Business and Technology · Microeconomic factors

Demand, Supply and Market Equilibrium for ACCA BT

Updated 11 October 2026 · Fact-checked

Demand shows how much buyers want at each price. Supply shows how much sellers offer at each price. Equilibrium is the price where the two quantities are equal. To solve questions, identify what changed, decide which curve shifts and in which direction, then read off the new equilibrium price and quantity.

Understand Demand, Supply and Market Equilibrium

A market is where buyers and sellers meet. Price is the signal that connects them. The demand curve slopes downwards: as price falls, buyers want to buy more. The supply curve slopes upwards: as price rises, sellers are willing to supply more.

The point where the curves cross is the equilibrium. At that price, the quantity buyers want equals the quantity sellers offer. There is no pressure for price to change. If price is above equilibrium, supply exceeds demand (a surplus), so sellers cut prices. If price is below equilibrium, demand exceeds supply (a shortage), so price is bid up.

The most tested idea is the difference between a movement along a curve and a shift of the curve. A change in the good's own price causes a movement along the curve. This is a change in quantity demanded or quantity supplied. A change in any other factor shifts the whole curve. This is a change in demand or supply.

Factors that shift demand: consumer income, prices of substitutes and complements, tastes and fashion, population size, and expectations about future prices. Factors that shift supply: costs of production, technology, taxes and subsidies, the prices of other goods the firm could make, and the number of sellers.

When a curve shifts, the equilibrium moves. An increase in demand shifts the demand curve right and raises both price and quantity. A fall in supply shifts the supply curve left, raising price and cutting quantity.

Key formulas to remember

Equilibrium condition: Quantity demanded = Quantity supplied; The equilibrium price is the price at which this holds.
Surplus (excess supply): Surplus = Quantity supplied − Quantity demanded, when price is above equilibrium; Price tends to fall.
Shortage (excess demand): Shortage = Quantity demanded − Quantity supplied, when price is below equilibrium; Price tends to rise.
Shift rules: Demand ↑ → price ↑, quantity ↑; Demand ↓ → price ↓, quantity ↓; Supply ↑ → price ↓, quantity ↑; Supply ↓ → price ↑, quantity ↓; Holds when only one curve shifts and the curves have normal slopes.
Own price change: Price change → movement along the curve, not a shift; Only non-price factors shift a curve.

How to solve Demand, Supply and Market Equilibrium questions

Use this method for any demand and supply question, whether it asks about a shift, a movement or a new equilibrium.

1Identify the good and the market affected.
2Find the event described, such as a tax, a rise in income or a new technology.
3Decide whether it affects buyers (demand) or sellers (supply). If it affects both, treat each separately.
4Check whether it is the good's own price changing. If so, it is a movement along the curve, not a shift.
5Decide the direction: increase shifts the curve right, decrease shifts it left.
6Work out the effect on equilibrium price and quantity using the shift rules.
7If numbers are given, calculate where quantity demanded equals quantity supplied, or compare quantities at a given price to find a surplus or shortage.
8Check your answer matches the wording of the question, for example price only, or quantity only.

Quickest way: Three-question shortcut

When to use it: Use this for multiple choice questions where you have about two minutes or less.

Ask: is the cause the good's own price? If yes, it is a movement along the curve.
If not, ask: does it change what buyers want or what sellers can produce? That tells you which curve shifts.
Sketch a quick cross in your head or on the scratch pad. Move the curve right for more, left for less, and read the new price and quantity.
For related goods: a substitute's price rise raises demand for this good; a complement's price rise lowers it.

Common mistakes in Demand, Supply and Market Equilibrium

Treating a change in the good's own price as a shift in the demand curve.
Students link any change in demand to price changes.
Fix: Own price changes only move you along the curve. Only other factors shift it.
Mixing up the effect of a rise in the price of a substitute and of a complement.
Both are 'related goods' and look similar.
Fix: Substitutes compete, so a rise in the price of one increases demand for the other. Complements are used together, so it reduces demand.
Shifting the wrong curve for a cost change or tax.
Students think of higher prices and jump to demand.
Fix: Costs, taxes, subsidies and technology affect sellers, so they shift supply.
Shifting a curve in the wrong direction, treating a rightward shift as a decrease.
Confusing the direction on the graph with the direction of price.
Fix: Right means an increase in quantity at every price. Left means a decrease.
Forgetting that both price and quantity change at a new equilibrium.
Focusing only on price.
Fix: State both price and quantity effects unless the question asks for one.

Worked examples

Example 1

A rise in consumer incomes increases demand for new cars, a normal good. State the effect on the equilibrium price and quantity.

Show the solution

Higher income affects buyers, so the demand curve shifts.
For a normal good, higher income increases demand, so the curve shifts right.
At the old price, quantity demanded now exceeds quantity supplied, creating a shortage.
The shortage pushes price up. As price rises, quantity supplied increases along the supply curve.
A new equilibrium forms at a higher price and higher quantity.

Answer: Equilibrium price rises and equilibrium quantity rises.

Example 2

At a price of $10, quantity demanded is 500 units and quantity supplied is 700 units. At $8, quantity demanded is 650 units and quantity supplied is 650 units. State the equilibrium price and describe the market at $10.

Show the solution

Equilibrium is where quantity demanded equals quantity supplied.
At $8, both are 650 units, so equilibrium price is $8.
At $10, quantity supplied is 700 and quantity demanded is 500.
Surplus = 700 − 500 = 200 units.
A surplus means sellers cut prices, moving the market towards $8.

Answer: Equilibrium price is $8 with 650 units. At $10 there is a surplus of 200 units.

Exam tips

Read the first words of the question for the cause. If it is the good's own price, the answer is a movement along the curve, not a shift.
Under time pressure, name the curve first, then the direction, then the result. This avoids most errors.
In multiple response questions, check each option against the shift rules separately before selecting.
Watch for 'change in demand' versus 'change in quantity demanded'. They are different answers.
When a number entry question gives quantities at prices, find where the two quantities match.

Practice questions from Microeconomic factors

Demand, Supply and Market Equilibrium in other exams

The same ground in other exams, if you are preparing for more than one or want another angle on it.

Demand, Supply and Market Equilibrium: frequently asked questions

What is the difference between a change in demand and a change in quantity demanded?

A change in quantity demanded is caused by a change in the good's own price and is shown as a movement along the demand curve. A change in demand is caused by another factor, such as income or tastes, and shifts the whole curve.

How is equilibrium price determined in a market?

It is the price at which quantity demanded equals quantity supplied. If price is higher, a surplus pushes it down. If price is lower, a shortage pushes it up.

What factors shift the demand curve?

Income, prices of substitutes and complements, tastes, population and expectations of future prices. Each changes how much buyers want at every price.

What factors shift the supply curve?

Costs of production, technology, taxes and subsidies, prices of other goods that producers could make, and the number of sellers.