Market equilibrium is the price where quantity demanded equals quantity supplied, shown where the demand and supply curves cross. When the price is too high you get a surplus, when it is too low you get a shortage, and market forces push the price back to equilibrium. When a determinant of demand or supply shifts a curve, you get a new equilibrium price and quantity.

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Why This Matters for the AP Macroeconomics Exam

Supply and demand is the base model for almost everything later in AP Macroeconomics, so getting comfortable with equilibrium now pays off in later units. You will need to draw correct supply and demand graphs, label the equilibrium price and quantity, and show how a shift changes the outcome. On multiple-choice questions you will predict whether price and quantity rise, fall, or are indeterminate. On free-response questions, properly labeled graphs and clear cause-and-effect explanations are where points are earned, so practice that habit here.

A big skill this topic builds is telling the difference between a movement along a curve (caused by a price change) and a shift of a curve (caused by a determinant). That distinction shows up again in the AD-AS model, the money market, the loanable funds market, and the foreign exchange market.

Key Takeaways

Equilibrium happens at the single price where quantity demanded equals quantity supplied; on a graph it is the intersection of the supply and demand curves.
A surplus (excess supply) happens when price is above equilibrium; a shortage (excess demand) happens when price is below equilibrium.
Market forces fix imbalances: shortages push price up, surpluses push price down, until the market clears.
A price change moves you along a curve; a change in a determinant shifts the whole curve.
A single shift gives a predictable change in both price and quantity; simultaneous shifts often leave either price or quantity indeterminate.
Always label axes (price and quantity), both curves, and the equilibrium point.

Market Equilibrium

Market equilibrium is the point where quantity supplied equals quantity demanded at the equilibrium price, sometimes called the market-clearing price. At this price, the plans of buyers and sellers match, so there is no pressure for the price to move. On a graph, equilibrium sits at the intersection of the demand curve and the supply curve.

Market Disequilibrium

Prices do not always sit at equilibrium. When they do not, the market is in disequilibrium, which shows up as a surplus or a shortage.

When the price is above equilibrium, firms want to supply more to earn higher profits, but consumers want to buy less. Quantity supplied is greater than quantity demanded, which creates a surplus (also called excess supply).

When the price is below equilibrium, consumers want to buy more because the good is cheaper, but firms want to supply less. Quantity demanded is greater than quantity supplied, which creates a shortage (also called excess demand).

Here is how to read a typical disequilibrium graph with equilibrium at P1 and quantity 150:

When price falls from P1 to a lower price P3, quantity demanded rises (for example, 150 to 200) and quantity supplied falls (for example, 150 to 100). At P3, quantity demanded is 200 and quantity supplied is 100, so there is a shortage of 100 units. Consumers want more than producers will make.
When price rises from P1 to a higher price P2, quantity demanded falls (for example, 150 to 100) and quantity supplied rises (for example, 150 to 200). At P2, quantity demanded is 100 and quantity supplied is 200, so there is a surplus of 100 units.

To calculate a surplus or shortage at a given price, find quantity supplied and quantity demanded at that price and take the difference.

In a competitive market, these imbalances do not last because they create pressure on the price:

If there is a shortage, buyers compete for the limited good, so the price rises. As price rises, quantity supplied increases and quantity demanded decreases until Qs = Qd at equilibrium.
If there is a surplus, sellers lower the price to clear excess output. As price falls, quantity demanded increases and quantity supplied decreases until Qs = Qd at equilibrium.

Topic 1.6 Changes in Equilibrium

A change in a determinant of demand or a determinant of supply shifts a curve, which gives a new equilibrium price and quantity. Some students use memory tricks for the determinants, such as a list of demand shifters (income, tastes, related goods, expectations, number of buyers) and a list of supply shifters (input/resource prices, technology, taxes and subsidies, number of sellers, expectations). The exact mnemonic does not matter; what matters is shifting the correct curve in the correct direction.

Single shifts give clear results:

Increase in demand: equilibrium price up, equilibrium quantity up.
Decrease in demand: equilibrium price down, equilibrium quantity down.
Increase in supply: equilibrium price down, equilibrium quantity up.
Decrease in supply: equilibrium price up, equilibrium quantity down.

When both demand and supply shift at the same time, move both curves on the graph. The new equilibrium is where the new demand and new supply curves cross. With simultaneous shifts, one outcome is often indeterminate without more information:

Demand increases and supply decreases: equilibrium price increases, equilibrium quantity is indeterminate.
Demand decreases and supply increases: equilibrium price decreases, equilibrium quantity is indeterminate.
Demand and supply both increase: equilibrium quantity increases, equilibrium price is indeterminate.
Demand and supply both decrease: equilibrium quantity decreases, equilibrium price is indeterminate.

How to Use This on the AP Macroeconomics Exam

MCQ

Read whether the change is a price change (movement along a curve) or a determinant change (shift of a curve). This single distinction trips up many students.
For a single shift, recall the predictable result for price and quantity.
For simultaneous shifts, decide which variable is determinate and which is indeterminate before picking an answer.

Free Response

Draw a correctly labeled supply and demand graph: price on the vertical axis, quantity on the horizontal axis, both curves labeled, and the equilibrium point marked.
When something changes, show the shift with a new curve and label the new equilibrium price and quantity.
Explain the cause and effect in words: name the determinant, state which curve shifts and in which direction, then state what happens to price and quantity.

Problem Solving

To find a surplus or shortage, compare quantity supplied and quantity demanded at the stated price and subtract.
A surplus means price is above equilibrium; a shortage means price is below equilibrium. Use that to check your work.

Common Trap

Shifting the wrong curve, or shifting a curve when the prompt only changed the price. Slow down and identify the cause first.

Common Misconceptions

Surplus and shortage are not the same as supply and demand. A surplus means quantity supplied is greater than quantity demanded at the current price, not that supply went up.
A change in price does not shift a curve. Price changes cause movement along the existing curves; only determinants shift a curve.
Equilibrium is not a price that never changes. It is stable only until a determinant shifts a curve, which creates a new equilibrium.
"Quantity demanded" and "demand" are different. Quantity demanded is one point on the curve at a given price; demand is the entire curve.
With two simultaneous shifts, you usually cannot pin down both price and quantity. One of them is typically indeterminate, so do not guess a direction for it without more information.

Vocabulary

The following words are mentioned explicitly in the College Board Course and Exam Description for this topic.

Term	Definition
demand	The quantity of a good or service that consumers are willing and able to buy at various price levels.
determinants of demand	Factors that influence and cause changes in the quantity of a good or service that consumers are willing and able to buy at various price levels.
determinants of supply	Factors that influence the quantity of goods and services producers are willing and able to supply at various price levels.
disequilibrium	A market condition in which the quantity supplied does not equal the quantity demanded, causing imbalances that create surpluses or shortages.
equilibrium	A market condition in which the quantity supplied equals the quantity demanded at a particular price, with no tendency for change.
equilibrium price	The price at which the quantity demanded equals the quantity supplied, resulting in no tendency for change.
equilibrium quantity	The quantity bought and sold at market equilibrium, where quantity demanded equals quantity supplied.
market equilibrium	The price and quantity at which the quantity demanded equals the quantity supplied in a market.
market forces	The supply and demand pressures that drive prices toward equilibrium in response to surpluses and shortages.
quantity demanded	The amount of a good or service that consumers are willing and able to purchase at a specific price.
quantity supplied	The amount of a good or service that producers are willing and able to offer for sale at a given price.
shortage	A situation in which the quantity demanded of a good exceeds the quantity supplied at a given price, resulting in insufficient supply to meet consumer demand.
supply	The quantity of a good or service that producers are willing and able to offer for sale at various price levels.
surplus	A situation in which the quantity supplied of a good exceeds the quantity demanded at a given price, resulting in excess inventory in the market.

Frequently Asked Questions

What is market equilibrium in AP Macro?

Market equilibrium is the price where quantity demanded equals quantity supplied. On a supply and demand graph, it is the intersection of the demand curve and the supply curve.

What is disequilibrium in a market?

Disequilibrium happens when the current price is not the equilibrium price. A price above equilibrium creates a surplus, while a price below equilibrium creates a shortage.

How do shortages and surpluses return a market to equilibrium?

A shortage creates upward pressure on price because buyers want more than sellers provide. A surplus creates downward pressure on price because sellers have excess output. These price adjustments move the market back toward equilibrium.

What happens when demand increases in AP Macro?

An increase in demand shifts the demand curve right, raising equilibrium price and equilibrium quantity. A decrease in demand shifts left, lowering both equilibrium price and equilibrium quantity.

What happens when supply increases in AP Macro?

An increase in supply shifts the supply curve right, lowering equilibrium price and raising equilibrium quantity. A decrease in supply shifts left, raising price and lowering quantity.

What is the difference between demand and quantity demanded?

Quantity demanded is one point on the demand curve at a specific price. Demand is the whole curve. A price change moves along the curve, while a determinant change shifts the entire curve.