Source: Microeconomic Theory, Texas A&M University
Tags: consumer surplus, producer surplus, deadweight loss, welfare, price ceiling, price floor, tax, subsidy, government policy, market efficiency
Consumer surplus and producer surplus together measure welfare in a competitive market. Government interventions (price ceilings, price floors, taxes, subsidies) all cause a deadweight loss by moving the market away from the efficient competitive equilibrium. The size and distribution of that loss depend on the policy and on the elasticities of supply and demand.
Consumer surplus (CS)
The total benefit to all consumers, measured as the area below the demand curve and above the market price.
Producer surplus (PS)
The total benefit to all producers, measured as the area above the supply curve and below the market price.
Total welfare
CS + PS (plus government revenue, if a tax or subsidy is in place). In a free competitive market, welfare = CS + PS.
Deadweight loss (DWL)
The reduction in total welfare relative to the efficient (competitive equilibrium) level. DWL = maximal welfare − current welfare.
Price ceiling
A regulation that prevents the price from rising above a set maximum (Pmax), which is below the equilibrium price.
Price floor
A regulation that prevents the price from falling below a set minimum (Pmin), which is above the equilibrium price.
At the competitive equilibrium, total welfare (CS + PS) is maximised. This is why competitive equilibrium is described as efficient.
Before the ceiling, the market is at equilibrium: Q₀ is produced.
CS = B + D
PS = A + C + E
Welfare = A + B + C + D + E
After the ceiling, quantity falls to Q₁ (determined by supply at Pmax):
CS = A + D
PS = E
Welfare = A + D + E
Deadweight loss = B + C
The price ceiling creates a shortage (quantity demanded exceeds quantity supplied at Pmax). Area A is transferred from producers to consumers, but the triangles B and C are lost to both sides.
Before the floor, the market is at equilibrium: Q₀ is produced.
CS = A + B + D
PS = C + E
Welfare = A + B + C + D + E
After the floor, quantity falls to Q₃ (determined by demand at Pmin):
CS = D
PS = A + E
Welfare = A + D + E
Deadweight loss = B + C
The price floor creates a surplus (quantity supplied exceeds quantity demanded at Pmin). Area A is transferred from consumers to producers, and the triangles B and C are deadweight loss.
A tax of t per unit drives a wedge between the price buyers pay (Pb) and the price sellers receive (Ps): Pb − Ps = t.
Before the tax, at equilibrium Q₀:
CS = A + B + E
PS = C + D + F
Welfare = A + B + C + D + E + F
After the tax, quantity falls to Q₁:
CS = E
PS = F
Government revenue = A + D
Welfare = CS + PS + Gov. revenue = A + D + E + F
Deadweight loss = B + C
Tax incidence and elasticity
If demand is very inelastic relative to supply, the burden falls mostly on buyers (Pb rises nearly by the full tax)
If demand is very elastic relative to supply, the burden falls mostly on sellers (Ps falls nearly by the full tax)
A subsidy of s per unit creates a gap between the price sellers receive (Ps) and the price buyers pay (Pb): Ps − Pb = s.
Before the subsidy, at equilibrium Q₀:
CS = A + B
PS = C + D
Welfare = A + B + C + D
After the subsidy, quantity rises to Q₁:
CS = A + B + C + E + F
PS = B + C + D + G
Government revenue = −(B + C + G + H + E + F), i.e. a cost to the government
Welfare = CS + PS + Gov. revenue = A + B + C + D − H
Deadweight loss = H
A subsidy pushes output above the efficient level, creating overproduction and a deadweight loss of H.
CS = area below demand, above price
PS = area above supply, below price
Welfare = CS + PS (+ government revenue if applicable)
DWL = efficient welfare − actual welfare
Tax wedge: Pb − Ps = t
Subsidy wedge: Ps − Pb = s
⚠️ All four policies (ceiling, floor, tax, subsidy) produce deadweight loss. The exam will ask you to identify CS, PS, DWL, and government revenue using labelled areas on a diagram.
⚠️ For price ceilings and floors, the quantity traded is determined by the short side of the market (whichever of supply or demand is smaller at the regulated price).
⚠️ With a tax, welfare now includes government revenue. Do not forget to add it. Welfare = CS + PS + government revenue.
⚠️ With a subsidy, government revenue is negative (it is a cost). Include it as a negative term when computing welfare.
⚠️ Tax incidence depends on relative elasticities, not on whether the tax is legally imposed on buyers or sellers. The more inelastic side bears more of the burden.
⚠️ Deadweight loss under a tax or subsidy is the triangle between the supply and demand curves, from the new quantity to the old equilibrium quantity.
Q: A price ceiling is set below the equilibrium price. What happens to quantity traded?
A: Quantity traded falls to the amount producers are willing to supply at the ceiling price (the supply side of the market). There is a shortage.
Q: With a per-unit tax, why does deadweight loss arise?
A: The tax raises the price buyers pay and lowers the price sellers receive, reducing the quantity traded below the efficient level. The units no longer produced would have generated surplus for both sides, and that surplus is now lost.
Q: If demand is perfectly inelastic, who bears the entire burden of a per-unit tax?
A: Buyers bear the entire burden. Price rises by the full amount of the tax, and quantity does not change.
Q: A subsidy increases quantity traded. Why does this still produce a deadweight loss?
A: The extra units cost more to produce than consumers value them. The subsidy pushes output past the efficient level, and the cost to the government of those extra units exceeds the combined surplus they generate.
Q: Before a price floor, CS = A + B + D and PS = C + E. After the floor, CS = D and PS = A + E. What is the deadweight loss?
A: Before welfare = A + B + C + D + E. After welfare = A + D + E. DWL = B + C.
consumer surplus, producer surplus, total welfare, deadweight loss, welfare loss, price ceiling, price floor, per-unit tax, per-unit subsidy, tax incidence, tax burden, elasticity and tax incidence, government revenue, market efficiency, competitive equilibrium efficiency, shortage, surplus, overproduction