Definition
Supply and demand is the model through which economists explain how prices and quantities are determined in markets. Demand summarises buyers' willingness and ability to pay at each possible price; supply summarises sellers' willingness and ability to produce at each possible price. Where the two curves intersect is equilibrium — the price at which the quantity buyers want to purchase exactly equals the quantity sellers want to sell.
The model is the workhorse of economics not because markets are always in perfect equilibrium but because it captures the directional logic of how markets respond to change: who bears the cost of a tax, who benefits from a subsidy, what happens when a hurricane destroys a region's crop, why rent control creates housing shortages. The supply-demand framework provides a consistent, falsifiable way to trace those effects.
Why it matters
How equilibrium forms and shifts
Shifts versus movements
A critical distinction
The most common error in supply-demand analysis is confusing a movement along a curve with a shift of the curve. Only a change in the good's own price causes movement along the curve. Everything else — consumer income, prices of related goods, expectations, input costs, technology, number of sellers — shifts the entire curve.
Example: the price of gasoline rises. This does not shift the demand curve for gasoline — it causes a movement along it, reducing quantity demanded. But if consumer incomes fall, the entire demand curve shifts left, reducing quantity demanded at every price.
What shifts demand
- Income (for normal goods, higher income shifts demand right)
- Prices of substitutes (higher substitute price → higher demand for this good)
- Prices of complements (higher complement price → lower demand for this good)
- Consumer expectations (expecting future price rises → buy now, shift right)
- Tastes and preferences
What shifts supply
- Input prices (higher input costs → shift left)
- Technology improvements (shift right — same inputs produce more)
- Number of sellers (more sellers → shift right)
- Expectations of future prices (sellers may hold inventory, shifting current supply left)