Supply and Demand
Markets are conversations between buyers and sellers, conducted entirely in the language of price. This unit builds the single most important model in economics — and gives you the tools to measure exactly how sensitive that conversation is to change.
Demand
Why does a higher price make people want less of something?
Learning Objective
- Define key terms and factors related to consumer decision-making and the law of demand.
- Explain the relationship between price and quantity demanded and how buyers respond to incentives and constraints.
Essential Knowledge
- Economic agents respond to incentives but face constraints — income, time, and regulation.
- The law of demand: a change in own-price causes an opposite change in quantity demanded, moving along a fixed demand curve.
- The downward slope is explained by the income effect, substitution effect, and diminishing marginal utility.
- The market demand curve is the horizontal summation of every individual's demand curve.
- Changes in the determinants of demand shift the entire curve.
Demand is the willingness and ability of consumers to purchase various quantities of a good at various prices, over a given period. Notice both words matter: wanting a yacht isn't demand if you can't afford one, and being able to afford broccoli isn't demand if you have no interest in eating it.
The law of demand captures an almost universal regularity: as price rises, quantity demanded falls, holding everything else constant. Three forces explain this inverse relationship. The substitution effect — as a good gets pricier, consumers switch toward now-relatively-cheaper alternatives. The income effect — a higher price shrinks your purchasing power, so your same paycheck buys less. And diminishing marginal utility — each additional unit delivers less satisfaction than the last, so consumers are only willing to buy more units at progressively lower prices.
It is critical to distinguish a movement along the curve (caused only by a change in the good's own price) from a shift of the entire curve (caused by anything else — income, the price of related goods, tastes, the number of buyers, or expectations).
Supply
Why does a higher price make producers willing to offer more?
Learning Objective
- Define the law of supply and explain the relationship between price and quantity supplied.
- Explain producers' responses to changes in incentives and technology.
Essential Knowledge
- A change in own-price causes a change in quantity supplied in the same direction — a movement along the supply curve.
- Market supply is the horizontal sum of individual firms' supply curves and is upward-sloping.
- Changes in the determinants of supply shift the entire curve.
Supply is the willingness and ability of producers to sell various quantities of a good at various prices. The law of supply states this relationship is direct: higher prices make production more profitable at the margin, so firms bring more output to market — and, over time, new firms are drawn into a profitable industry.
Just as with demand, changes in the good's own price only move you along a fixed supply curve. A genuine shift comes from the determinants: input prices, technology, the number of sellers, taxes and subsidies, and producer expectations. Notice input prices and taxes work the same direction — both raise the cost of producing each unit, discouraging supply — while technological improvement and subsidies do the opposite.
Price Elasticity of Demand
Slope tells you the direction. Elasticity tells you how much it matters.
Learning Objective
- Define measures of elasticity.
- Explain the impact of a price change on total revenue.
- Calculate measures of elasticity from graphs or tables.
Essential Knowledge
- Elasticity measures the magnitude of percentage changes in quantity relative to a given change in price.
- PED = %ΔQd / %ΔP. Elasticity varies along a linear demand curve — slope is not elasticity.
- |E|>1 elastic, |E|<1 inelastic, |E|=1 unit elastic.
- Determinants: necessity vs. luxury, availability of substitutes, time to adjust, share of budget.
- The total revenue test links elasticity directly to what happens to firm revenue.
Two demand curves can look similarly steep on a graph and still behave completely differently as price changes — which is exactly why economists don't rely on slope alone. Price elasticity of demand asks a sharper question: for a given percentage change in price, what percentage change in quantity demanded results?
The most reliable shortcut on the exam is the total revenue test, since it needs no formula at all: calculate TR = P × Q at each price and watch what happens as price changes.
| Price change causes TR to... | Demand is... |
|---|---|
| move in the same direction as price | INELASTIC |
| move in the opposite direction from price | ELASTIC |
| not change at all | UNIT ELASTIC |
The determinants of PED all boil down to one idea: how easily can a consumer walk away? Necessities (insulin) have few good alternatives, so demand is inelastic; luxuries (a second vacation home) are easy to skip, so demand is elastic. Ready substitutes (name-brand vs. generic aspirin) make demand elastic; no substitutes (addictive goods) make it inelastic. More time to shop around increases elasticity; an emergency purchase decreases it. A purchase that eats a large share of your budget (a car) gets more elastic; a trivial one (a pack of gum) stays inelastic.
Price Elasticity of Supply
How quickly can producers respond to a new price?
Learning Objective
- Calculate and interpret the price elasticity of supply.
Essential Knowledge
- PES = %ΔQs / %ΔP — the responsiveness of quantity supplied to price.
- Same elastic/inelastic/unit-elastic benchmarks as demand, separated at magnitude 1.
- PES depends heavily on the price of alternative inputs and, above all, on time.
The single biggest determinant of supply elasticity is the amount of time producers have to adjust. Given a year, an apple farmer can plant more trees; given an afternoon, they can only pick what's already ripe. This is why economists often distinguish the momentary run (fixed supply — perfectly inelastic) from the short and long run (increasingly elastic as firms can add machinery, then build entirely new factories, then let new competitors enter).
Other Elasticities
Price isn't the only thing demand responds to.
Learning Objective
- Calculate income elasticity and cross-price elasticity of demand.
Essential Knowledge
- Elasticity can be measured for any determinant of demand, not just the good's own price.
- Income elasticity classifies goods as normal or inferior.
- Cross-price elasticity classifies pairs of goods as substitutes, complements, or unrelated.
Income elasticity of demand measures how quantity demanded responds to a change in consumer income:
| Value of Ei | Classification |
|---|---|
| Ei < 0 | Inferior good — demand falls as income rises |
| 0 ≤ Ei < 1 | Normal good, necessity |
| Ei > 1 | Normal good, luxury |
Cross-price elasticity of demand measures how the quantity demanded of good X responds to a change in the price of a related good Y:
Market Equilibrium and Consumer/Producer Surplus
Where supply and demand agree — and why that point is special.
Learning Objective
- Define market equilibrium, consumer surplus, and producer surplus.
- Calculate areas of consumer and producer surplus at equilibrium.
Essential Knowledge
- Equilibrium occurs where Qd = Qs — the market "clears" with no shortage or surplus.
- Consumer surplus and producer surplus measure the benefits markets create for buyers and sellers.
- Total surplus is maximized at equilibrium in the absence of market failure — perfectly competitive markets are efficient.
At the equilibrium price and quantity, every mutually beneficial trade that could happen, does happen — and no trade that would make anyone worse off is forced through. That's what economists mean by allocative efficiency.
Consumer surplus is the gap between what a buyer was willing to pay and what they actually paid — the area below the demand curve and above price. Producer surplus is the mirror image: the gap between what a seller actually received and the minimum they'd have accepted — the area above supply and below price.
Market Disequilibrium and Changes in Equilibrium
Markets don't stay still — but they do settle down.
Learning Objective
- Define surplus and shortage.
- Explain and calculate how shocks to a competitive market alter price, quantity, and surplus.
Essential Knowledge
- Whenever markets experience imbalances, market forces drive price and quantity back toward equilibrium.
- Shifts in demand or supply change price, quantity, and both surpluses — and how much they change depends on elasticity.
A shortage (excess demand) exists when Qd > Qs at the current price — this happens whenever price sits below equilibrium. A surplus (excess supply) exists when Qs > Qd — whenever price sits above equilibrium. Left alone, market forces close both gaps: sellers facing unsold inventory cut prices; buyers facing empty shelves bid prices up. Either way, the market self-corrects back to Qe, Pe.
Use the tool below to see a shortage and a surplus at a glance, then how each of the four possible shifts moves the market to a brand new equilibrium.
| Supply increase | Supply decrease | |
|---|---|---|
| Demand increase | P: ? Q: ↑ | P: ↑ Q: ? |
| Demand decrease | P: ↓ Q: ? | P: ? Q: ↓ |
When demand and supply shift simultaneously, one variable (price or quantity) becomes ambiguous without knowing the relative size of each shift — a classic AP free-response trap.
The Effects of Government Intervention in Markets
What happens when policy overrides the market-clearing price?
Learning Objective
- Define price ceilings, price floors, taxes, and subsidies.
- Calculate changes in market outcomes resulting from government policy, including deadweight loss.
Essential Knowledge
- Price floors, price ceilings, taxes, and subsidies affect incentives and outcomes in every market structure.
- Any policy that pushes quantity away from the efficient level can only decrease allocative efficiency.
- Deadweight loss is the surplus destroyed — trades that would have benefited both sides but no longer happen.
- Tax incidence depends on the relative price elasticities of supply and demand.
Government policies can shift prices away from equilibrium — and whenever they do, the market loses some of the surplus that buyers and sellers used to share. Below are deep-dive explanations for each policy you'll see on the AP exam.
💸 DEADWEIGHT LOSS (DWL)
What it is: Deadweight loss is the value of trades that would have happened in a free market but don't because a policy pushes quantity away from equilibrium. It's surplus that simply vanishes — no one gets it.
Real-world example: A farmer would happily sell an apple for $1, and you'd happily pay $1.50 — that trade creates $0.50 of value for society. Now a $1 tax is added. The farmer needs at least $2 to sell, but you won't pay $2. The trade dies. That $0.50 of potential value is gone forever. That's deadweight loss — value destroyed, not transferred.
Key ideas: DWL always appears as a triangle on a supply-and-demand graph. It gets bigger the more elastic supply and demand are, because more people avoid the policy by simply not trading. Both too little production (tax, price ceiling) and too much production (subsidy, price floor) cause DWL.
Easy to remember: "DWL = deals that died." Those are trades that used to happen at Q* but no longer do because price got pushed off course.
AP Exam tip: DWL is always the triangle between the old quantity (Q*) and the new quantity (Qt or Qs). Shade it pointing toward Q*, not away from it. The bigger the gap between Q* and the new quantity, the larger the DWL.
💰 SUBSIDY
What it is: A subsidy is a government payment that lowers the cost of producing or buying something. The government covers part of the price on behalf of buyers or sellers, so more of the good gets produced and consumed than the free market would provide on its own.
Real-world example: Electric vehicles (EVs) are expensive to make, so without help, fewer people would buy them. The federal government offers up to $7,500 per EV — meaning the government pays part of the cost. Buyers pay a lower price (Pc), producers receive a higher price (Pp), and more EVs get sold than the market would produce alone. The catch: society is now producing more EVs than the economically "efficient" amount, which wastes resources on units that aren't worth their true cost.
Key ideas: A subsidy shifts the supply curve down (or the demand curve up), increasing quantity beyond Q*. The difference between what producers receive (Pp) and what consumers pay (Pc) equals the per-unit subsidy. Even though both consumer surplus and producer surplus expand, the government's total expenditure — (Pp − Pc) × Qs — is larger than the combined surplus gains. The leftover is the DWL triangle.
Easy to remember: "A subsidy buys too much of a good thing." The government is paying to overproduce, and the DWL triangle represents wasted resources on those extra units beyond Q*.
AP Exam tip: Government expenditure on a subsidy = (Pp − Pc) × Qs. This is the full rectangle on the graph. The net welfare loss (DWL) = that rectangle minus the gains in consumer surplus and producer surplus.
🔒 PRICE CEILING
What it is: A price ceiling is a government-imposed maximum price for a good or service. The goal is to keep prices "affordable" for consumers. A ceiling only matters — it only "binds" — when it's set below the equilibrium price. Set it above equilibrium, and the market ignores it completely.
Real-world example: Rent control in New York City caps what landlords can charge. If equilibrium rent is $3,000/month but the ceiling is $1,500, two things happen: many more people want apartments at that low price (quantity demanded soars), but landlords don't want to rent at that price (quantity supplied shrinks). The result is a shortage — not enough apartments for everyone who wants one. Long waiting lists, under-the-table payments, and neglected maintenance follow. The policy meant to help renters ends up hurting the very people it was designed for.
Key ideas: A binding price ceiling (Pc < Pe) always creates a shortage: Qd > Qs. It transfers some producer surplus to the consumers who do manage to get the good at the low price — but a DWL triangle forms because other mutually beneficial trades no longer happen.
Easy to remember: "Ceilings are low — they push prices down and cause shortages." Below equilibrium → shortage → DWL.
AP Exam tip: If a price ceiling is set above equilibrium, it is non-binding and has zero effect on the market. Only ceilings below Pe matter. The shortage = Qd − Qs at the ceiling price. The DWL triangle lies between Qs and Q*.
🏛️ PRICE FLOOR
What it is: A price floor is a government-imposed minimum price for a good or service. The goal is to keep prices "high enough" for sellers. A floor only matters — it only "binds" — when it's set above the equilibrium price. Set it below equilibrium, and the market ignores it.
Real-world example: The federal minimum wage is a price floor on labor. If the equilibrium wage would be $8/hour, but the government sets a floor of $15/hour, employers hire fewer workers (quantity demanded for labor falls) while more people want to work at that wage (quantity supplied of labor rises). The difference is a surplus of labor — better known as unemployment. Agricultural price supports work the same way: the government sets a minimum price for wheat above equilibrium, so farmers grow more wheat than buyers actually want — mountains of surplus grain that someone has to buy and store (usually the government).
Key ideas: A binding price floor (Pf > Pe) always creates a surplus: Qs > Qd. It transfers some consumer surplus to the producers who do sell at the higher price — but a DWL triangle forms because some mutually beneficial trades no longer happen (buyers who'd pay between Pe and Pf simply exit the market).
Easy to remember: "Floors are high — they push prices up and cause surpluses." Above equilibrium → surplus → DWL.
AP Exam tip: If a price floor is set below equilibrium, it is non-binding and has zero effect. Only floors above Pe matter. The surplus = Qs − Qd at the floor price. The DWL triangle lies between Q* and Qd.
International Trade and Public Policy
What happens when the domestic market meets the world price?
Learning Objective
- Define tariffs and quotas.
- Explain and calculate how markets are affected by trade policy.
Essential Knowledge
- Opening an economy to trade can move price above or below the autarky (no-trade) equilibrium; the domestic supply-demand gap is filled by trade.
- Tariffs affect domestic price, quantity, government revenue, and total surplus.
- Quotas restrict quantity, raising domestic price and increasing domestic production relative to imports.
Compare the domestic no-trade (autarky) equilibrium price to the world price. If the world price is higher than the domestic price, domestic producers have an incentive to sell abroad — the country exports, producer surplus rises, and consumer surplus falls as the domestic price is bid up to match the world price. If the world price is lower, domestic consumers gain access to cheaper goods abroad — the country imports, consumer surplus rises, and producer surplus falls.
🌐 1. TRADE CAN CHANGE THE DOMESTIC PRICE
What it means: When a country opens its economy to international trade, the world price can be different from the domestic no-trade price (called the autarky price). The world price — not domestic supply and demand alone — now determines what buyers pay and sellers receive.
The two scenarios:
- World price > domestic price → the country exports. Domestic producers earn more by selling abroad. The domestic price rises to match the world price. Producers gain surplus; consumers lose surplus (they pay more).
- World price < domestic price → the country imports. Domestic consumers buy cheaper goods from abroad. The domestic price falls to match the world price. Consumers gain surplus; producers lose surplus (they receive less).
The gap between domestic supply and demand at the world price is filled by trade: if Qs > Qd, the surplus is exported; if Qd > Qs, the shortfall is imported.
Simple idea: The world price determines whether a country buys from or sells to the rest of the world. Compare Pw to Pe — that comparison tells you everything.
AP Exam tip: You will almost always be asked to identify who wins and who loses from trade. Remember: whoever faces a higher price gains surplus and whoever faces a lower price loses surplus — apply that to producers and consumers separately.
🛃 2. TARIFFS AFFECT PRICE, QUANTITY, REVENUE, AND TOTAL SURPLUS
What it is: A tariff is a tax on imports. The government charges foreign producers extra to sell in the domestic market, which drives the domestic price back up toward the pre-trade autarky level.
Real-world example: In 2018, the U.S. placed a 25% tariff on imported steel. American steel producers could now charge more because foreign competition was taxed. U.S. car manufacturers — who buy steel to make cars — paid higher input costs, which got passed on to consumers. The government collected billions in tariff revenue. But overall, society lost more than it gained: the deadweight loss triangles were the price of "protecting" domestic steel workers.
A tariff does six things at once:
- ↑ Domestic price (rises above Pw, toward the autarky price)
- ↑ Domestic production (domestic producers supply more at the higher price)
- ↓ Domestic consumption (consumers buy less at the higher price)
- ↓ Imports (the gap between Qd and Qs at the tariff price shrinks)
- ↑ Government revenue (tariff rate × quantity imported)
- ↓ Total surplus — two DWL triangles appear: one from domestic overproduction (inefficient domestic firms now producing), one from lost consumption (consumers who would have bought at Pw but won't at the higher price)
Simple idea: A tariff protects domestic producers but makes imported goods more expensive for everyone else — and always creates a net efficiency loss for society.
AP Exam tip: On the tariff graph, you'll need to identify four areas: (1) consumer surplus lost, (2) producer surplus gained, (3) government revenue rectangle, and (4) the two DWL triangles. Net welfare change = producer gain + government revenue − consumer loss. The two DWL triangles are the leftover loss.
📦 3. QUOTAS RESTRICT THE QUANTITY OF IMPORTS
What it is: A quota is a legal limit on the quantity of a good that can be imported. Instead of taxing imports like a tariff does, a quota simply caps the number of units that can enter the country.
Real-world example: The U.S. has long maintained a sugar quota — only a limited amount of foreign sugar can be imported each year. Because the supply of sugar in the U.S. is restricted, domestic sugar prices stay well above world prices. American candy companies famously moved production to Canada and Mexico where sugar is cheaper. The quota benefits domestic sugar farmers, hurts domestic food manufacturers and consumers, and generates no government revenue (unlike a tariff) — the gains go to whoever holds the import licenses.
What a quota does:
- ↓ Imports (capped at the quota amount)
- ↑ Domestic price (rises above Pw because supply is restricted)
- ↑ Domestic production (producers supply more at the higher price)
- ↓ Domestic consumption (consumers buy less at the higher price)
- ❌ No government revenue (unlike a tariff — this is the key difference)
Simple idea: A quota limits how much can be imported, which pushes the domestic price up — same effect on price as a tariff, but the government collects no revenue. The value that would have been tariff revenue goes to whoever holds the import licenses instead.
AP Exam tip: The big testable difference between a tariff and a quota: a tariff generates government revenue; a quota does not (the equivalent gain goes to license holders). Both create DWL triangles. Both raise domestic price and domestic production while reducing consumption and imports.
⭐ THE THREE IDEAS TO REMEMBER
| Policy / Situation | Main effect | Who benefits |
|---|---|---|
| World price > domestic price | Exports · domestic price rises to Pw | Domestic producers |
| World price < domestic price | Imports · domestic price falls to Pw | Domestic consumers |
| Tariff | Tax on imports → higher price + government revenue + 2 DWL triangles | Domestic producers + government |
| Quota | Limit on imports → higher price + restricted quantity + no gov't revenue | Domestic producers + license holders |