Demand, Supply and the Price Mechanism (A Level Economics)

Demand and supply (A Level Economics) is the first diagram you draw and the one you will draw most. Demand is what buyers want at each price. Supply is what sellers offer at each price. Where the two meet is the equilibrium, and the price mechanism is the story of how the market gets there. This page gives you the three diagrams every board sets, the shift table, consumer and producer surplus with the areas worked out, the three jobs of the price mechanism, a worked example in pounds and three exam-style questions with the answers derived.

Spec map: where demand and supply sits on your board

Section numbers are from the AQA specification (the PDF numbers these sections 4.1.x, the web version 3.1.x), the Edexcel specification (Issue 2), the OCR specification (v3.0, June 2026) and the Cambridge syllabus for 2026 to 2028, all checked in September 2026. Elasticity has its own page: PED, YED, XED and PES. Indirect taxes sit with market failure and price controls with maximum and minimum prices.

What you must be able to do

• Draw demand and supply curves with labelled axes, and explain why each slopes the way it does.
• Tell a movement along a curve from a shift, and name the causes of each shift.
• Find the equilibrium price and quantity, on a diagram and from two equations.
• Show excess demand and excess supply, and explain how market forces remove them.
• Draw and calculate consumer surplus and producer surplus, before and after a shift.
• Explain the rationing, signalling and incentive functions of the price mechanism with one example.

Demand: the law and the shifts

Demand is the quantity buyers are willing and able to buy at each price over a period. Price goes up. Quantity demanded falls. That is the law of demand, and it is why the curve slopes down. Two reasons the exam accepts: each extra unit is worth less to the buyer than the last (diminishing marginal utility), and a higher price means the buyer’s income stretches less far.

A change in the good’s own price is a movement along the curve. Everything else is a shift. The exam calls the shifters the conditions of demand.

Supply: the law and the shifts

Supply is the quantity sellers are willing and able to sell at each price. Price goes up. Quantity supplied rises. A higher price covers the higher cost of the extra units and pulls new sellers in. So the curve slopes up. Again, the good’s own price moves you along the curve. Anything that changes the cost of making a unit shifts it.

Equilibrium: where the market clears

Equilibrium is the price at which quantity demanded equals quantity supplied. Nothing more. Draw both curves, mark the crossing, drop the dashed lines to each axis and label P* and Q*. That is the base diagram for every question on this page.

Below the equilibrium price there is excess demand. Buyers want more than sellers offer. The price rises. Above it there is excess supply. Sellers offer more than buyers want. The price falls. The market clears itself, and that self-correction is the price mechanism at work.

Consumer surplus and producer surplus

Consumer surplus is the gap between what buyers would have paid and what they did pay. On the diagram it is the triangle under the demand curve and above the price. Producer surplus is the gap between the price sellers got and the lowest price they would have accepted. It is the triangle above the supply curve and below the price. Together they measure the welfare the market creates.

Shifting the curves: the two diagrams the exam wants

Every “explain the effect on the market for…” question is one of two shifts. Decide which curve moves, draw the new curve, mark the new equilibrium, then read off the change in price and quantity.

A rise in demand raises both price and quantity. A fall in demand lowers both. Look at the size. Demand rose by 10 at every price, but quantity only rose by 5. The higher price rationed away the other 5.

A fall in supply raises price and lowers quantity. A rise in supply does the opposite. Look at the size again. Costs rose £10 a unit, but the price rose only £5. Buyers cut back, so sellers could not pass the whole rise on. Who pays what share is the elasticity page’s job.

The price mechanism: rationing, signalling and incentive

The price mechanism is the market’s way of allocating resources with nobody in charge. Edexcel and Cambridge name its three functions in the specification, and every board expects them in a chain of reasoning. Learn all three with the same example, and use them in order.

The same three jobs run in reverse when demand falls. The price drops, which signals a glut, removes the incentive to produce, and stops rationing because nothing is scarce.

Worked example — wireless earbuds at a town’s phone shops

Small round numbers, in pounds, with Q the pairs sold a week across the town.

1. Demand: P = 60 − Q. Supply: P = 20 + Q.
2. Equilibrium: 60 − Q = 20 + Q, so Q* = 20 pairs and P* = £40.
3. Consumer surplus = ½ × 20 × (60 − 40) = £200. Producer surplus = ½ × 20 × (40 − 20) = £200. Total surplus £400.
4. A viral video sends demand up by 10 pairs at every price. New demand: P = 70 − Q.
5. New equilibrium: 70 − Q = 20 + Q, so Q = 25 and P = £45. Price up £5, quantity up 5. Only half of the extra 10 pairs demanded get sold. The higher price rations away the rest.
6. Why the price had to rise: at the old £40, buyers now want 30 pairs but shops still supply 20. A shortage of 10. Shops raise the price, and keep raising it until the shortage is gone at £45.
7. Surplus after: consumer surplus = ½ × 25 × (70 − 45) = £312.50, producer surplus = ½ × 25 × (45 − 20) = £312.50. Both up by £112.50. More trades happen at a price both sides accept.

Change the 10 in step 4 and the whole chain moves. That is Edexcel 1.2.6’s “real-world situations” in numbers.

Exam-style questions

Q1: Costs rise by £10 a pair, so supply becomes P = 30 + Q while demand stays at P = 60 − Q. Calculate the new equilibrium, the new consumer and producer surplus, and the change in total surplus. (6 marks)
A: 60 − Q = 30 + Q, so Q = 15 at P = £45 (2). Consumer surplus = ½ × 15 × (60 − 45) = £112.50 (1). Producer surplus, from the new supply curve, = ½ × 15 × (45 − 30) = £112.50 (1). Total surplus falls from £400 to £225, a loss of £175 (1). Only £5 of the £10 cost rise reached the price, because buyers cut back (1).

Q2: Using the original curves (P = 60 − Q and P = 20 + Q), calculate the excess demand or excess supply at £30 and at £50, and explain how the market removes each. (6 marks)
A: At £30: quantity demanded 30, quantity supplied 10, excess demand of 20 (2). Sellers raise the price, demand contracts and supply extends until they meet at £40 (1). At £50: quantity demanded 10, quantity supplied 30, excess supply of 20 (2). Unsold stock forces the price down to £40 (1).

Q3: Electric scooters become cheaper to hire. Using a diagram, discuss the likely effect on the market for bus journeys in a town. (10 marks)
A: Define the terms, use the diagram, then weigh up the outcome.

• Knowledge and application: bus journeys and scooter hire are substitutes for short trips. A cheaper substitute shifts the demand for bus journeys left, D₁ to D₂.
• Analysis: at the old fare there is excess supply of seats, so fares fall or services are cut. Lower price, lower quantity, and both surpluses in the bus market fall. Signalling: fewer buses run. Incentive: fewer drivers hired.
• Evaluation: the size depends on cross elasticity of demand. Scooters replace short dry-weather trips, not the school run in January. Capped fares mean the adjustment comes through quantity, not price. Judgement: the direction is clear, the size is not.

Examiner’s checklist: the mistakes that lose demand and supply marks

• A movement drawn as a shift. Own price moves you along the curve. Only the conditions of demand or supply shift it.
• The wrong curve moved. The buyers’ side is demand. The cost of making the good is supply. A cheaper substitute is demand, not supply.
• No new equilibrium. Mark E₂, drop the dashed lines, and say what happened to P and Q.
• Surplus measured from the wrong curve. After a supply shift, producer surplus sits above the new supply curve.
• The price mechanism without the three words. Rationing, signalling, incentive. Name them, then show each in the example.
• A diagram with no words. “At £40 there is now a shortage of 10, so price rises” is where the analysis marks are.

Key takeaways

• Demand slopes down, supply slopes up, the crossing is the equilibrium. Excess demand pushes price up, excess supply pushes it down.
• Own price moves you along a curve. The conditions of demand and supply shift it.
• A rise in demand raises P and Q. A fall in supply raises P and lowers Q. Draw it, mark both equilibria, read off the change.
• Consumer surplus: the triangle under D above P. Producer surplus: the triangle above S below P. Both ½ × base × height.
• The price mechanism rations, signals and gives incentives. Use all three in a chain.

FAQ

Q: What is the difference between a movement along the demand curve and a shift?
A: A movement along the curve comes from a change in the good’s own price: an extension when price falls, a contraction when it rises. A shift comes from anything else: income, other prices, tastes, population, expectations. Supply works the same way, shifted by costs, technology, taxes and subsidies.

Q: How do I calculate consumer surplus and producer surplus at A Level?
A: With straight-line curves each is a triangle: ½ × base × height. The base is the quantity sold. The height is the demand intercept minus the price for consumer surplus, and the price minus the supply intercept for producer surplus. In figure 1 both come to £200.

Q: What are the three functions of the price mechanism at A Level?
A: Rationing, signalling and incentive. Price allocates scarce goods to those willing to pay, tells buyers and sellers the market has changed, and rewards extra supply. Edexcel names them in 1.2.7 and Cambridge in 2.4.4.

Q: Which demand and supply diagrams do I need for the exam?
A: Three: equilibrium with consumer and producer surplus, a demand shift and a supply shift. The A Level economics diagrams page has all three with a one-line “use it when”.

Q: How many marks is the demand and supply diagram worth?
A: Usually 2 to 4 inside a 4 to 12-mark question, for the axes, curves, shift and both equilibria. The rest is for explaining and evaluating it. Board detail is on the AQA, Edexcel and OCR pages.

Q: Where does demand and supply come up again later in the course?
A: Everywhere in microeconomics: elasticity, indirect taxes, price controls, the labour market. The revision hub lists every topic that builds on it.

Turn the diagram into marks

If the shift still goes the wrong way under exam pressure, or the surplus triangle ends up on the wrong side of the price, one lesson fixes that. Send your exam board and year; a first session time comes back within a day. See the A Level economics tutor page for how lessons are run and priced.