---
title: "Perfectly Elastic — AP Micro Definition & Exam Guide"
description: "Perfectly elastic means quantity responds infinitely to any price change, shown as a horizontal demand curve. It's why firms in perfect competition are price takers."
canonical: "https://fiveable.me/ap-micro/key-terms/perfectly-elastic"
type: "key-term"
subject: "AP Microeconomics"
unit: "Unit 3"
---

# Perfectly Elastic — AP Micro Definition & Exam Guide

## Definition

Perfectly elastic describes demand or supply where quantity responds infinitely to any price change, graphed as a horizontal line. In AP Micro, it's the demand curve an individual firm in perfect competition faces: it can sell any amount at the market price, but nothing at even one cent higher.

## What It Is

Perfectly elastic means the [price](/ap-micro/unit-2/supply/study-guide/6Q4OmUPc9RVRr9R7JmFS "fv-autolink") elasticity coefficient is infinite. The tiniest price change causes [quantity](/ap-micro/key-terms/quantity "fv-autolink") to swing to its extreme. On a graph, a perfectly elastic demand (or supply) curve is a **horizontal line** at one specific price. Buyers will take any quantity at that price, and zero units at anything above it.

In [AP Micro](/ap-micro "fv-autolink"), this concept matters most in Topic 3.7. A firm in a perfectly competitive market is a **price taker** because it's tiny relative to the market and sells an identical product to everyone else. If it raises its price even slightly, customers buy from a competitor instead and the firm sells nothing. So the individual firm faces a perfectly elastic demand curve at the market equilibrium price, even though the *market* demand curve still slopes downward. That horizontal line is where the famous identity P = MR = D = AR comes from.

## Why It Matters

Perfectly elastic demand is the bridge between [Unit 2](/ap-micro/unit-2 "fv-autolink") elasticity and the Unit 3 perfect competition model. It directly supports learning objective 3.7.A (defining the characteristics of perfectly competitive markets using graphs) and 3.7.B (explaining firm decision making and efficient outcomes). EK PRD-3.A.3 says firms in perfect competition can sell all their output at a constant price determined by the market. "Perfectly elastic [demand curve](/ap-micro/key-terms/demand-curve "fv-autolink")" is just the graphical way of saying that. It also explains why marginal revenue equals price for these firms, which is the whole reason the profit-maximizing rule P = MR = MC works, and why perfect competition lands at allocative efficiency (EK PRD-3.A.2, price equals marginal cost).

## Connections

### Perfect Competition (Unit 3)

The perfectly elastic demand curve IS the firm-level picture of perfect competition. Many sellers plus identical products plus no [barriers to entry](/ap-micro/key-terms/barriers-to-entry "fv-autolink") means no firm has market power, so each one takes the market price as given and faces a flat demand line at that price.

### [Marginal Revenue (Unit 3)](/ap-micro/key-terms/marginal-revenue)

When demand is perfectly elastic, every extra unit sells at the same price, so [marginal revenue](/ap-micro/key-terms/marginal-revenue "fv-autolink") equals price. That's why the firm's horizontal line gets labeled D = MR = AR = P, and it's the setup for profit maximization at MR = MC.

### Price Elasticity of Demand (Unit 2)

Perfectly elastic is one extreme of the [elasticity](/ap-micro/unit-2/price-elasticity-demand/study-guide/kCV9eYI16fBj1nx9STCs "fv-autolink") spectrum you learn in Unit 2, where the coefficient equals infinity. The other extreme is perfectly inelastic, a vertical curve where quantity doesn't budge no matter what price does.

### [Allocative Efficiency (Units 1 & 3)](/ap-micro/key-terms/allocative-efficiency)

Because the firm's perfectly elastic demand makes P = MR, profit maximization at MR = MC automatically gives P = MC. Price equals marginal cost is the definition of allocative efficiency, which is why perfect competition is the efficiency benchmark on the exam.

## On the AP Exam

Multiple-choice questions love the side-by-side graph setup. The market shows normal downward-sloping demand, and you're asked what the demand curve for an individual firm looks like (answer: horizontal, perfectly elastic, at the market price). Other common stems ask what happens to a firm's quantity sold or total revenue if it raises its price when demand is perfectly elastic (quantity and revenue both drop to zero). On FRQs, you'll typically draw the side-by-side market and firm graphs for perfect competition, and that horizontal D = MR line at the market price is exactly where points get earned or lost. The 2022 FRQ Q3 worked with individual buyers' willingness to pay in a competitive market for Good X, the kind of price-taking setup this concept underlies. Make sure you can draw it, label it, and explain in one sentence why the firm faces it.

## Perfectly Elastic vs Perfectly Inelastic

These are opposite extremes and the graphs are easy to flip under pressure. Perfectly elastic is a HORIZONTAL line (elasticity = infinity, any price change wipes out all quantity). Perfectly inelastic is a VERTICAL line (elasticity = 0, quantity never changes no matter the price, like a life-saving medication). Memory trick: perfectly Inelastic looks like the letter I, standing straight up.

## Key Takeaways

- Perfectly elastic means the price elasticity coefficient is infinite, and the curve is a horizontal line at one price.
- An individual firm in perfect competition faces a perfectly elastic demand curve at the market price because it's a price taker selling an identical product.
- The market demand curve in perfect competition still slopes downward; only the single firm's demand curve is horizontal.
- Because the curve is flat, price equals marginal revenue for the firm, which is why the line is labeled P = MR = D = AR.
- If a firm with perfectly elastic demand raises its price even slightly, it sells zero units and total revenue falls to zero.
- Perfectly elastic demand is the reason P = MC holds at the profit-maximizing output in perfect competition, making the market allocatively efficient.

## FAQs

### What does perfectly elastic mean in AP Micro?

It means quantity demanded or supplied responds infinitely to any price change, so the curve is a horizontal line at a single price. Buyers take any quantity at that price and zero at any higher price.

### Is a perfectly elastic demand curve horizontal or vertical?

Horizontal. A vertical demand curve is perfectly inelastic (elasticity of zero). Remember that perfectly Inelastic looks like the letter I standing upright.

### Why does a firm in perfect competition face a perfectly elastic demand curve?

Because it's a price taker. With many sellers offering identical products and no barriers to entry, the firm has zero market power. Raise the price a penny and every customer switches to a competitor, so the firm can only sell at the market price.

### Is the whole market demand curve perfectly elastic in perfect competition?

No, and this is the classic trap. The market demand curve slopes downward like normal. Only the individual firm's demand curve is horizontal, drawn at the price set by market supply and demand. That's why exam graphs show the market and the firm side by side.

### What happens to total revenue if price increases and demand is perfectly elastic?

Total revenue falls to zero. With perfectly elastic demand, any price above the going price means quantity sold drops to nothing, so there's no revenue at all. This shows up regularly as a multiple-choice stem.

## Related Study Guides

- [3.7 Perfect Competition](/ap-micro/unit-3/perfect-competition/study-guide/T08vY2meNhtpbLCT83uH)

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