Supply and Demand: How Markets Set Prices

Two Curves, One Price

Demand describes how much of a good or service buyers want to purchase at each possible price. Generally, the lower the price, the more people want to buy, which is why demand is typically drawn as a downward-sloping line as price rises. Supply describes how much sellers are willing to produce and sell at each possible price. Generally, the higher the price, the more sellers are willing to produce, since it becomes more profitable to do so, which is why supply typically slopes upward.

The price where these two forces meet, where the quantity buyers want to purchase exactly equals the quantity sellers want to sell, is called the equilibrium price.

What Happens Away From Equilibrium

If a price is set above equilibrium, sellers want to sell more than buyers want to buy at that price, creating a surplus: unsold inventory piles up, which pressures sellers to lower prices to move it. If a price is set below equilibrium, buyers want more than sellers are willing to supply, creating a shortage: buyers compete for limited goods, which tends to push prices back up.

This self-correcting tendency is why, absent outside interference like price controls, markets tend to drift toward the equilibrium price over time rather than staying stuck at a surplus or shortage.

A Worked Example

Suppose a farmers' market sells a particular apple variety. At $3 per pound, farmers are willing to bring 200 pounds to market, but shoppers only want to buy 120 pounds, an 80-pound surplus. Unsold apples start to spoil, so farmers drop the price to $2 per pound. At $2, shoppers want to buy 180 pounds, but farmers are only willing to supply 150 pounds, a 30-pound shortage, and impatient buyers start offering a bit more to secure apples.

At $2.50 per pound, suppose farmers supply exactly 165 pounds and shoppers want to buy exactly 165 pounds. That's the equilibrium: no surplus pushing prices down, no shortage pushing prices up.

Shifts vs. Movements

It's worth distinguishing a movement along a curve from a shift of the entire curve. A movement along the demand curve happens when price changes and buyers respond, as in the apple example above. A shift happens when something other than price changes buyers' or sellers' behavior at every price level. For example, a heat wave destroying part of the apple harvest would shift the supply curve left, meaning less available at every price, pushing the equilibrium price up even if buyer preferences never changed at all.

Common shift factors include changes in income, consumer tastes, the price of related goods, production costs, and the number of buyers or sellers in the market.

Why This Matters Beyond Apples

The same logic applies to labor markets, housing markets, currency markets, and virtually anywhere buyers and sellers interact. Understanding supply and demand doesn't let you predict exact prices, but it does explain why prices move the way they do in response to real-world events, such as a supply disruption, a surge in popularity, or a new competitor entering the market, without needing to track every transaction individually.