Perfectly inelastic means quantity demanded or supplied stays fixed no matter how price changes. In Principles of Macroeconomics, it shows up as a vertical demand or supply curve.
Perfectly inelastic is the extreme case where a price change does not change quantity at all. In Principles of Macroeconomics, that means demand or supply is completely unresponsive, with an elasticity of 0.
If demand is perfectly inelastic, consumers buy the same amount whether the price goes up or down. The graph is a vertical demand curve, because quantity demanded stays fixed while price can move. A common example is a lifesaving medication with no close substitute. If you need it, a higher price does not change the quantity you need in the short run.
Perfectly inelastic supply works the same way from the producer side. Quantity supplied stays fixed even when price changes, so the supply curve is vertical. A classic macro-style example is a fixed amount of land in a city or a short-run situation where a firm cannot quickly expand output, though true perfect inelasticity is rare in real markets.
This term matters because it is not just about “not very elastic.” It is the limit case where responsiveness disappears completely. That makes it useful for reading graphs, interpreting tax effects, and seeing why some markets do not react the way regular supply and demand examples do.
A common mistake is mixing up perfectly inelastic with simply inelastic. Inelastic means quantity changes a little when price changes. Perfectly inelastic means quantity does not move at all, so the line is vertical and the elasticity coefficient is exactly 0.
Perfectly inelastic gives you a clean way to spot markets where price changes do not change behavior. That matters in macroeconomics because many policy questions depend on how much buyers or sellers can adjust. If demand is nearly fixed, a tax, subsidy, or price shock changes who pays, not how much gets bought.
This term also helps you read graphs correctly. A vertical line tells you quantity is locked in, so any movement in price moves you up or down the same curve without changing output. That is a fast visual clue on homework, quizzes, and class problem sets.
It also connects to real policy examples. Life-saving medicine, emergency water, or some short-run fixed resources can behave this way more than ordinary goods do. When you see a scenario with no substitutes and no easy way to adjust quantity, perfectly inelastic is the strongest possible description.
Once you know this term, you can compare it with inelastic and elastic cases instead of treating all demand changes the same. That makes your analysis sharper when you explain tax incidence, shortages, or why a market price may rise without a drop in quantity.
Keep studying Principles of Macroeconomics Unit 5
Visual cheatsheet
view galleryElasticity
Perfectly inelastic is the zero point on the elasticity scale. Instead of asking whether quantity changes a little or a lot, this case says quantity does not change at all. That makes it the easiest benchmark for recognizing the difference between responsiveness and no responsiveness.
Inelastic Demand
Inelastic demand still changes when price changes, just not by much. Perfectly inelastic demand goes one step farther and shows no quantity response at all. If you are deciding between the two on a graph, inelastic curves slope steeply, while perfectly inelastic demand is vertical.
Marginal Revenue
A perfectly inelastic good has a special revenue pattern because quantity sold stays fixed. If a seller can raise price without losing units sold, total revenue rises with price, and marginal revenue equals the price of the added unit. That makes revenue questions easier to trace on problems about fixed-quantity goods.
Determinants of Elasticity
The determinants of elasticity help explain why a market might be close to, but not exactly, perfectly inelastic. Few substitutes, necessity, and a tiny share of income all push demand toward being less responsive. Perfect inelasticity is the extreme version of those forces, not the usual everyday result.
A quiz item may show a vertical demand or supply curve and ask you to identify it as perfectly inelastic. You might also get a scenario about a lifesaving drug, a fixed resource, or a market with no substitutes and need to explain why quantity does not change when price changes. In problem sets, you may be asked to interpret elasticity as 0, compare it with inelastic demand, or predict what happens to revenue when price rises. The skill is usually not memorizing the phrase alone, but linking the graph, the elasticity value, and the market behavior in the same answer. If the question asks about taxes or policy, focus on who bears the burden when quantity cannot adjust.
These sound similar, but they are not the same. Inelastic demand means quantity changes only a little when price changes, while perfectly inelastic demand means quantity does not change at all. On a graph, inelastic demand slopes steeply, but perfectly inelastic demand is a vertical line.
Perfectly inelastic means quantity demanded or supplied stays the same even when price changes.
On a graph, perfectly inelastic demand or supply is a vertical line.
The elasticity coefficient for a perfectly inelastic curve is 0.
This idea is useful for markets with no close substitutes, fixed resources, or very short-run quantity limits.
Do not confuse perfectly inelastic with inelastic, because inelastic still has some quantity response.
Perfectly inelastic means quantity demanded or supplied does not change when price changes. In macroeconomics, you usually see it as a vertical demand or supply curve. The elasticity value is 0, which tells you there is no quantity response at all.
It looks like a vertical line. That shape shows quantity demanded stays fixed while price can move up or down. If a question gives you a graph with the same quantity at every price, you are looking at perfectly inelastic demand.
Inelastic demand still changes a little when price changes, just not much. Perfectly inelastic demand does not change at all. The difference shows up on graphs, too, since inelastic demand slopes steeply but perfectly inelastic demand is vertical.
This happens when buyers have no close substitutes and the good is a necessity, like a lifesaving medication in the short run. It can also happen when supply is fixed, such as a limited resource that cannot be increased quickly. Real markets are rarely perfectly inelastic, but some come close.