Buyers want to pay less. Sellers want to charge more. Somewhere between those two wishes is one price that keeps both sides happy enough to trade. That price is the market equilibrium, and it is the single most useful point on any diagram you will draw this year.
📘 What you need to know
A market is anywhere buyers and sellers meet — a corner shop, a car boot sale, an app on your phone.
Equilibrium is the price where quantity demanded equals quantity supplied. Nothing is left on the shelf and nobody is left waiting.
That price has a second name in the syllabus: the market-clearing price, because it clears the shelves.
Price too high → excess supply (a surplus). Price too low → excess demand (a shortage). Both are called disequilibrium.
A surplus pushes price down; a shortage pushes price up. Nobody orders this to happen — sellers just react to what they see.
On a diagram, equilibrium is where the demand and supply curves cross. Always label Pe and Qe.
What equilibrium really means
Think about a bakery. If the price of a loaf is set too high, trays of bread are still sitting there at closing time. If it is set too low, the bread is gone by nine in the morning and customers walk out annoyed. Somewhere in between there is a price where the last loaf is sold to the last customer who wants one. That is equilibrium.
Notice what equilibrium is not. It is not the price everyone is delighted with, and it is not “fair”. It is simply the price at which the plans of buyers and the plans of sellers match up.
The dashed lines are not decoration — examiners want to see them, because they show you can read both the price and the quantity off the diagram.
Draw the dashed lines every single time, even in rough work. Marks for “identifies equilibrium price and quantity” are given for the labelled lines, not for a neat crossing.
When the price is not at equilibrium
Any price above or below the equilibrium leaves one side of the market unhappy. Economists call this disequilibrium, and there are only two versions of it.
Same market, two wrong prices. The thick coloured bar is the size of the surplus or the shortage.
Price too high: excess supply
At a price above equilibrium, sellers happily bring lots of stock to the market but buyers back away. Quantity supplied is bigger than quantity demanded, so goods pile up unsold. Sellers hate unsold stock — it ties up money and, in the case of food or fashion, it goes off or goes out of style. So they cut the price. As the price falls, some buyers who said no now say yes (an extension of demand) and some sellers decide it is no longer worth supplying so much (a contraction of supply). The gap closes.
Price too low: excess demand
At a price below equilibrium, buyers rush in but sellers do not want to supply much. Quantity demanded is bigger than quantity supplied, so there is a shortage. Sellers notice their stock vanishing in an hour and realise they are charging too little. They raise the price. Higher prices push some buyers out (a contraction of demand) and tempt more sellers in (an extension of supply). Again the gap closes.
🧩 How a market clears itself — the four steps to write in an answer
Spot the disequilibrium. Compare QD and QS at the given price.
Say what sellers see. Unsold stock, or empty shelves.
Say what sellers do. Lower the price if there is a surplus, raise it if there is a shortage.
Follow both curves. One side extends, the other contracts, until QD = QS again at PeQe.
Speed matters in the real world. A market stall clears its disequilibrium in an afternoon by slashing prices at closing time. The housing market can take years, because houses take years to build and sellers hold out for the price they wanted. Same theory, very different timescale — a good evaluation point.
Shortage and surplus are measured at a stated price. If a question says “at $8 there is a shortage of 40 units”, it wants the horizontal gap between the two curves at $8 — nothing else.
Finding equilibrium with numbers
Paper 1 and Paper 2 often give you a schedule or two equations instead of a picture. The method is the same: set demand equal to supply, solve, then check your answer by putting the price back into both equations.
WORKED EXAMPLE
In the market for cinema tickets, QD = 120 − 2P and QS = 20 + 3P, where P is in dollars. Find the equilibrium price and quantity.
Step 1: equilibrium means QD = QS120 − 2P = 20 + 3PStep 2: get the P terms on one side120 − 20 = 3P + 2P → 100 = 5P → P = 20Step 3: put P back in to get the quantityQD = 120 − 2(20) = 80 QS = 20 + 3(20) = 80 ✓Equilibrium: P = $20, Q = 80 ticketsBoth equations giving 80 is your proof that the answer is right.
WORKED EXAMPLE
Using the same market, the cinema decides to charge $10 a ticket. Explain what happens.
Step 1: work out both quantities at $10QD = 120 − 2(10) = 100 QS = 20 + 3(10) = 50Step 2: compare them100 > 50, so there is excess demand of 50 ticketsStep 3: say what the market does next
Seats sell out fast, so the cinema raises the price. Demand contracts, supply extends, and the shortage shrinks until the price is back at $20.
Shortage of 50 tickets → price rises towards $20$10 is below equilibrium, so a shortage is exactly what you should expect.
💡 Exam tip
Label everything: both axes with units, both curves (D and S), and Pe and Qe with dashed lines. Unlabelled diagrams lose easy marks.
Remember the rule: shortage below equilibrium, surplus above it. Sketch a tiny diagram in the margin if you blank.
Use the right words. Curves shift when a non-price factor changes; you only move along a curve when the price changes.
Measure a surplus or shortage horizontally, as a quantity. It is never a price gap.
When you solve equations, always substitute your price back into both equations as a check.
Explain the adjustment as a story: sellers see something, sellers do something, both curves respond. That is what “explain” marks are for.
⚠ Common mix-up
Mixing up demand and quantity demanded. A price change does not shift the demand curve. It moves you along it.
Saying “supply falls” when the price rises after a shortage. Supply extends. The curve stays exactly where it is.
Thinking equilibrium is the “best” or “fairest” price. It is only the price where the two plans match. Someone can still be priced out.
Drawing the shortage or surplus as a vertical gap. It is horizontal, because it is a quantity.
Forgetting units on the axes. “Price” alone is not enough; write “Price ($)” and “Quantity (tickets per week)”.
Assuming markets clear instantly. Some do, some take years. Saying which, and why, is worth evaluation marks.
Up next: How the Price Mechanism Allocates Resources — now that you can find equilibrium, we look at the three jobs prices are quietly doing while they get there.
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