Chapter 4 found the equilibrium; this one asks whether it was any good. Two triangles turn the diagram into a measure of value created — and then show exactly how much a well-meant price control burns.
This is Chapter 1's single buyer and seller, scaled up to a whole market. The curves you already know turn out to be lists of valuations.
| Term | Is | On the diagram |
|---|---|---|
| Willingness to pay | The most a buyer would give up for a unit | The height of the demand curve at that unit |
| Consumer surplus | Willingness to pay − price actually paid | Area under demand and above the price |
| Willingness to accept | The least a seller would take, i.e. their cost | The height of the supply curve at that unit |
| Producer surplus | Price received − cost | Area above supply and below the price |
The demand curve slopes down because buyers are sorted: the keenest buyer is served first, and each later unit goes to someone who values it less. Supply slopes up for the mirror reason — the cheapest producer goes first. So the vertical gap between the curves at any unit is the value that unit creates, exactly as in Chapter 1.
Since both areas are triangles here, you can usually compute them with
½ × base × height — base is the quantity, height is the distance from the price to the
curve's intercept.
Total surplus = consumer surplus + producer surplus. The competitive equilibrium maximises it, and the reason is a marginal argument you already own from Chapter 1.
Every unit where willingness to pay exceeds cost is worth making — it creates surplus. Every unit beyond that destroys surplus, because it costs more to make than anyone will pay. The quantity where the curves cross is precisely where those two conditions meet. It is MB = MC again, drawn at market scale.
Maximum total surplus says nothing about how that surplus is split, or about who could afford to participate at all. "The market outcome is efficient" and "the market outcome is just" are different claims, and the second is normative. An exam answer that slides from one to the other is making a value judgement it has not earned.
A control only does anything if it binds. When it does, trades that would have created value stop happening, and that lost value is deadweight loss.
| Control | Binds when | Produces | Example |
|---|---|---|---|
| Price ceiling — a legal maximum | Set below equilibrium | A shortage | Rent control |
| Price floor — a legal minimum | Set above equilibrium | A surplus | Minimum wage |
A ceiling is a maximum, so it only matters if it sits below where the price wanted to be — a ceiling above the equilibrium is non-binding and changes nothing. A floor is a minimum, so it bites from above. Draw the line before you reason about it and the direction stops being a guess.
Deadweight loss is the surplus that simply disappears — not transferred to anyone, just not created, because mutually beneficial trades were blocked. Watch the triangle open up.
Four shapes, and the error each is built around.
| Asked as | Do this | Trap |
|---|---|---|
| "Calculate consumer surplus" | ½ × quantity × (choke price − price paid) | Using the price instead of the distance from the price to the intercept. |
| "Is this ceiling binding?" | Compare it with P*. Below → binding. | Assuming every control binds. A ceiling above P* does nothing at all. |
| "Who gains from the ceiling?" | Buyers who still get the good. Buyers shut out lose; sellers lose. | Saying "consumers gain" flatly — some consumers are rationed out entirely. |
| "Find the deadweight loss" | ½ × (Q* − Q traded) × (vertical gap between the curves at Q traded) | Counting a transfer as a loss. Surplus moved from sellers to buyers is not destroyed. |
A binding ceiling does two different things. It transfers surplus from producers to the consumers who still buy — that surplus still exists, just in different hands. And it destroys the surplus on units no longer traded — that is the deadweight loss, and nobody receives it. Only the second is a loss to society. Confusing the two is the most common error in the whole chapter.
Shuffled. Two items are area calculations — draw the triangle before you reach for a formula.
| Model | Genuinely computed | Simplified |
|---|---|---|
| The two triangles | Equilibrium from the linear system, then consumer and producer surplus as the exact areas between each curve and the price, and the buyers' share of the total. | Linear curves, so the areas really are triangles. Curved demand needs calculus for the same idea. |
| Price controls | Whether the control binds, the quantity traded as the short side, both surpluses at the controlled price by integration, and deadweight loss as the drop in total surplus. | No black market, no queueing cost, no quality degradation — all real responses to a binding ceiling, none of them on this diagram. |