Check whether the maximum is below equilibrium.
A price ceiling is a legal maximum. It is binding when below the unrestricted equilibrium price. At that low price, quantity demanded exceeds quantity supplied. If sellers comply and no extra supply is provided, the smaller quantity supplied limits trade; the shortage must be resolved through some form of rationing rather than a legal price rise.
- Binding
- Binding means the rule constrains the original market outcome. A ceiling binds below the equilibrium price, where buyers and sellers would otherwise trade equal quantities. A maximum above that price leaves the original outcome legal.
- Shortage
- At a binding ceiling, buyers want more units than sellers offer. This shortage is quantity demanded (Qd) minus quantity supplied (Qs), measured at the same price.
- Sales
- With compliance and no added supply, only the available quantity supplied can be sold. The shortage is unmet demand, not the number of sales.
Draw and interpret the ceiling
Horizontal maximum
Draw the ceiling below initial equilibrium. Read Qs on S and Qd on D at the same controlled price; label the gap.
Restricted adjustment
Upward price pressure remains, but the legal ceiling prevents the ordinary clearing adjustment. The price control itself is not a shift of D or S.
Rationing
Rationing means deciding who receives a limited supply. Queues, purchase limits, lotteries or eligibility rules may do this. Illegal resale above the ceiling or changes in quality are possible responses, not inevitable outcomes.
Time matters
If sellers can withdraw capacity over time, quantity supplied at the low price may fall further. Use the relevant supply response, not a fixed shortage forever.
| Control | Qd | Qs | Gap | Traded Q without added buyers/supply |
|---|---|---|---|---|
| Ceiling 4 | 120 | 40 | Shortage 80 | 40 |
| Floor 8 | 40 | 120 | Excess supply 80 | 40 |
Worked example: A $4 maximum price
In the common model, the unrestricted price is $6 and quantity is 80 per day. Government sets a $4 ceiling.
- At $4, demand P=10-0.05 Q gives Qd=120.
- At $4, supply P=2+0.05 Q gives Qs=40. The shortage is 120-40=80 units.
- Only 40 units can trade in this model. Buyer spending and seller revenue are 4x 40=$160, not 4x 120.
- Successful buyers pay less, but some willing buyers cannot buy. Queues, eligibility rules or another allocation method determine access.
Watch out for this
The shortage of 80 means 80 units are sold.
Shortage is unmet demand. The available quantity is 40; that limits sales under the stated assumptions.
Check your understanding
If equilibrium price is $6 and a ceiling is set at $8, what happens with no other change?
- Price must rise to $8.
- Price stays $6; the ceiling is non-binding.
- A shortage must arise.