Taxes and subsidies nudge the market. Price controls overrule it. The government simply declares a price illegal, and the market is left permanently out of balance — on purpose.
📘 What you need to know
A price ceiling (maximum price) is set below equilibrium to help buyers. Sellers may not legally charge more.
A ceiling causes a shortage: quantity supplied contracts, quantity demanded extends, and QsQd is the excess demand.
A price floor (minimum price) is set above equilibrium to help sellers. Buyers may not legally pay less.
A floor causes a surplus: quantity supplied extends, quantity demanded contracts, and QdQs is the excess supply.
A control only does anything if it is on the correct side of equilibrium. A maximum price above the market price changes nothing.
A minimum wage is a price floor in the labour market, and the surplus it creates is unemployment.
Both controls cause welfare loss, because fewer units are traded than the efficient amount.
Price ceilings
Governments cap prices when they think buyers cannot afford something essential — rents, basic food, fuel during a crisis. The intention is good, but the low price cuts the incentive to supply at exactly the moment it boosts the incentive to buy.
The quantity actually bought is Qs, not Qd. Buyers cannot purchase what nobody is producing, however much they want it.
What follows from a ceiling
Someone has to be turned away. With price no longer rationing, goods get shared out by queues, waiting lists or luck.
Black markets appear. Buyers who missed out are willing to pay far more than the legal price, so illegal reselling becomes profitable.
Quality often falls. A landlord who cannot raise the rent has little reason to repair the building.
Producer surplus falls, and total surplus falls too: fewer units are traded than at equilibrium, so there is a welfare loss.
Consumer surplus is mixed. Those who get the good gain, but many who wanted it get nothing at all.
WORKED EXAMPLE
The market rent for a flat is $800, with 5,000 flats let. A ceiling of $600 is imposed. At $600, landlords offer 3,500 flats and tenants want 6,200. Find the shortage and the change in landlords’ revenue. [4]
Step 1: the shortage6,200 − 3,500 = 2,700 flatsStep 2: revenue before800 × 5,000 = $4,000,000Step 3: revenue after (only 3,500 are supplied)600 × 3,500 = $2,100,000Step 4: the change2,100,000 − 4,000,000 = −$1,900,000Shortage of 2,700 flats; landlord revenue falls by $1.9mUse the quantity actually supplied, not the quantity demanded. Only 3,500 flats exist to rent.
Price floors
A floor does the opposite. It guarantees sellers a price above the market level, usually to protect farmers’ incomes or to discourage consumption of something harmful like cheap alcohol.
Notice the awkward result: producers get a better price per unit but sell fewer units to consumers. Whether they gain overall depends on what happens to the surplus.
WORKED EXAMPLE
Milk sells at $0.80 a litre with 40,000 litres traded. A minimum price of $1.00 is set. At that price consumers buy 30,000 litres and farmers supply 52,000. Find the surplus, the cost of buying it up, and the change in farmers’ revenue from consumers. [5]
Step 1: the surplus52,000 − 30,000 = 22,000 litresStep 2: cost if the government buys it all1.00 × 22,000 = $22,000Step 3: revenue from consumers before0.80 × 40,000 = $32,000Step 4: revenue from consumers after1.00 × 30,000 = $30,000Surplus 22,000 litres; $22,000 to buy it; consumer revenue falls by $2,000Farmers only end up better off because the government steps in and buys the rest.
Minimum wages: a price floor for labour
The labour market works like any other, with two sides swapped: firms demand labour and workers supply it. A national minimum wage is a price floor set above the market wage, and the surplus of labour it creates has a familiar name.
Exactly the same diagram as a price floor. Only the axis labels change, which is why this one is quick marks if you have practised the floor.
WORKED EXAMPLE
The market wage for cleaners is $9 an hour with 12,000 employed. A minimum wage of $12 is introduced. Firms then want 9,500 workers and 14,000 people want the jobs. Find the excess supply and comment. [3]
Step 1: excess supply of labour14,000 − 9,500 = 4,500 workersStep 2: how many actually lose work12,000 − 9,500 = 2,500 jobs goneStep 3: comment
The 9,500 still employed earn $3 an hour more. The cost is 2,500 lost jobs plus 2,000 extra people now looking for work at the higher wage.
Excess supply of 4,500 workers, of whom 2,500 lost existing jobsWinners and losers in the same policy — ideal material for an evaluation paragraph.
Evaluating price controls
Control
Arguments for
Arguments against
Price ceiling
Makes essentials affordable for those who get them; useful in a short crisis; raises the surplus of the consumers who do buy.
Creates shortages; encourages black markets and queuing; producer surplus and total surplus fall; quality and investment decline over time.
Price floor
Protects producer incomes and stabilises farm earnings; reduces consumption of demerit goods like very cheap alcohol.
Creates surpluses; costly if the government buys the excess; consumers pay more; resources are wasted producing things nobody buys.
Minimum wage
Guarantees a basic income; higher pay may raise motivation and spending in the economy.
Raises firms’ costs, which can mean higher prices or job losses, especially for the lowest-skilled workers.
Elasticity changes the size of the problem. The more elastic demand and supply are, the bigger the shortage or surplus a control creates. If both are very inelastic, the control does far less damage — a neat way to add depth to an evaluation.
Always ask which side of equilibrium the control sits on. A maximum price above the market price and a minimum price below it are both completely useless, and questions sometimes test exactly that.
🧩 Drawing a price control step by step
Draw the free market and label PeQe.
Draw the control as a horizontal line: below Pe for a ceiling, above it for a floor.
Read across to both curves at that price to find Qs and Qd.
Mark the gap between them and name it: shortage or surplus.
State the quantity actually traded — always the smaller of the two.
Say who gains and who loses, and mention the welfare loss.
💡 Exam tip
Label the control line clearly as Pmax or Pmin and draw it right across the diagram.
Mark three quantities: Qs, Qe and Qd. The gap is the answer to most calculation parts.
The quantity traded is always the shorter side of the market. Use it in any revenue calculation.
Bring in consumer and producer surplus if the question mentions efficiency — both controls shrink total surplus.
Name a real example: rent controls, minimum alcohol pricing, farm support, minimum wages.
For evaluation, ask who gains, who loses, and whether the policy is short term or permanent.
⚠ Common mix-up
Drawing the ceiling above equilibrium. A maximum price only bites if it is below the market price.
Saying Qd units are sold under a ceiling. Only Qs exists to be sold.
Shifting a curve. A price control moves neither curve; it just outlaws part of the price range.
Assuming a ceiling always helps consumers. Many end up unable to buy at all.
Assuming a floor always helps producers. They sell less, and may only gain if the surplus is bought up.
Forgetting the minimum wage is just a price floor. Same diagram, same logic.
Ignoring black markets. Shortages create them, and examiners like to see it mentioned.
Up next: Regulation, Direct Provision and Nudges — the tools that work without touching prices at all.
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