What is supply and demand? The curves, the equilibrium and worked examples
Demand describes how much buyers want at each price and slopes downwards; supply describes how much sellers will offer at each price and slopes upwards. Equilibrium is the price where the two quantities are equal and the market clears. The distinction that carries the most marks in any first-year economics exam is between a movement along a curve, caused by a change in the good's own price, and a shift of the whole curve, caused by anything else.
Key points
- Price changes move you along a curve; everything else shifts the curve.
- Equilibrium is where quantity demanded equals quantity supplied.
- Above equilibrium there is surplus; below it there is shortage.
- Elasticity says how much quantity responds to a price change.
- A price ceiling below equilibrium creates a permanent shortage, not a lower price for everyone.
What is demand?
Demand is the quantity of a good buyers are willing and able to purchase at each possible price, holding everything else constant. It is a whole schedule of price-quantity pairs, not a single number, and the clause "holding everything else constant" is what makes the rest of the model work.
The demand curve slopes downwards for two reasons. The substitution effect means that when a good gets more expensive, some buyers switch to alternatives; the income effect means the same money now buys less, so real purchasing power falls. Alfred Marshall (1890) set out this apparatus in Principles of Economics, and the diagram has barely changed since.
What is supply?
Supply is the quantity producers are willing to sell at each price, again holding everything else constant. It slopes upwards because a higher price covers the cost of output that was previously unprofitable, and because it makes it worth bringing less efficient capacity into use.
Supply is more time-dependent than demand. In the very short run a bakery cannot make more bread today whatever the price, so supply is nearly vertical; over a year it can hire and buy ovens, and the curve becomes much flatter. When an exam question says "in the short run", it is usually telling you which curve to draw steeply.
What is equilibrium?
Equilibrium is the price at which the quantity demanded equals the quantity supplied, so there is neither a queue of unsatisfied buyers nor a pile of unsold stock. On a diagram it is the intersection of the two curves.
The market reaches it by trial and error, not by calculation. If the price is above equilibrium, sellers hold surplus stock and cut prices; if it is below, buyers compete and the price is bid up. This is why the model predicts a direction of change even when nobody in it knows the equilibrium price.
How do you calculate equilibrium with numbers?
Set the two equations equal and solve for price. Take demand Qd = 120 − 3P and supply Qs = 20 + 2P, where P is the price in euros:
| Step | Working | Result |
|---|---|---|
| Set Qd = Qs | 120 − 3P = 20 + 2P | |
| Collect terms | 100 = 5P | |
| Solve for price | P = 100 / 5 | P = 20 |
| Substitute back | Qd = 120 − 3(20) | Q = 60 |
| Check | Qs = 20 + 2(20) | Q = 60 ✓ |
Always substitute into both equations. Getting the same quantity from each is the only check that catches an arithmetic slip, and it takes ten seconds.
What is the difference between a shift and a movement?
A change in the good's own price moves you along the curve; a change in anything else shifts the whole curve. Mixing these up produces the single most common wrong answer in introductory economics — "higher prices reduce demand" describes a movement, not a fall in demand.
Demand shifts when incomes change, when the price of a substitute or complement changes, when tastes change, when the population changes, or when buyers expect a future price change. Supply shifts when input costs change, when technology improves, when taxes or subsidies change, or when the number of sellers changes. Coffee becoming more expensive moves you along the tea demand curve's neighbour and shifts the tea demand curve itself — one good's price is another good's "everything else".
What is elasticity and why does it matter?
Price elasticity of demand measures how much quantity responds to a price change: the percentage change in quantity divided by the percentage change in price. If the answer is greater than one in absolute terms, demand is elastic; less than one, inelastic.
It matters because it determines what happens to revenue. Raise the price of an inelastic good and revenue rises, because the quantity lost is proportionally smaller than the price gained; raise the price of an elastic good and revenue falls. This is why insulin and cinema tickets are priced along completely different logics, and why a tax on tobacco raises revenue while a tax on a particular restaurant chain would mostly just move customers next door.
What happens with a price ceiling or a price floor?
A price ceiling set below equilibrium creates a shortage, because at the lower price buyers want more than sellers will supply. Rent control is the standard example: the rent is lower for whoever holds a contract, and there are fewer flats available for everyone else. The good is then rationed by something other than price — queues, waiting lists, or who you know.
A price floor set above equilibrium creates a surplus, which is what agricultural support schemes have historically produced. Note the conditional in both statements: a ceiling above the equilibrium price and a floor below it change nothing at all, and exam questions regularly test exactly that case. If prices in general are rising across the whole economy, that is a different mechanism — see our explainer on inflation.
How should you revise supply and demand?
Draw the diagram from memory before reading the answer, because the skill being examined is producing the diagram under pressure, not recognising it. Take a scenario, sketch both curves, shift the right one, mark the new equilibrium, and only then check.
Build a bank of scenarios and work through them cold: a poor harvest, a subsidy for electric cars, a new competitor entering, a rise in the minimum wage, a fall in consumer confidence. Retrieval practice of this kind is what Dunlosky (2013) found to be among the highest-utility study techniques, and the effect is stronger when you spread the attempts out rather than doing them all in one sitting (Cepeda et al., 2006). Turning a lecture PDF into a quiz of such scenarios gives you the retrieval attempts; interleaving them with other topics is what makes the practice stick.
Frequently asked questions
What is the law of demand?
The law of demand states that, all else equal, the quantity demanded falls as price rises. The "all else equal" is not decoration — the law says nothing about what happens when incomes or tastes also change, which is why real-world data often shows price and quantity rising together.
Can a demand curve slope upwards?
In rare cases, yes. Giffen goods and Veblen goods are the textbook exceptions, where a higher price raises quantity demanded because of income effects on a staple, or because price signals status. They are worth naming in an essay as exceptions and never worth assuming in a numerical question.
What shifts the supply curve?
Input costs, technology, taxes and subsidies, the number of sellers, and producer expectations. Weather counts as an input cost for agriculture. The price of the good itself is the one thing that never shifts the supply curve — it moves you along it.
What is consumer surplus?
Consumer surplus is the difference between what buyers were willing to pay and what they actually paid, shown as the area between the demand curve and the price line. Producer surplus is the mirror image below the price line. Together they measure the total gain from trade at that price.
Why do queues form under rent control?
Because a price ceiling below equilibrium leaves demand higher than supply, and something other than price has to do the rationing. Waiting lists, informal payments and personal connections are what fill the gap — the shortage does not disappear just because the price is not allowed to clear it.