30-Second Overview

The law of supply says that as the price of a good rises, producers offer more of it for sale, and as the price falls, they offer less, assuming nothing else that affects production changes. It works because price is what the seller receives, so a higher price makes more units worth producing. The key consequence is a clean split: a change in the good's own price moves you along the supply curve, while a change in anything else, like input costs, technology, weather, or taxes, moves the entire curve.

Related Terms and Concepts

Related: Supply Curve, Quantity Supplied, Law of Demand, Supply and Demand, Market Equilibrium, Price Elasticity of Supply, Producer Surplus, Ceteris Paribus

Law of Supply

Supply is the amount of a good or service that producers are willing to make and bring to market at each possible price. The law of supply describes how that amount responds to price. A higher price leads to a higher quantity supplied, and a lower price leads to a lower one. OpenStax's Principles of Economics calls this a positive relationship, meaning price and quantity move in the same direction.

The textbook's gasoline example shows what that looks like in practice. When the price of gas rises, oil companies expand exploration, drill more wells, invest in pipelines and tankers, build refineries, and keep gas stations open longer. None of those decisions were worth making at the old price. At the new one, they are.

Supply Versus Quantity Supplied

Economists use two terms that sound interchangeable and are not. Supply is the whole relationship between price and quantity, the entire curve. Quantity supplied is one point on that curve: how much producers will offer at one specific price.

The textbook's supply schedule for gasoline makes the difference concrete. Quantities are in millions of gallons:

• At $1.00 per gallon, producers supply 500
• At $1.40, they supply 600
• At $1.80, they supply 680
• At $2.20, they supply 720

The full table is the supply. Any single row is a quantity supplied. Plot the rows with price on the vertical axis and quantity on the horizontal axis, connect them, and you get the supply curve, which slopes upward from left to right. Supply curves differ by product. Some are steeper, some flatter, some curved. Nearly all of them share that upward slope.

When the price rises and the quantity supplied rises with it, the curve itself has not moved. Producers have moved along it, from one point to another.

Why Higher Prices Bring Out More Output

Firms are motivated by profit, which is the difference between revenue and costs. Price is the revenue side of every unit sold. When price goes up and costs stay the same, each unit becomes more profitable and producing more becomes more attractive.

The textbook offers a second way to read the curve. It shows the minimum price a firm will accept to produce a given quantity. That price has two parts. The first is the cost of producing at the margin, which for a pizza shop means ingredients, the oven, rent, and wages. The second is the profit the firm wants for its trouble. Add them together and you get the price the seller needs.

Read that way, a higher market price clears the bar for more production. A bakery selling bread at $3 a loaf might only find it worth running one shift. At $5 a loaf, a second shift paid at overtime rates starts to make sense, and so does firing up the older, slower oven in the back. The extra output was always physically possible. The higher price is what made it worth doing.

What Shifts the Whole Curve (Determinants)

The supply curve relates two variables, price and quantity, and holds everything else constant. Economists call that assumption ceteris paribus, Latin for "other things being equal." When something other than the good's own price changes, the assumption no longer holds and the whole curve moves.

Almost every supply shifter works through the same channel: the cost of production. The textbook groups them like this:

• Input prices. When steel gets more expensive, making a car costs more. In the textbook's example, car makers who would supply 18 million cars at $20,000 now supply only 16.5 million at that same price. If steel gets cheaper, the quantity at $20,000 rises to 19.8 million.
• Natural conditions. A drought cuts the supply of crops at every price. Unusually good weather does the opposite.
• Technology. A new method that lowers production costs shifts supply to the right. The textbook points to the Green Revolution seeds that doubled wheat and rice harvests per acre in many low-income countries.
• Government policy. Firms treat taxes and the cost of complying with regulations as costs of production, so both shift supply left. Subsidies lower costs and shift supply right.

A rightward shift means producers offer more at every price. A leftward shift means they offer less at every price.

The textbook also shows the shift from the price side. If cheese costs a pizza shop an extra $0.75 per pizza, the shop needs $0.75 more to produce the same quantity. The curve moves up by that amount, which is the same thing as the curve moving left.

The Mistake the Law Is Built to Prevent

The most common error with supply is letting price shift the curve. The textbook walks through a student who reasons like this: a drought reduces the supply of wheat, which raises the price, and then the higher price makes farmers supply more, so the curve shifts back to the right. The first step is correct. The second is wrong.

A change in a good's own price never shifts that good's supply curve. It only moves producers to a different point on the curve they already have. The drought shifted supply. The higher price that followed is a movement along the new curve, not a reason to redraw it.

A quick test is to ask what changed first. If it was the price of the good itself, you are moving along the curve. If it was anything that changes what it costs to produce the good, the curve moves.

What the Law Does Not Tell You

The law of supply gives you direction, not size. It says quantity supplied rises with price. It does not say by how much. Whether a 10% price increase brings a flood of new output or barely any is a separate question, called the price elasticity of supply, which the textbook covers in Chapter 5.

The textbook also says a higher price almost always increases quantity supplied, and the hedge is deliberate. The law describes producer behavior with everything else held constant, and in real markets everything else is rarely constant. Costs, technology, and regulation change while price does. The law is useful precisely because it lets you separate those effects and look at them one at a time.

TLDR

Higher prices bring out more supply because they make more production worth doing, and the supply curve shows the lowest price at which each quantity gets made. The part people get wrong is the difference between moving along the curve and moving the curve. The good's own price only ever moves you along it. Costs, weather, technology, and government policy are what move the curve itself. Keep those two categories separate and most supply questions answer themselves.