Supply & Demand Simulator — Feel Equilibrium, Surplus, and Elasticity by Doing
Supply and Demand — The Heart of Economics
When you first study economics, supply and demand curves look simple. But the moment you genuinely feel that the intersection of those two lines explains price, quantity, tax burden, and welfare distribution — economics starts to look different.
The goal of this article is to let you move the textbook graph with your own hands.
1. The Demand Curve and the Supply Curve
Demand Curve
When price rises, the quantity buyers want falls. Plotted on a graph, this relationship produces a downward-sloping line.
Factors that shift demand:
- Income changes
- Changes in prices of substitutes and complements
- Consumer preference shifts
- Expected future prices
Supply Curve
When price rises, suppliers want to produce more. This relationship produces an upward-sloping line.
Factors that shift supply:
- Changes in production costs
- Technological improvements
- Changes in the number of producers
- Weather and raw material shocks
2. Equilibrium — Where the Curves Intersect
Equilibrium price (P*) and equilibrium quantity (Q*) are where the quantity demanded equals the quantity supplied.
Markets tend to move toward equilibrium automatically:
- Price above P*: excess supply → downward price pressure
- Price below P*: excess demand → upward price pressure
This is how the Invisible Hand operates.
3. Consumer Surplus and Producer Surplus
Surplus is the social gain generated by market transactions.
Consumer surplus: The difference between what consumers were willing to pay and what they actually paid. It equals the triangular area between the demand curve and the equilibrium price.
Producer surplus: The difference between what producers actually received and the minimum they needed to accept. It equals the triangular area between the supply curve and the equilibrium price.
Total surplus = Consumer surplus + Producer surplus = Social welfare
Free market equilibrium maximizes total surplus. This is the core logic of market efficiency.
4. Explore with the Simulator
Use the sliders below to see how the model responds.
- Shift curves: Move supply or demand left or right and watch the equilibrium point relocate
- Elasticity: Change the slope of the demand curve to see how responsive quantity is to price
- Show surplus: Toggle the consumer and producer surplus triangles on and off
- Per-unit tax: Apply a tax to shift the supply curve upward and observe deadweight loss
5. Elasticity — Measuring How Much Things React
If price rises 10% and quantity demanded falls 20%, elasticity is −2. If it falls only 5%, elasticity is −0.5.
Elasticity classifications:
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6. The Economic Effects of Taxation
When a government imposes a per-unit tax:
- The supply curve shifts upward by the amount of the tax
- New equilibrium: higher price, lower quantity
- Consumers pay a higher price → consumer surplus falls
- Producers receive a lower net amount → producer surplus falls
- Tax revenue = Tax per unit × New equilibrium quantity
- Deadweight Loss: The welfare that disappears because trades that would have occurred at the old equilibrium no longer happen
Tax incidence: Who legally pays a tax and who economically bears it are different questions. The less elastic side absorbs more of the burden. Cigarette taxes are remitted by producers, but smokers bear most of the cost — because demand for cigarettes is inelastic.
The supply-demand model is just two intersecting lines, but within those lines lies a vast amount of insight about price, quantity, taxation, welfare, and efficiency. Move the sliders, and that insight becomes intuition.
Oiyo
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