Chapter 4 · Government intervention in microeconomics
Price floor
Explain why a price floor above equilibrium creates excess supply. Show the quantity traded and welfare loss when government does not buy the surplus.
The economic idea
A price floor is a minimum legal price. Governments may use it to support producer incomes, but a higher price does not guarantee that every producer can sell more or earn more revenue.
A price control is a legally imposed limit on the price at which a good, service or factor of production may be exchanged. Unlike a tax or subsidy, it does not initially shift demand or supply; it prevents the market price from moving beyond the legal limit.
A price floor, also called a minimum price, makes it illegal to charge below the stated price. It is binding only when set above the competitive equilibrium price. A floor below equilibrium is non-binding because the market already charges a permitted higher price.
A binding floor creates excess supply because the higher minimum price decreases quantity demanded while increasing quantity supplied. Without government purchases, actual trade is limited to quantity demanded. Quantity supplied describes what firms are willing to offer, not necessarily what they ultimately produce or sell.
Possible consequences include unsold stocks, storage or disposal costs, inefficient overproduction when government guarantees purchases, barriers to market entry and fiscal expenditure. The stakeholder and welfare effects depend on the policy design.
Reading a binding price floor
“Price” is measured vertically and “Quantity” horizontally.
“D” and “S” establish the competitive equilibrium “E”, with price “Pₑ” and quantity “Qₑ”.
“Price floor” is the horizontal minimum-price line. Its position relative to “Pₑ” determines whether it binds.
Benchmark: E
Before the control constrains the market, “D” intersects “S” at “E”. Planned purchases and planned sales are equal at “Qₑ” for price “Pₑ”.
Legal minimum: Price floor
“Price floor” is currently binding because it lies above “Pₑ”. At “Pꜰ”, quantity supplied “Qs” exceeds quantity demanded “Qd”, creating excess supply—a surplus of planned supply.
The planned surplus between Qd and Qs
Consumers are willing and able to buy “Qd”, while firms are willing to supply “Qs”. Without government purchases, only “Qd” is traded. The difference “Qs” minus “Qd” is planned excess supply, not necessarily output already produced.
Read producer support alongside the cost to others
Without another buyer, sales stop at “Qd”. Consumer surplus is below “D” and above “Pꜰ”, up to that quantity. If the lowest-cost producers make those sales, producer surplus is above “S” and below “Pꜰ”, also up to “Qd”. A higher price helps successful sellers, but fewer sales mean producer surplus need not rise overall.
Under that allocation assumption, the lost gains from trade lie between “D” and “S”, from “Qd” to “Qₑ”. This welfare-loss area is different from planned excess supply: one measures lost net benefit, while the other measures a quantity gap.
How the market responds to the minimum price
Begin at “E” and compare “Pₑ” with the higher legal minimum “Pꜰ”.
At the higher minimum price “Pꜰ”, consumers buy less, so quantity demanded decreases to “Qd” along “D”. The higher price gives firms an incentive to offer more, so quantity supplied increases to “Qs” along “S”. Neither curve shifts.
Because “Qs” exceeds “Qd”, planned excess supply emerges and would normally place downward pressure on price. The legal minimum prevents that adjustment.
Without government purchases, only “Qd” is traded. Firms cannot sell all of the quantity they are willing to supply at the minimum price.
What the current state establishes
Actual legal-market trade is “Qd”, the short side of the market. The distance to “Qs” is planned excess supply, not necessarily output that has already been produced and discarded.
Common mistakes to avoid
- A price floor is not automatically binding. It must lie above the competitive equilibrium price.
- Without government purchasing, “Qs” is planned quantity supplied; actual trade is limited to household demand “Qd”.
- A surplus caused by a price floor is excess supply. It is not consumer surplus or producer surplus.
- Government expenditure on purchased stocks is not automatically equal to deadweight loss; their value, use, storage and financing matter.
- A higher minimum price does not guarantee higher total revenue for every producer. Explain the quantity sold and whether government buys the excess supply.
How to draw this diagram
- Title the diagram “Price floor”. Label the horizontal axis “Quantity” and the vertical axis “Price”.
- Draw D first and label it clearly.
- Draw S next and label it clearly.
- Draw Price floor next and label it clearly.
- Mark each relevant intersection and distinguish the initial, market and socially optimum or policy outcomes where applicable.
- Add the relevant guide lines and shade the economic area only after its boundaries are established.
- Check that every symbol used in the explanation appears on the diagram and add arrows showing the direction of change.
Assumptions and limitations
- The model assumes a competitive market, enforceable regulation and unchanged product quality.
- Planned quantity supplied need not equal actual production when firms cannot sell their intended output.
- With government purchases, welfare depends on whether stocks are stored, redistributed, exported or destroyed and on the cost of public finance.
- Long-run entry, exit, technological change and international spillovers can alter the size and distribution of the effects.