Price Ceilings and Price Floors

Governments sometimes decide that the market price is too high for buyers or too low for sellers, and respond by setting a legal limit. A price ceiling is a legal maximum — sellers may not charge above it. A price floor is a legal minimum — sellers may not charge below it. Rent control is a classic price ceiling: a city caps what landlords can charge. The minimum wage is a classic price floor: the government sets a lower bound on what employers can pay.

Both policies are meant to help someone — rent control protects tenants, the minimum wage protects workers. The supply-and-demand model does not say whether those goals are worth pursuing. What the model shows is what else happens when a price is held away from its market-clearing level.

Take the market from the previous section: demand is $Q_d = 100 - 2P$ and supply is $Q_s = -20 + 4P$. Free-market equilibrium is at $P^* = 20$ and $Q^* = 60$. Now suppose the government imposes a price ceiling at $15 — a legal maximum. Because $15 < 20 = P^*$, the ceiling is binding.

At a price of $15, buyers want to purchase more than the free-market quantity — the lower price makes the good more attractive. At the same time, sellers are willing to supply fewer units — $15 covers fewer producers' costs than $20 did. Those two forces move in opposite directions and produce a shortage: the quantity demanded exceeds the quantity supplied.

Because sellers can only offer 40 units, only 40 units are actually traded — the quantity traded is determined by the short side of the market, which is supply. The free market delivered 60 units; the ceiling reduces that to 40.

This gap between what buyers want and what sellers offer shows up in the real world as queues, waiting lists, and black markets. In cities with strict rent control, apartments can sit on years-long waiting lists because the legal price is too low to induce enough new supply. When official channels cannot clear the market, unofficial ones often fill the void.

The shaded triangle in the diagram is the deadweight loss — the surplus that is destroyed by the price control. Between $Q = 40$ and $Q = 60$ there are 20 units that buyers value more than they cost sellers to produce: buyers' willingness to pay runs from $30 down to $20, while sellers' minimum acceptable price runs from $15 up to $20. Under the free market those 20 trades would happen; under the ceiling they do not.

The deadweight loss triangle has a base of $(60 - 40) = 20$ units and a height of $30 - $15 = $15 — the gap between the demand price and the supply price at $Q = 40$. The area of the triangle gives the total value destroyed:

That $150 is not transferred to anyone — it is gone. It represents the net benefit that would have been created by the 20 trades that the price control prevents, and it is pure economic waste.

Now consider the opposite intervention: a price floor at $30, a legal minimum. Because $30 > 20 = P^*$, this floor is binding. At $P = 30$, sellers want to supply $Q_s(30) = -20 + 4(30) = 100$ units, while buyers only want $Q_d(30) = 100 - 2(30) = 40$ units. The result is an excess supply of $100 - 40 = 60$ units — more goods than anyone wants to buy at that price.

Because buyers limit purchases to 40 units, only 40 units trade — again, the short side of the market determines the quantity. The deadweight loss triangle is geometrically identical to the ceiling case: a base of 20 units and a height of $15, giving$\text{DWL} = 150$. The same amount of value vanishes whether the price is forced too low or too high.

Excess supply appears in the real world as unsold stocks or persistent unemployment. The European Union's agricultural price supports — which once produced famous "butter mountains" and "wine lakes" — are a textbook example: floors set above equilibrium induced farmers to produce far more than consumers would buy at the controlled price. When the labor market is the market and the wage is the price, a binding minimum wage creates excess supply of labor — that is, unemployment among the workers the policy is designed to help.

The controls below let you set the type of price control (ceiling or floor) and drag the controlled price to any level. The plot and metrics update in real time so you can see exactly how the market responds.