Lesson preview · Market Equilibrium

Equilibrium

~8 min · Free to read

Alfred Marshall had a favorite metaphor: supply and demand are like two blades of a pair of scissors. You can’t cut with just one. Ask whether price is set by supply or demand and Marshall would have laughed — it’s set by both, simultaneously, at the single point where the two blades meet. That meeting point has a name: equilibrium.

Key Term

Market Equilibrium

The price and quantity where the supply curve and demand curve intersect. At this point, the quantity buyers want to buy equals the quantity sellers want to sell — the market clears.

The equilibrium condition: quantity demanded equals quantity supplied.

Equilibrium has two coordinates, usually written with a star: • $P^{*}$ — the equilibrium price • $Q^{*}$ — the equilibrium quantity

At $P^{*}$ , every buyer willing to pay that price finds a seller, and every seller willing to accept that price finds a buyer. Nobody goes home empty-handed; nobody is stuck with leftover inventory.

Interactive — Croissant market equilibrium

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Demand and supply for the bakery’s croissants, built up step by step until they cross at E*.

Tip

Why it's called equilibrium

Just like a marble at the bottom of a bowl, a market at equilibrium has no reason to move. Push it away (with a price control or a shock) and it tends to roll back. Equilibrium is the resting state.

Finding equilibrium with math. Graphs are great for intuition, but you’ll often need a number. The recipe has three steps and never changes:

Key Term

The Equilibrium Recipe

1. Set $Q_{d} = Q_{s}$ . 2. Solve for $P$ — that’s your $P^{*}$ . 3. Plug $P^{*}$ back into either equation to find $Q^{*}$ . Both equations should give the same $Q^{*}$ — if not, you’ve made an algebra error.

Worked Example

A local bakery estimates daily croissant demand at $Q_{d} = 20 - 2 P$ and supply at $Q_{s} = 4 + 2 P$ (P in dollars, Q in dozens). Find $P^{*}$ and $Q^{*}$ .

Tip

When equations use P = ... (inverse form)

Sometimes you’ll see inverse demand like $P = 50 - 0.5 Q$ . Either solve for $Q$ first, or set the two $P$ equations equal: $50 - 0.5 Q = 10 + Q$ , then solve for $Q$ . Same logic, same answer.

Key Term

Why Equilibrium Matters: Allocative Efficiency

At $P^{*}$ , everyone who values the good at LEAST $P^{*}$ buys it, and everyone who can produce it for AT MOST $P^{*}$ sells it. Goods flow to those who value them most, made by those who can make them cheapest. Competitive equilibrium achieves allocative efficiency automatically — without central planning.

Friedrich Hayek called this ‘the marvel of the price system.’ No single person knows why wheat prices rose — just that they did. But because they did, bakers economize, farmers plant more, and consumers switch to rice. A problem nobody understands gets solved by millions of people following local incentives.

Trap · Common Misconception

Equilibrium ≠ 'fair' or 'best'

Equilibrium just means the market clears. It doesn’t mean everyone can afford the good, or that the price is morally right. Allocative efficiency (resources flowing to highest-value users) is a separate question from equity. A life-saving drug at its equilibrium price can be efficient and still priced out of reach for poor patients.

Check yourself · no marks

Why plug $P^{*}$ back into BOTH equations at the end?

Commit to one first. Guessing and being wrong beats reading the answer cold.

Practice · 1 / 5

No penalty for a wrong answer

Find and interpret equilibrium:

At market equilibrium, which is TRUE?

Multistep Problem0/4 · +20 XP

A market for widgets has the following equations: • Demand: $Q_{d} = 200 - 5 P$ • Supply: $Q_{s} = - 40 + 3 P$ Work through the following steps to find the equilibrium, then see how the market changes when a per-unit tax is imposed.

Find the equilibrium price $P^{*}$ . Set $Q_{d} = Q_{s}$ and solve.

Using $P^{*} = 30$ , find the equilibrium quantity $Q^{*}$ .

An 8-dollar per-unit tax is placed on sellers, shifting the supply curve up by 8. The new supply is $Q_{s} = - 40 + 3 (P - 8)$ . Find the new equilibrium price consumers pay, $P_{c}$ .

With $P_{c} = 33$ , how much of the 8-dollar tax is paid by consumers (in dollars)?

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