How do I analyse a tax or subsidy using demand and supply diagrams?
- Expert answer
- Undergraduate
- Asked
The question
My microeconomics assignment asks me to show the effect of a per-unit tax on a market using a demand and supply diagram.
I need to explain price, quantity, consumer surplus, producer surplus and deadweight loss.
Short answer
To analyse a tax or subsidy, show how the supply or demand curve shifts, identify the new equilibrium, compare prices paid and received, and explain who bears the burden or receives the benefit using elasticity.
Full expert answer
Economics tutor
MSc Economics
Tax and subsidy questions test whether you can move from a diagram to economic interpretation. The diagram is not decoration. It should show what happens to price, quantity, surplus and incentives after government intervention.
What the question is asking
The question is asking you to compare the market before and after the policy. For a per-unit tax, the key issue is the wedge between the price paid by buyers and the price received by sellers. For a subsidy, the wedge works in the other direction because the government pays part of the cost.
Diagram method for a per-unit tax
- 1Draw the original demand and supply curves.
- 2Mark the original equilibrium price and quantity.
- 3Shift the supply curve upward by the amount of the tax if the tax is imposed on sellers.
- 4Mark the new equilibrium quantity.
- 5Show the price paid by consumers and the price received by producers.
- 6Identify tax revenue as tax per unit multiplied by quantity sold after the tax.
- 7Explain deadweight loss from trades that no longer happen.
If the tax is legally imposed on buyers, you can shift demand down instead. The economic incidence depends on elasticity, not simply on who sends the tax to the government.
Mini worked example
Suppose the original price of a sugary drink is 2.00 and the original quantity is 1,000 bottles per week. The government adds a 0.50 per-unit tax. After the tax, consumers pay 2.30, sellers receive 1.80 after tax, and quantity falls to 850.
Interpretation:
- Consumers pay 0.30 more than before.
- Producers receive 0.20 less than before.
- The 0.50 tax burden is split between consumers and producers.
- Tax revenue is 0.50 x 850 = 425.
- Quantity falls by 150, so some mutually beneficial trades disappear.
If demand is relatively inelastic, consumers usually bear more of the tax. If supply is relatively inelastic, producers bear more.
Subsidy version
For a per-unit subsidy to producers, shift supply downward by the subsidy amount. Quantity normally rises. Consumers pay a lower price, producers receive a higher effective price, and the government pays subsidy per unit multiplied by quantity. The welfare analysis should include government cost, not only the gain to buyers and sellers.
Sample university-style questions and how to answer them
| Sample question | What a strong answer should do |
|---|---|
| Use a diagram to show the effect of a tax on cigarettes. | Shift supply up, show lower quantity, tax wedge, tax revenue and deadweight loss. Explain inelastic demand if consumers bear much of the tax. |
| Who bears the burden of a fuel tax: drivers or fuel companies? | Compare elasticity of demand and supply. The more inelastic side bears more burden. |
| Analyse a subsidy for electric vehicles. | Shift supply down or demand up depending on how the subsidy is paid, show higher quantity and discuss government cost. |
| Explain why a tax creates deadweight loss. | Identify lost trades where buyer value exceeded seller cost before the tax but no trade occurs after it. |
| Compare a tax on buyers with a tax on sellers. | Show that legal incidence differs from economic incidence; final burden depends on elasticity. |
Common mistakes
- Shifting both demand and supply for the same simple tax
- Saying sellers bear the tax because the law says they pay it
- Forgetting the price received by producers after tax
- Drawing a deadweight loss triangle without explaining what it means
- Ignoring elasticity
- Treating subsidy benefits as free without considering government cost
What earns higher marks
Add an elasticity paragraph. The tax burden falls more heavily on the side of the market that is less responsive to price. That is why cigarette or fuel taxes can be politically sensitive: quantity may fall only modestly, while consumers pay much of the tax through higher prices.
Academic use note
This guide is for economics assignment support. Draw the diagram required by your module and label axes, curves, equilibrium points, tax wedge and welfare areas clearly.
Sources and further reading
This answer explains a method for you to apply to your own work. Copying it into a submission would count as plagiarism, and it is indexed by similarity checkers.
All questions