Put the demand curve and the supply curve on the same axes and something rather clever happens. Nobody is in charge, nobody announces the right price, and yet the market finds one anyway. This page is about how it does that, and what goes wrong when the price is stuck at the wrong level.
📚 What you need to know
A market is anywhere buyers and sellers meet, physical or online.
Equilibrium is where quantity demanded = quantity supplied. There is no pressure on the price to change.
The equilibrium price is also called the market-clearing price: everything offered is sold.
Any other price gives disequilibrium.
Price below equilibrium → excess demand (a shortage) → price is bid up.
Price above equilibrium → excess supply (a surplus) → price is cut down.
Markets clear at different speeds: clothing in days, housing in years.
Where the two curves meet
The demand curve says what buyers will take at each price. The supply curve says what sellers will offer. There is exactly one price where those two plans match, and that is equilibrium.
The market clears. Sellers are happy with the rate of sales and buyers who valued the good at that price have all been served, so nothing pushes the price either way.
Equilibrium does not mean fair, and it does not mean everyone got what they wanted. Plenty of people wanted the good and were priced out. It only means there is no pressure on the price to move. Keep that distinction ready for evaluation questions.
Disequilibrium 1: price too low, excess demand
Suppose the price sits below equilibrium. Buyers love it, so quantity demanded is large. Sellers do not, so quantity supplied is small. The gap between them is a shortage.
Watch the two movements as the price rises: buyers slide up the demand curve and sellers slide up the supply curve. Both are movements along, not shifts.
🧩 How a shortage clears itself
Stock runs out. Sellers see product flying off the shelves at a price that is obviously too low.
Some buyers go unserved and would happily pay more, so sellers start raising the price.
Quantity demanded contracts as the higher price puts some buyers off.
Quantity supplied extends as the higher price makes extra production worth it.
The gap closes and the market settles back at P(e) and Q(e).
Disequilibrium 2: price too high, excess supply
Every end-of-season sale you have ever seen is this diagram. The shop set a price above what buyers would pay, watched stock sit there, and cut it.
The rule worth memorising. Shortages happen below equilibrium; surpluses happen above it. If you can only remember one line from this page, make it that one.
How fast do markets clear?
The diagram makes it look instant. In reality, how quickly a market gets back to equilibrium varies enormously.
Market
Roughly how long
Why
Fresh produce at a street market
Hours
Sellers can cut prices instantly and stock will not keep
Clothing retail
Days to weeks
Sales and markdowns clear stock quickly, but stock has to be reordered
Housing
Months to years
New homes take years to build, and sellers resist cutting asking prices
Skilled labour
Years
Training a nurse or an engineer takes years, so shortages persist
This table is pure evaluation gold. Any time you write “the market will return to equilibrium”, follow it with “but in this market that may take years, because supply cannot respond quickly”. That single sentence lifts an answer.
Worked examples
WORKED EXAMPLE 1
In the market for concert tickets, Qd = 900 − 10P and Qs = 100 + 10P (P in dollars). Find the equilibrium price and quantity, and state what happens at a price of $30. [4]
Step 1: set Qd = Qs900 − 10P = 100 + 10PStep 2: solve for P800 = 20P, so P = 40Step 3: substitute backQs = 100 + 10(40) = 500 ticketsStep 4: test $30
Qd = 900 − 300 = 600; Qs = 100 + 300 = 400 → excess demand of 200 ticketsP(e) = $40, Q(e) = 500; at $30 a shortage of 200Always check your answer by putting P back into both equations. They must give the same number.
WORKED EXAMPLE 2
A shop prices umbrellas at $18 and finds them sitting unsold for weeks. Using a diagram, explain what is happening and what the shop is likely to do. [4]
Step 1: name the situation
The price is above equilibrium, so there is excess supply — a surplus.
Step 2: read it off the diagram
At $18, Qs is greater than Qd. The horizontal gap between the two curves is the surplus.
Step 3: what the shop does
It cuts the price to shift stock, because unsold umbrellas earn nothing and take up space.
Step 4: how the gap closes
As the price falls, QS contracts and QD extends — both movements along their own curves — until the market clears.
Excess supply, so the price is cut back to P(e)Do not shift either curve. Nothing changed except the price the shop chose.
WORKED EXAMPLE 3
A city has a long-running shortage of rental flats even though rents are high and rising. Explain why the market has not cleared. [4]
Step 1: describe the mechanism that should work
Excess demand pushes rents up, which should extend quantity supplied and contract quantity demanded.
Step 2: explain why supply cannot respond
New housing takes years to plan and build, and land is limited, so QS extends very slowly.
Step 3: explain why demand does not fall away
Housing is a necessity with few substitutes, so higher rents shrink QD only a little.
Step 4: conclude
The market is moving towards equilibrium but very slowly, so the shortage persists.
Slow supply response, not a broken marketA great place to mention that time period matters: short run and long run behave differently.
💡 Exam tip
Label P(e) and Q(e) with dashed lines to both axes on every equilibrium diagram.
Mark the disequilibrium price as P₁ and show the horizontal gap between Qd and Qs.
Say the gap is horizontal. Shortages and surpluses are measured in quantity, not in price.
Describe both movements when a price adjusts: one along demand, one along supply.
Use “excess demand” and “excess supply”, then add “shortage” or “surplus” in brackets.
Add a time-lag point whenever you say a market will return to equilibrium.
⚠ Common mix-up
Shifting a curve to fix disequilibrium. The price adjusts; the curves stay put.
Getting the direction backwards. Low price = shortage. High price = surplus.
Measuring the gap vertically. It is the horizontal distance between Qd and Qs.
Thinking equilibrium means everyone is satisfied. Buyers who could not afford the price are still shut out.
Assuming markets clear instantly. Housing and skilled labour can take years.
Confusing a shortage with scarcity. Scarcity is permanent; a shortage is a temporary disequilibrium at one price.
Up next: How Prices Signal, Ration and Motivate — the three jobs a price quietly does every time it moves.
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