When you buy something for less than you would have paid, you have gained. When a seller gets more than the minimum they would have accepted, they have gained too. Both gains can be measured, both show up as triangles on the market diagram, and both turn out to be at their largest exactly at equilibrium.
📚 What you need to know
Consumer surplus = what a buyer was willing to pay − what they actually paid.
Producer surplus = what a seller actually receives − the minimum they were willing to accept.
On a diagram, consumer surplus is the area below the demand curve and above the price.
Producer surplus is the area above the supply curve and below the price.
Community (social) surplus = consumer surplus + producer surplus.
Community surplus is maximised at equilibrium, where MB = MC — this is allocative efficiency.
Areas are triangles, so use area = ½ × base × height.
What the two surpluses mean
Imagine you would happily have paid $18 for a cinema ticket. The price is $12. You have gained $6 of value that never appears on the receipt. That $6 is your consumer surplus.
Now the cinema. The minimum it would have accepted for that seat, given its costs, was $7. It got $12. That $5 is producer surplus.
The two definitions
Consumer surplus = willing to pay − actually paid
Producer surplus = actually received − willing to accept
Both sides win. Trade is not one person’s gain and another’s loss. At $12 the buyer gained $6 and the seller gained $5, from the same ticket. That is the whole reason voluntary exchange happens.
Finding them on the diagram
Every point on the demand curve shows what somebody was willing to pay. Every point on the supply curve shows the minimum somebody would accept. Draw the price as a horizontal line and the two surpluses appear as triangles either side of it.
Point A is the highest price anyone would have paid; point C is the lowest any seller would have accepted. Everyone between those two points and the price line gains from trading.
Make sure both curves reach the vertical axis when you draw this. If they stop short, your triangles have no top and bottom corners and you cannot calculate the areas. It is the single most common reason students lose marks on surplus questions.
Calculating the areas
Both shapes are right-angled triangles sitting against the vertical axis, so:
Area of a surplus triangle
Surplus = ½ × base × height
🧩 The method, every time
Find the equilibrium price and quantity.
Draw the price line horizontally from the equilibrium across to the vertical axis.
Check both curves touch the vertical axis, so both triangles are complete.
Base = the equilibrium quantity (the horizontal side, lying along the price line).
Height = the vertical gap: for consumer surplus, from P(e) up to where D meets the axis; for producer surplus, from P(e) down to where S meets the axis.
Read the units off the axes and give the answer as a money amount.
The units trap. If the horizontal axis says “thousands of units”, your base is in thousands too, and the answer must be scaled. Losing a factor of a thousand is a classic Paper 1 error.
What happens when the market changes
Shift either curve and both triangles change size. This is why surplus is such a useful tool later on, when you look at taxes, subsidies and price controls.
The shaded areas are the new surpluses. Compare them with the smaller triangles that would sit above and below the old price line at P₁ and you can see the gain.
Community surplus and allocative efficiency
Add the two triangles together and you get community surplus (also called social surplus): the total gain society makes from this market existing.
Here is the important result. Community surplus is at its maximum at the equilibrium quantity. Produce less and some trades that would have benefited both sides never happen. Produce more and you are making units that cost more to produce than anyone values them at.
There is a second way to say the same thing. The demand curve shows the marginal benefit (MB) of the last unit to consumers. The supply curve shows the marginal cost (MC) of the last unit to producers. Where they cross, MB = MC — and that is allocative efficiency.
At Q(e) every unit whose benefit exceeded its cost has been produced, and no unit costing more than it is worth has been. That is what “efficient” means here.
Notice what allocative efficiency does not claim. It says the total gain is as large as possible — not that it is shared fairly, and not that everyone who needed the good got it. Efficiency and equity are separate ideas, and pointing that out is a reliable evaluation move.
Worked examples
WORKED EXAMPLE 1
In a market, demand is P = 60 − 0.1Q and supply is P = 10 + 0.1Q, with P in dollars. Calculate the consumer surplus and the producer surplus at equilibrium. [4]
Step 1: find equilibrium60 − 0.1Q = 10 + 0.1Q → 50 = 0.2Q → Q = 250P = 60 − 0.1(250) = $35Step 2: find where each curve meets the vertical axis
Demand at Q = 0 gives $60; supply at Q = 0 gives $10.
Step 3: consumer surplus½ × 250 × (60 − 35) = ½ × 250 × 25 = $3 125Step 4: producer surplus½ × 250 × (35 − 10) = $3 125CS = $3 125, PS = $3 125, community surplus = $6 250Equal triangles here only because the two curves have the same steepness. That is a coincidence, not a rule.
WORKED EXAMPLE 2
New technology changes supply in the same market to P = 5 + 0.1Q. Calculate the change in consumer surplus. [4]
Step 1: new equilibrium60 − 0.1Q = 5 + 0.1Q → 55 = 0.2Q → Q = 275P = 60 − 0.1(275) = $32.50Step 2: new consumer surplus½ × 275 × (60 − 32.50) = ½ × 275 × 27.50 = $3 781.25Step 3: subtract the old one3 781.25 − 3 125 = 656.25Consumer surplus rises by $656.25Two things helped buyers at once: a lower price on every unit, and more units traded.
WORKED EXAMPLE 3
A regulator forces a market to produce below its equilibrium quantity. Explain the effect on community surplus. [4]
Step 1: what is lost
Units between the new quantity and Q(e) are not produced, even though for each of them MB > MC.
Step 2: name the loss
Those trades would have benefited buyer and seller, so the gain they would have created disappears — a loss of community surplus.
Step 3: on the diagram
The shaded triangle shrinks; the missing slice sits between the two curves to the right of the new quantity.
Step 4: conclude
The market is no longer allocatively efficient, because MB no longer equals MC.
Community surplus falls: welfare is lostBalance it: the regulation might still be worthwhile if the market was ignoring costs to third parties, such as pollution.
💡 Exam tip
Extend both curves to the vertical axis or your triangles will be incomplete.
Shade and label both areas clearly, and name the corner points if the question uses letters.
Write the formula before you substitute. Method marks are available even if the arithmetic slips.
Check the axis units before you write the final number, especially with “000s”.
Always give the answer as money, with the right currency.
Bring surplus into evaluation whenever a policy changes a market — it lets you say precisely who gained and who lost.
⚠ Common mix-up
Swapping the two triangles. Consumer surplus is above the price line, producer surplus below it.
Using quantity as the height. Quantity is the base; the height is the vertical price gap.
Forgetting the ½. These are triangles, not rectangles.
Ignoring “thousands” on the axis and giving an answer a thousand times too small.
Thinking allocative efficiency means fairness. It only means total surplus is at its maximum.
Assuming a lower price always raises consumer surplus. It usually does, but only if the quantity traded is not restricted at the same time.
Confusing allocative with productive efficiency. Allocative is MB = MC; productive is producing at the lowest average cost.
Up next: Price Elasticity of Demand — how sharply buyers actually react when a price moves, and why that number decides almost every policy question in this course.
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