Fundamentals of Business Economics and Management · Theory of Demand and Supply

Market Equilibrium and Price Determination Explained

Updated 10 October 2026 · Fact-checked

Market equilibrium is the price at which quantity demanded equals quantity supplied. To solve a numerical, set Qd = Qs and find the price, then put it back in either equation to get the quantity. For shifts, move the curve first, then compare the new and old equilibrium.

Understand Market Equilibrium and Price Determination

Buyers want a low price. Sellers want a high price. A market brings them together, and the price settles where both sides agree on the quantity. That point is called equilibrium.

At the equilibrium price, the quantity buyers want to buy equals the quantity sellers want to sell. This common quantity is the equilibrium quantity. On a graph, it is the point where the demand curve cuts the supply curve.

If the price is above equilibrium, sellers offer more than buyers want. This is excess supply (surplus). Unsold stock builds up, so sellers cut prices. If the price is below equilibrium, buyers want more than sellers offer. This is excess demand (shortage). Buyers compete, so the price rises. In both cases the price moves back towards equilibrium.

Equilibrium changes when a curve shifts. A shift in demand comes from factors other than price, such as income, tastes or the price of related goods. A shift in supply comes from factors like input costs, technology or taxes. A change in the good's own price only moves you along a curve and does not shift it.

Remember four basic results. Demand rises (right shift): price and quantity both rise. Demand falls: both fall. Supply rises (right shift): price falls, quantity rises. Supply falls: price rises, quantity falls.

Key formulas to remember

Equilibrium condition: Qd = Qs; Solve this equation for P to get the equilibrium price. Then substitute P into either equation for the equilibrium quantity.
Excess demand: Excess demand = Qd − Qs (when Qd > Qs); Occurs at a price below equilibrium. Pressure on price is upward.
Excess supply: Excess supply = Qs − Qd (when Qs > Qd); Occurs at a price above equilibrium. Pressure on price is downward.
Effect of a demand shift (supply unchanged): Demand ↑ → P ↑, Q ↑; Demand ↓ → P ↓, Q ↓; Price and quantity move in the same direction.
Effect of a supply shift (demand unchanged): Supply ↑ → P ↓, Q ↑; Supply ↓ → P ↑, Q ↓; Price and quantity move in opposite directions.

How to solve Market Equilibrium and Price Determination questions

Use this method for numerical and theory questions on equilibrium.

1Identify what is given: demand and supply equations, a table of prices and quantities, or a description of a change.
2For equations, write Qd = Qs and solve for P. This is the equilibrium price.
3Put this price into the demand or supply equation to find the equilibrium quantity. Check using the other equation.
4For a given price, compare Qd and Qs. If Qd > Qs there is excess demand. If Qs > Qd there is excess supply.
5For a shift question, decide which curve moves and in which direction. Ignore the good's own price as a cause.
6Apply the four-result rule, and if both curves shift, work out the effect on price and quantity separately.
7Pick the option that matches your result and check that the direction of price and quantity is correct.

Quickest way: Substitute the options or use the shift rule

When to use it: Use when you have 1 minute per question and the options are numbers or direction words.

For a numerical, subtract the equations: if Qd = a − bP and Qs = c + dP, then P = (a − c) ÷ (b + d).
Check the quantity by putting P in one equation. A quick check in the other catches slips.
For a table, find the row where Qd equals Qs. No calculation is needed.
For shifts, remember: demand moves P and Q together, supply moves them opposite.
If both curves shift, the effect on one variable is certain and the other is unclear, so eliminate options that claim certainty about both.

Common mistakes in Market Equilibrium and Price Determination

Treating a price change as a shift in the curve
Students link any change in the market to a shifting curve.
Fix: A change in the good's own price causes movement along a curve. Only other factors shift the curve.
Mixing up the effect of demand and supply shifts on price
Students memorise results without picturing the graph.
Fix: Demand shift: P and Q move the same way. Supply shift: they move opposite ways.
Calling a surplus at a low price
Students forget which side of equilibrium they are on.
Fix: Price above equilibrium means excess supply. Price below equilibrium means excess demand.
Algebra errors when solving Qd = Qs
Signs go wrong when moving terms across the equals sign.
Fix: Collect P terms on one side and numbers on the other, then verify that Qd equals Qs at your answer.
Claiming both price and quantity change definitely when both curves shift
Students apply single-shift rules to a double shift.
Fix: When both shift, one variable is certain and the other depends on which shift is larger.

Worked examples

Example 1

The demand for a product is Qd = 100 − 5P and supply is Qs = 20 + 3P, where P is in ₹. Find the equilibrium price and quantity.

Show the solution

Set Qd = Qs: 100 − 5P = 20 + 3P.
Collect terms: 100 − 20 = 3P + 5P, so 80 = 8P.
P = 80 ÷ 8 = 10.
Quantity from demand: 100 − 5 × 10 = 50.
Check with supply: 20 + 3 × 10 = 50. It matches.

Answer: Equilibrium price is ₹10 and equilibrium quantity is 50 units.

Example 2

Using Qd = 100 − 5P and Qs = 20 + 3P, what happens at a price of ₹14?

Show the solution

Qd at ₹14 = 100 − 5 × 14 = 100 − 70 = 30.
Qs at ₹14 = 20 + 3 × 14 = 20 + 42 = 62.
Qs is more than Qd, so there is excess supply.
Excess supply = 62 − 30 = 32 units.
Price is above the equilibrium of ₹10, so it will tend to fall.

Answer: There is excess supply of 32 units, and the price will tend to fall.

Exam tips

Questions often ask for the direction of change. Decide which curve shifts, then use the four-result rule.
In numericals, always check the answer in both equations. It takes seconds.
Read whether the question says price is above or below equilibrium before naming excess demand or excess supply.
Watch for wording that separates movement along a curve from a shift of the curve.
If both curves shift, expect an option saying one variable is uncertain. That is often correct.

Practice questions from Theory of Demand and Supply

Market Equilibrium and Price Determination in other exams

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

Market Equilibrium and Price Determination: frequently asked questions

What is equilibrium price?

It is the price at which the quantity buyers want equals the quantity sellers offer. At this price there is no shortage or surplus, so there is no pressure for the price to change.

What is the difference between excess demand and excess supply?

Excess demand means quantity demanded is more than quantity supplied, which happens below the equilibrium price. Excess supply means quantity supplied is more than quantity demanded, which happens above the equilibrium price.

What happens to equilibrium if demand increases?

If supply stays the same, the demand curve shifts right. Equilibrium price rises and equilibrium quantity also rises.

What happens if supply increases?

If demand stays the same, the supply curve shifts right. Equilibrium price falls and equilibrium quantity rises.