Supply and Demand, Explained
How prices are set in a market — the law of demand, the law of supply, how they meet at equilibrium, and what makes the whole curve shift — the single most important model in economics.
Updated 2026-06-06
How markets set prices
Supply and demand is the model economists use to explain how prices and quantities are determined in a market. It rests on two simple behavioral tendencies: buyers want more of something when it is cheaper, and sellers want to provide more of it when the price is high. Where those two forces meet sets the market price.
Almost every topic in microeconomics — taxes, shortages, surpluses, minimum wages, the effect of a new technology — is an application of this one framework. Master how the two curves move and you can reason about a huge range of real-world questions.
The core concepts
Two laws, one meeting point, and the forces that move them.
Law of demand
As the price of a good falls, the quantity buyers want rises, and vice versa — the demand curve slopes downward.
Law of supply
As the price rises, the quantity sellers want to provide rises — the supply curve slopes upward.
Equilibrium
The price where the quantity demanded equals the quantity supplied; the market clears with no shortage or surplus.
Surplus
When price is above equilibrium, sellers offer more than buyers want, and downward pressure on price results.
Shortage
When price is below equilibrium, buyers want more than sellers offer, pushing the price upward.
Shifts vs. movements
A price change moves you along a curve; a change in another factor (income, tastes, costs) shifts the whole curve.
What shifts the curves
A common point of confusion is the difference between moving along a curve and shifting it. A change in the good's own price moves you along the existing curve. A change in anything else shifts the entire curve to a new position.
Demand shifts with consumer income, tastes, the prices of related goods, expectations, and the number of buyers. Supply shifts with production costs, technology, the prices of inputs, expectations, and the number of sellers. When a curve shifts, the equilibrium price and quantity both move to a new point.
How to study it (the efficient way)
Internalize the two laws
Demand slopes down, supply slopes up. Everything else builds on these two directions, so make them automatic.
Find equilibrium on a graph
Practice locating the intersection point and reading off the equilibrium price and quantity.
Master shift vs. movement
For any scenario, ask first: did the good's own price change (movement) or did something else change (shift)?
Predict the new equilibrium
Shift one curve, then state how both the price and the quantity respond. This is the single most-tested skill.
Common questions
What is the difference between demand and quantity demanded?
Quantity demanded is the amount wanted at one specific price (a point); demand is the whole relationship across all prices (the entire curve).
What causes a shortage?
A price set below equilibrium, where buyers want more than sellers are willing to supply. The gap pushes the price up toward equilibrium.
What shifts a demand curve?
Changes in consumer income, tastes and preferences, the prices of related goods, expectations about the future, and the number of buyers in the market.
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