Consumer and producer surplus: their definition and measurement on a demand-and-supply diagram, how they change when price or the curves shift, and their use in welfare analysis.

What this dot point is asking

Eduqas wants you to define consumer and producer surplus, measure them as areas on a demand-and-supply diagram, show how each changes when the price or the curves move, and use them to judge the welfare effects of market changes and government intervention. Surplus analysis is the language examiners expect when you evaluate who gains and who loses, so the diagram and the two definitions must be exact.

Consumer surplus

Consumer surplus exists because all consumers pay the same market price, yet many would have paid more. The demand curve traces the marginal valuation of each successive unit: the first units are worth a lot to consumers (high willingness to pay), while later units are worth less. The gap between that valuation and the single market price is the welfare consumers gain from being able to trade. The more inelastic demand is, the steeper the demand curve and the larger the consumer surplus for a given price.

Producer surplus

The supply curve traces the marginal cost of each successive unit. Low-cost producers (or the first units) would supply at a low price, but they too receive the market price, so they earn a surplus. Producer surplus is the producers' counterpart to consumer surplus: the welfare producers gain from selling at a price above their minimum acceptable price.

Total economic welfare

How surplus changes

A change in price or in the curves moves the surplus areas:

• A fall in price (for example from a rightward supply shift) raises consumer surplus and lowers the per-unit producer surplus, although more units are now sold.
• A rise in price (for example from a rightward demand shift) lowers consumer surplus per unit but tends to raise producer surplus.
• An indirect tax raises the price consumers pay and lowers the price producers keep, shrinking both surpluses and creating a deadweight welfare loss (the triangle of trades no longer made).
• A subsidy lowers the price, raising both surpluses, at the cost of the subsidy to taxpayers.

Examples in context

• Falling technology prices. As the cost of flat-screen televisions and smartphones has fallen, the equilibrium price has dropped and consumer surplus has risen sharply.
• Minimum prices (alcohol, agriculture). A minimum price above equilibrium raises producer surplus on units sold but cuts consumer surplus and creates a surplus of unsold output.
• Ticket resale. When face-value prices sit below equilibrium, the large consumer surplus is captured by resellers in the secondary market.

Try this

Q1. On a demand-and-supply diagram, shade and label consumer surplus and producer surplus at the market equilibrium. [3 marks]

• Cue. Consumer surplus = triangle below demand, above price; producer surplus = triangle above supply, below price; both meet at the equilibrium price and quantity.

Q2. Explain why total economic welfare is said to be maximised at the free-market equilibrium of a competitive market. [4 marks]

• Cue. Every unit where willingness to pay (demand) exceeds marginal cost (supply) is produced, so consumer plus producer surplus is as large as possible; producing more or less reduces it.