where total revenue is maximized. Above the midpoint demand is elastic, so a price cut still raises revenue; below the midpoint demand is inelastic, so a price cut lowers it; at the midpoint the two effects cancel and revenue is at its peak. If a question asks where total revenue is greatest on a straight-line demand curve, the answer is the midpoint, where demand is unit elastic. You can watch total revenue rise to its peak and fall again in the elasticity sandbox.

Practice and connect

Elasticity is the measurement layer on top of the ordinary law of demand, so make sure the demand curve is solid first, then layer the ratio on top. Compare the two cases side by side in elastic versus inelastic demand, extend the idea to related goods with cross-price elasticity, and work the full toolkit in the elasticity module. Drill the midpoint method and the total revenue test until you can classify any demand as elastic, inelastic, or unit elastic from a table alone.