---
title: "Law of Demand — Principles of Microeconomics Guide"
description: "The law of demand says price and quantity demanded move in opposite directions, which is why demand curves slope downward and markets reach equilibrium."
canonical: "https://fiveable.me/principles-microeconomics/key-terms/law-demand"
type: "key-term"
subject: "Principles of Microeconomics"
unit: "Unit 5"
---

# Law of Demand — Principles of Microeconomics Guide

## Definition

The law of demand states that, holding everything else constant, when the price of a good rises the quantity demanded falls, and when the price falls the quantity demanded rises. This inverse relationship is what gives the demand curve its downward slope.

## What It Is

The law of demand is one of the first building blocks you learn in microeconomics. It says that price and quantity demanded move in opposite directions: raise the price and people buy less, drop the price and people buy more, as long as nothing else changes (that "nothing else changes" condition is called *ceteris paribus*).

Why does this happen? Two reasons. First, **diminishing marginal utility**: each extra unit of a good gives you a little less satisfaction than the one before, so you're only willing to buy more if the price drops. Second, the substitution and income effects, when a good gets pricier, you switch to substitutes and your real buying power shrinks. Put those together and you get the classic downward-sloping demand curve, with price on the vertical axis and quantity on the horizontal axis.

## Why It Matters

The law of demand shows up everywhere in Principles of Microeconomics. It's the foundation of Topic 3.1 (Demand, Supply, and Equilibrium), because you can't find a market equilibrium without a downward-sloping demand curve meeting an upward-sloping supply curve. It connects to Topic 3.2, where you separate a movement *along* a demand curve (caused by a price change) from a *shift* of the whole curve (caused by income, preferences, or related goods). It underpins Topic 5.1 on price elasticity, since elasticity measures exactly how much quantity demanded responds to a price change. And in Topic 6.2, it explains consumer behavior through the substitution and income effects. Get this right and the rest of the demand-and-supply unit clicks into place.

## Connections

### Demand Curve (Unit 3)

The law of demand is the reason the demand curve slopes downward. The law is the principle; the curve is the picture of that principle on a graph.

### Diminishing Marginal Utility (Unit 6)

This is the deeper reason the law of demand holds. Because each extra unit gives less added satisfaction, you'll only buy more when the price falls to match that lower value.

### [Ceteris Paribus (Unit 3)](/principles-microeconomics/key-terms/ceteris-paribus)

The law of demand only works when all other factors are held constant. Drop that assumption and a price change could be masked by shifts in income, tastes, or substitute prices.

### [Availability of Substitutes (Unit 5)](/principles-microeconomics/key-terms/availability-substitutes)

Substitutes strengthen the law of demand. When a good has many close alternatives, a price increase pushes buyers away faster, making quantity demanded more responsive (more elastic).

## On the AP Exam

Expect the law of demand on quizzes, problem sets, and exam multiple-choice as a graph-reading skill. You'll be asked to identify the downward slope, predict how quantity demanded changes when price changes, and explain why the relationship is inverse using diminishing marginal utility or the substitution and income effects. A common trap on free-response or short-answer questions: distinguish a movement along the demand curve (a price change) from a shift of the whole curve (a change in a demand shifter). When you draw a graph, label the axes correctly (price vertical, quantity horizontal) and use Qd, not Q, when you mean quantity demanded.

## Law of Demand vs Shift in Demand

The law of demand describes a movement ALONG a fixed demand curve caused by the good's own price changing. A shift in demand moves the entire curve and is caused by something else, like income, preferences, or the price of a related good. If price is what changed, you move along; if anything else changed, you shift.

## Key Takeaways

- The law of demand says that, holding all else constant, price and quantity demanded move in opposite directions.
- This inverse relationship is exactly why the demand curve slopes downward.
- Diminishing marginal utility and the substitution and income effects explain why the law of demand holds.
- A change in the good's own price causes a movement along the curve, not a shift of the curve.
- The law of demand is the foundation for finding market equilibrium and for measuring price elasticity of demand.

## FAQs

### What is the law of demand in microeconomics?

It's the principle that when the price of a good rises, the quantity demanded falls, and when the price falls, the quantity demanded rises, assuming everything else stays the same. This inverse relationship is what makes demand curves slope downward.

### Does a price change shift the demand curve?

No. A change in the good's own price causes a movement along the existing demand curve (that's the law of demand). The curve only shifts when a non-price factor changes, like income, preferences, the number of buyers, or the price of a related good.

### How is the law of demand different from a shift in demand?

The law of demand is about moving along one fixed curve as the good's price changes. A shift in demand moves the whole curve left or right because of something other than the good's own price, such as higher income or a change in tastes.

### Why does the law of demand work?

Two main reasons: diminishing marginal utility (each extra unit is worth less to you, so you only buy more at a lower price) and the substitution and income effects (a higher price pushes you toward substitutes and shrinks your real buying power).

### Are there exceptions to the law of demand?

Rare ones exist, like Giffen goods and certain status or luxury goods, where quantity demanded can rise as price rises. For your intro course, treat the law of demand as holding in virtually all standard cases.

## Related Study Guides

- [5.1 Price Elasticity of Demand and Price Elasticity of Supply](/principles-microeconomics/unit-5/1-price-elasticity-demand-price-elasticity-supply/study-guide/BkL2P33arvPHWYj5)
- [4.3 The Market System as an Efficient Mechanism for Information](/principles-microeconomics/unit-4/3-market-system-efficient-mechanism-information/study-guide/UEJrG1YoEyNa9tqw)
- [6.2 How Changes in Income and Prices Affect Consumption Choices](/principles-microeconomics/unit-6/2-income-prices-affect-consumption-choices/study-guide/WxztKVU1FFMz0wis)
- [3.1 Demand, Supply, and Equilibrium in Markets for Goods and Services](/principles-microeconomics/unit-3/1-demand-supply-equilibrium-markets-goods-services/study-guide/ZPsG4nXmzraVNUNa)
- [3.2 Shifts in Demand and Supply for Goods and Services](/principles-microeconomics/unit-3/2-shifts-demand-supply-goods-services/study-guide/oRurgzYFanZTjnTp)

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