--- title: "Perfectly Elastic Supply | Principles of Macroeconomics" description: "Perfectly elastic supply means sellers will supply any quantity at one market price, shown as a horizontal supply curve in Principles of Macroeconomics." canonical: "https://fiveable.me/principles-macroeconomics/key-terms/perfectly-elastic-supply" type: "key-term" subject: "Principles of Macroeconomics" unit: "Unit 5" --- # Perfectly Elastic Supply | Principles of Macroeconomics ## Definition Perfectly elastic supply is a situation where producers will supply any quantity at one market price, but none at a lower price. In macroeconomics, it is a horizontal supply curve and a useful extreme case for elasticity. ## What It Is Perfectly elastic supply is the extreme case in Principles of Macroeconomics where quantity supplied changes infinitely at one exact price. If the market price stays the same, producers are willing to sell any amount. If price falls even a little below that level, quantity supplied drops to zero. On a graph, perfectly elastic supply is drawn as a horizontal line. That shape shows that the supplier does not need a higher price to sell more output, at least within the relevant range. The market is facing a fixed price, and firms can expand production without running into capacity limits in the simple model. This is different from the everyday supply curve you usually see, which slopes upward. A normal upward-sloping supply curve says higher prices encourage firms to produce more, often because extra units require more workers, overtime pay, or more expensive inputs. Perfectly elastic supply skips that tradeoff and assumes firms can absorb all the quantity buyers want at the same price. In macroeconomics, you usually meet this term when the class is comparing extreme elasticity cases. It is not a realistic description of most markets, but it helps you see how responsive sellers can be and how price affects quantity supplied. A perfectly elastic supply curve is often treated as a benchmark, especially when a firm is a price taker in a highly competitive market. A good way to picture it is a small firm in a market where the going price is already set. The firm can sell as much as the market demands at that price, so long as it can keep producing at the same cost. If the price changes, the response is immediate and extreme, which is why the curve is flat rather than steep. The big idea is that perfectly elastic supply tells you quantity supplied is completely sensitive to price at one level. It is a theoretical edge case, but it makes the logic of elasticity much easier to compare with inelastic supply, elastic supply, and the midpoint method used to measure responsiveness. ## Why It Matters Perfectly elastic supply gives you a clean benchmark for reading supply behavior in Principles of Macroeconomics. When you see a horizontal supply curve, you know the seller is acting like a price taker and can expand output without changing the market price. That makes it easier to separate changes in price from changes in quantity supplied. It also sharpens your understanding of price elasticity of supply. If supply is perfectly elastic, even a tiny price change would create an enormous change in quantity supplied. That is the opposite of inelastic supply, where producers barely change output when price changes. Comparing the two helps you explain why some markets react quickly to shocks while others barely move. This term shows up any time your course asks about market adjustment. If demand rises and the supply side is perfectly elastic, quantity can increase without pushing price upward in the simple model. That gives you a very different market outcome than you would get with a steep supply curve. It also helps with pricing questions. When firms face a flat supply condition, they have little room to set prices above the market level, because buyers can go elsewhere or because the market price is already fixed. That makes the concept useful for thinking about competition, market power, and why some industries behave like constant-price suppliers. ## Connections ### Price Elasticity of Supply Perfectly elastic supply is the extreme end of price elasticity of supply. Instead of measuring a moderate response to price, this case shows an unlimited response at one price. It gives you a benchmark for thinking about how sensitive producers are when market conditions change. ### [Inelastic Supply](/principles-macroeconomics/key-terms/inelastic-supply) Inelastic supply is almost the opposite of perfectly elastic supply. With inelastic supply, quantity supplied changes only a little when price changes, often because firms cannot ramp up production quickly. Comparing the two makes the shape of the supply curve much easier to interpret. ### Elastic Supply Elastic supply means quantity supplied responds a lot to price changes, but not infinitely. Perfectly elastic supply takes that responsiveness to the limit by making the supply curve horizontal. Both concepts help you judge how easily firms can adjust output. ### [Law of Supply](/principles-macroeconomics/key-terms/law-supply) The law of supply says that higher prices usually lead to higher quantity supplied. Perfectly elastic supply fits that broad idea, but in a special way, because the quantity can rise without the price changing at all. It is a special case, not the normal upward-sloping pattern. ## On the AP Exam A quiz question or graphing problem may ask you to identify a horizontal supply curve and explain why quantity supplied is perfectly responsive to price. You may also need to compare it with a normal upward-sloping supply curve and tell what happens if price falls below the market price. On problem sets, you might label the curve, describe the firm as a price taker, or explain why output can expand without a higher price in the simplified model. ## Perfectly Elastic Supply vs Perfectly Inelastic Supply These two are easy to mix up because both are extreme cases. Perfectly elastic supply is a horizontal line, meaning quantity supplied changes a lot with no change in price. Perfectly inelastic supply is a vertical line, meaning quantity supplied does not change even if price changes. ## Key Takeaways - Perfectly elastic supply means sellers will provide any quantity at one specific market price. - On a graph, perfectly elastic supply is a horizontal line. - This term is an extreme case used to compare how responsive producers are to price changes. - It is the opposite of perfectly inelastic supply, where quantity supplied never changes with price. - In macroeconomics, it is most useful as a benchmark for reading supply curves and market adjustment. ## FAQs ### What is perfectly elastic supply in Principles of Macroeconomics? It is a supply situation where producers will sell any quantity at one exact price, but not below it. The supply curve is horizontal, showing that quantity supplied is infinitely responsive to price at that level. Macroeconomics uses it as an extreme case to compare with more realistic supply shapes. ### What does a perfectly elastic supply curve look like? It looks like a straight horizontal line. That means the market price stays fixed while quantity supplied can expand as much as buyers want. If price drops below that line, quantity supplied falls to zero in the simplified model. ### How is perfectly elastic supply different from elastic supply? Elastic supply is highly responsive to price, but not infinitely responsive. Perfectly elastic supply is the limit case where the response is unlimited at one price. If your graph is horizontal, that is perfectly elastic, not just elastic. ### Why do macroeconomics classes use perfectly elastic supply? It gives you a simple benchmark for comparing supply responsiveness and market outcomes. You can see how a market behaves when producers can increase output without changing price. That makes it easier to contrast with inelastic supply and normal upward-sloping supply. ## Related Study Guides - [5.3 Elasticity and Pricing](/principles-macroeconomics/unit-5/3-elasticity-pricing/study-guide/EGzdnMCS0vp8C915) - [5.1 Price Elasticity of Demand and Price Elasticity of Supply](/principles-macroeconomics/unit-5/1-price-elasticity-demand-price-elasticity-supply/study-guide/rrnH4IGsxQaS78mi) ## About This Document Canonical Fiveable pages are available as Markdown at the same path plus `.md`. - [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns - [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing - [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`) - [MCP server for AP teachers](https://fiveable.me/mcp/teachers): a teacher's classes, assignments and AP-rubric grading (`https://fiveable.me/api/mcp/teacher`) ## Structured Data ```json {"@context":"https://schema.org","@graph":[{"@type":"LearningResource","@id":"https://fiveable.me/principles-macroeconomics/key-terms/perfectly-elastic-supply#resource","name":"Perfectly Elastic Supply | Principles of Macroeconomics","url":"https://fiveable.me/principles-macroeconomics/key-terms/perfectly-elastic-supply","learningResourceType":"Concept explainer","educationalLevel":"AP® / High School","about":{"@id":"https://fiveable.me/principles-macroeconomics/key-terms/perfectly-elastic-supply#term"},"audience":{"@type":"EducationalAudience","educationalRole":"student"},"dateModified":"2026-07-03T02:24:03.605Z","isPartOf":{"@type":"Collection","name":"Principles of Macroeconomics Key Terms","url":"https://fiveable.me/principles-macroeconomics/key-terms"},"publisher":{"@type":"Organization","name":"Fiveable","url":"https://fiveable.me"}},{"@type":"DefinedTerm","@id":"https://fiveable.me/principles-macroeconomics/key-terms/perfectly-elastic-supply#term","name":"Perfectly Elastic Supply","description":"Perfectly elastic supply is a situation where producers will supply any quantity at one market price, but none at a lower price. 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