Every market has two sides. Chapter 6 studied the buyers — how the quantity they demand responds to price and how their behavior can be summarized in a demand curve. Chapter 7 turns to the other side: the sellers. Supply describes how producers respond to price. When the price of a good rises, producers generally become more willing and more able to offer larger quantities for sale.

When costs rise, or when conditions change, producers adjust their behavior — sometimes dramatically. Understanding supply helps explain why prices change, why some goods become scarce, why firms enter and exit industries, and why government policies that affect producers ripple through markets in predictable ways.

This chapter builds the concept of supply carefully, beginning with what supply means and what the law of supply says, then examining supply schedules and supply curves, the difference between movement along a curve and a shift of the curve, and the major factors that shift supply. By the end of this chapter, you will be ready to combine supply with demand in Chapter 8 and analyze how markets reach equilibrium.

In everyday language, "supply" sometimes means the total amount of something that exists in the world — as in, "the global supply of fresh water." Economists use the word differently.

In economics, supply refers to the relationship between the price of a good or service and the quantity that producers are willing and able to offer for sale during a particular time period, holding all other factors constant.

Three elements of this definition matter: willing — a producer must actually want to sell at a given price, since the price must be high enough to make selling worthwhile given the producer's costs and alternatives; able — a producer must have the capacity to produce and sell, meaning the inputs, technology, and time required; and at a given price — supply is not a single number, but a relationship between price and quantity, where different prices lead to different quantities supplied.

Supply describes producer behavior in the same way that demand describes consumer behavior. Just as demand focuses on the buyer's side, supply focuses on the seller's side. Both together will explain how markets work.

The central proposition about producer behavior is the law of supply: all else equal, when the price of a good or service rises, the quantity supplied increases; when the price falls, the quantity supplied decreases. Price and quantity supplied move in the same direction — the opposite of demand, where price and quantity demanded move in opposite directions.

Why does the law of supply hold? Higher prices make production more profitable. If a wheat farmer can sell wheat for $8 per bushel instead of $5, growing wheat becomes more financially rewarding, giving the farmer an incentive to plant more acres and produce a larger crop. Higher prices attract new producers who previously found the market unprofitable. Higher prices justify accepting higher costs, since producing more often means using less ideal inputs or less efficient equipment, and producers will supply these costlier units only if the price covers those costs. Lower prices make production less profitable, leading producers to reduce quantity supplied or exit the market.

The law of supply is not an absolute physical law — it is a generalization about producer incentives that holds reliably across a wide range of goods and markets, assuming other relevant factors stay constant.

A supply schedule is a table showing the quantities that producers are willing and able to supply at various prices during a given time period. Consider a small local bakery that produces loaves of sourdough bread.

Reading the table: at a price of $4 per loaf, the bakery is willing to supply 20 loaves per day. At $10, it is willing to supply 140 loaves. At $2, it supplies nothing — the price is too low to cover costs. Notice that as price rises, quantity supplied rises, expressing the law of supply numerically.

The schedule above is an individual supply schedule. When we add up the quantities that all producers in a market are willing to supply at each price, we get a market supply schedule.

At each price, market quantity supplied is simply the sum of all individual producers' quantities supplied. This horizontal addition of individual supply schedules produces the market supply schedule.

A supply curve is a graph showing the relationship between the price of a good and the quantity supplied, with price on the vertical axis and quantity on the horizontal axis. Each point on the curve shows a price-quantity pair from the supply schedule, and the curve slopes upward from left to right.

The upward slope of the supply curve reflects the law of supply: as price rises (moving up the vertical axis), quantity supplied increases (moving right along the horizontal axis). The supply curve is a simplified picture of producer behavior; in reality, supply relationships can be curved or kinked, but the essential feature — upward slope — captures the fundamental economic behavior we need to analyze.

Individual supply curves show the supply behavior of a single producer. Market supply curves show the supply behavior of all producers combined. Market supply curves are also upward sloping, but they are often flatter (more elastic) than individual curves, because as price rises, both existing producers supply more and new producers enter the market.

Individual supply describes a single firm's willingness and ability to sell at various prices. A single wheat farmer, a single software company, or a single bakery each has its own supply curve. Market supply describes the total supply from all firms in a market, constructed by adding, at each price, the quantities that all individual producers are willing to supply.

This horizontal summation is straightforward in a table: at each price, add up all producers' quantities. On a graph, market supply is drawn to the right of any individual curve because it represents a larger total quantity.

When we analyze markets — how prices are determined, how they respond to shocks, how policies affect producers — we almost always work with market supply. Individual supply curves matter when we study firm behavior in Part III. But for understanding market outcomes, the market supply curve is the relevant tool.

If new firms enter the market, market supply increases. If firms exit, market supply decreases. This is one of the important shifters of supply examined in Section 7.7.

A movement along a supply curve occurs when a change in price causes a change in the quantity supplied, with all other factors held constant. This movement is also called a change in quantity supplied. When the price of a good rises, we move up and to the right along the existing supply curve — quantity supplied increases. When price falls, we move down and to the left — quantity supplied decreases.

A shift in supply occurs when something other than price changes, causing producers to supply a different quantity at every price. When supply shifts, the entire supply curve moves — either to the right (supply increases) or to the left (supply decreases). A rightward (outward) shift means producers supply more at every price than before. A leftward (inward) shift means producers supply less at every price than before.

The factors that shift supply are called supply shifters or determinants of supply. The major ones are examined in Sections 7.8 and 7.9.

The two most fundamental determinants of supply are the costs of inputs and the state of technology available to producers.

Input costs. To produce any good or service, producers must purchase or hire inputs — also called factors of production — including labor, raw materials, energy, equipment, and land. When input costs fall, supply increases: lower costs mean higher profit margins, incentivizing more production at every price. When input costs rise, supply decreases: higher costs squeeze profit margins and make some production unprofitable.

Technology refers to the knowledge and methods used to turn inputs into outputs. A technological improvement means producers can get more output from the same quantity of inputs, or produce the same output at lower cost. When technology improves, supply increases — at every price, producers can now offer more output, shifting the supply curve right. Technological change rarely makes supply decrease; an older method of production does not typically displace a newer, more efficient one unless there are regulatory barriers or other constraints.

Taxes. A tax on production is an increase in costs for producers. When a government taxes a good — for example, an excise tax on each unit sold — the effective cost of supplying that good rises, shifting the supply curve to the left: producers supply less at every price. Examples include a gasoline tax, a corporate income tax on profits, or a carbon tax on emissions.

Subsidies. A subsidy is a payment by the government to producers, effectively reducing their cost of production. Subsidies shift the supply curve to the right — at every price, producers are willing and able to supply more, because their effective costs have been reduced. Examples include agricultural subsidies for corn, wheat, and cotton, and subsidies to electric vehicle manufacturers.

Expectations. Producers are forward-looking and make decisions today based partly on what they expect future prices and costs to be. If producers expect the price of their good to rise in the future, they may withhold current supply, reducing current supply. If they expect the price to fall, they may rush to sell more now, increasing current supply. If they expect input costs to rise, they may stockpile production now, temporarily increasing current supply. Expectations matter most in industries where output can be stored, such as oil, grain, and metals.

Number of sellers. Market supply is the sum of all individual supplies. When more firms enter a market, market supply increases — the supply curve shifts right. When firms exit, market supply decreases — the supply curve shifts left. In competitive markets, high prices attract new entrants, eventually pushing supply up and prices back down — a dynamic central to Chapter 12.

Natural conditions. For some industries, weather, natural disasters, disease, and environmental conditions are important supply shifters, particularly in agriculture, fishing, mining, and energy. A drought reduces the supply of crops; a hurricane disrupts oil production; a particularly good growing season increases supply.

The supply model introduced in this chapter is a simplification — a tool for reasoning — not a complete description of every real-world industry. But it captures the most important features of producer behavior in a wide range of markets.

Agricultural markets. Farmers decide how much to plant based on expected prices, input costs, and expectations about weather. A drought shifts supply left and raises food prices. A bumper crop shifts supply right and can actually harm farmers by driving prices so low that total revenue falls — a concept related to elasticity studied in Chapter 9. Government agricultural policies, including price supports, subsidies, and crop insurance, shift supply curves in predictable ways.

Energy markets. Oil, natural gas, coal, and electricity markets are intensely supply-driven. The discovery of new reserves, improvements in extraction technology, and geopolitical disruptions all shift supply. The shale revolution dramatically increased the supply of oil and natural gas in North America through hydraulic fracturing, lowering prices. Environmental regulations can shift supply leftward by raising extraction or use costs.

Technology and manufacturing. In electronics, computing, and manufacturing, supply has been shifting rightward for decades due to technological improvements, reducing prices and making products more widely available.

Labor markets. Labor supply, introduced fully in Chapter 15, behaves similarly to the supply of goods — higher wages generally attract more workers — though with important differences at very high wage levels.

Services. Service industries such as healthcare, education, financial services, and hospitality also have supply curves, though supply is harder to expand quickly when workers must be trained or licensed. A shortage of nurses, teachers, or software engineers reflects a supply constraint that wages alone cannot quickly resolve.

Graph 1: Individual and Market Supply Curves. Identify what each curve represents. Confirm upward slope and confirm that market supply lies to the right of each individual supply curve at every price.

Graph 2: Movement Along the Supply Curve. Given a supply curve for gasoline, show what happens to quantity supplied when price rises from $3.00 to $4.00 per gallon. Mark the two points and label the movement; the curve itself does not shift.

Graph 3: Shifts in Supply. Draw an original supply curve S₁ for wheat. Draw S₂ showing the effect of a drought (shift left). Draw S₃ showing the effect of improved seed technology (shift right). Label each curve and explain the direction of each shift.

Graph 4: Tax Effect on Supply. Draw a supply curve for cigarettes. Impose an excise tax and show how the curve shifts. Label the original curve S₁ and the new curve S₂.