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Supply and demand explain how buyers’ willingness and ability to purchase and sellers’ willingness to sell interact to determine a market price and quantity. In the basic microeconomic model, a change in a good’s own price causes movement along a curve; a change in another market condition shifts the curve and can create a new equilibrium.
What do supply and demand mean?
Supply and demand are relationships between price and the quantities buyers want to buy and sellers want to sell in a defined market over a defined period. Demand means willingness and ability to buy at each possible price—not simply wanting a product. Supply means willingness to sell at each possible price. In the standard introductory model, holding other relevant conditions constant, buyers generally demand less at a higher price, while sellers generally offer more.
Economists show these relationships on a graph with price on the vertical axis and quantity on the horizontal axis. The demand curve typically slopes downward; the supply curve typically slopes upward. These curves describe quantities at different prices, not fixed amounts independent of price. OpenStax summarizes the basic model and its market applications in its introduction to demand and supply.
How do supply and demand affect prices?
The curves’ intersection is the market equilibrium: the price at which quantity demanded equals quantity supplied. At a price below equilibrium, buyers want more than sellers offer, creating a shortage, or excess demand. At a price above equilibrium, sellers offer more than buyers want, creating a surplus, or excess supply. In the model, these imbalances put pressure on price and other market adjustments until the quantities demanded and supplied move toward balance. See OpenStax’s explanation of demand, supply, and market equilibrium.
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Equilibrium is a model’s balancing point, not a promise that every real market clears immediately. Adjustment can be slowed or altered by such factors as imperfect information, market power, policy constraints, or delays in changing production and purchasing.
What shifts demand or supply?
A change in the good’s own price changes quantity demanded or quantity supplied: it is a movement along the relevant curve. A change in another determinant can shift the entire curve. This distinction matters because a price movement along a curve is not, by itself, a new cause of demand or supply.
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| Curve | Examples of non-price determinants | Typical direction |
|---|---|---|
| Demand | Tastes, population, income, prices of substitutes or complements, and expectations | More willingness or ability to buy at each price shifts demand right; less shifts it left. |
| Supply | Input prices, natural conditions, technology, and government taxes, regulations, or subsidies | Greater willingness or ability to sell at each price shifts supply right; less shifts it left. |
These are ceteris paribus comparisons: they isolate one change while treating other relevant conditions as unchanged. OpenStax explains the determinants and the difference between a shift and movement along a curve in its guide to shifts in demand and supply.
How to work out a market change
- Establish the starting point. Identify the market, time period, initial equilibrium price, and initial equilibrium quantity.
- Classify the event. Decide whether it changes buyers’ willingness or ability to purchase, sellers’ willingness or ability to sell, or both.
- Choose the shift direction. A curve shifts right when more is demanded or supplied at each price; it shifts left when less is demanded or supplied at each price.
- Compare the new equilibrium. Use the shifted curve and the unchanged curve to determine how equilibrium price and quantity compare with their initial levels.
For example, a drought that reduces a crop’s supply shifts the supply curve left. With demand otherwise unchanged, the standard model predicts a higher equilibrium price and a lower equilibrium quantity. The new price then corresponds to movement along the demand curve; it does not create a second demand shift. OpenStax sets out this reusable four-step method for analyzing changes in equilibrium.
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When both curves shift, do not infer a unique result unless the direction and relative size of the changes are known. If demand rises and supply falls, both shifts put upward pressure on price, but their effects on equilibrium quantity oppose each other. If demand falls while supply rises, both put downward pressure on price, while their effects on quantity oppose each other. The quantity outcome in either case depends on the relative shifts. If the curves move in directions that oppose each other on price as well, the price outcome also depends on their relative sizes.
Why do elasticity and time matter?
Elasticity describes how strongly buyers or sellers respond to a change in price. A less elastic side of the market adjusts quantity less in response to price changes; following a shift, that side tends to be associated with a larger price change relative to the quantity change. The size of the effect therefore depends not only on which curve shifts, but also on the responsiveness of both sides.
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Responsiveness can also change with the time allowed to adjust. Demand and supply are often less elastic in the short run and more elastic over a longer horizon, as buyers and sellers find alternatives or change plans. That can make quantity adjust more readily over time. OpenStax discusses how elasticity affects the relative price and quantity response to shifts in its section on elasticity and pricing.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Where the model applies—and where it does not
Use introductory supply and demand to reason about a particular good or service in a specified market and time period. It is a framework for tracing how a change can affect equilibrium, not a guarantee of an exact price or quantity. Real markets may have frictions, imperfect information, market power, policy constraints, or delayed adjustment, and several conditions may change at once.
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This article concerns microeconomic markets for goods and services. Aggregate supply and aggregate demand belong to macroeconomics: they describe economy-wide relationships and do not use the same interpretation as an individual market’s price-and-quantity curves.
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