Supply and Demand Explained: How Markets Actually Set Prices

Understanding supply and demand — simply put

Supply and demand is the model of how a competitive market sets a price. Demand records how much buyers want at each price; supply records how much sellers offer at each price. The equilibrium is the price at which the two quantities match. The model's real power is not the crossing point itself but what it predicts when something moves one of the curves, and how far the price must travel to restore balance.

Why did the same fuel cost several times more in Europe than in America?

Natural gas is a single commodity, yet in 2022 its price in Europe rose to several times the U.S. price. A single world price exists only when goods can move freely between markets. Gas travels by pipeline or by liquefied-gas tanker, and both are limited in the short run, so Europe, North America and Asia behave as separate markets, each with its own supply and demand curves.

When Russian pipeline deliveries collapsed, only Europe's supply curve shifted. Prices in the United States rose far less, because domestic production was unaffected and export terminals were already running near capacity. Asian prices rose as European buyers competed for the same tankers: the shock travelled exactly as far as the transport links allowed.

How far must the price move to clear the market?

The size of a price spike depends less on the size of the shock than on how steep the curves are. With demand and supply elasticities ε_d and ε_s, a supply shift of s percent changes the equilibrium price by approximately:

In the short run, gas demand is highly inelastic (homes must be heated, industrial processes cannot switch fuel overnight) and supply is close to fixed. With ε_d = −0.1 and ε_s = 0.1, a 10% loss of supply requires a price rise of about 50% to cut demand by enough. A larger loss and steeper curves produce the multiples seen in 2022.

Why do price spikes fade even when the shock does not?

Elasticities grow with time. Within a year Europe had cut gas use by more than the 15% voluntary target agreed by EU governments, built new import terminals, refilled storage and switched some industry to other fuels. The same shock, faced with flatter long-run curves, needs a much smaller price change. This is the Le Chatelier principle in economics: long-run responses are always at least as large as short-run ones, so spikes are steepest at the start.

What exactly shifts when "demand falls"?

A movement along a curve is a response to the good's own price; a shift is a change in everything else, such as income, prices of substitutes or expectations. Formally, with linear curves:

Who really pays a tax?

The legal incidence of a tax says who hands money to the government; the economic incidence says who bears it. For a per-unit tax t, the share paid by buyers through a higher price is:

This is why taxes on fuel, tobacco and other inelastic goods fall mostly on consumers, and why they raise reliable revenue with little change in quantity.

When is the equilibrium the wrong benchmark?

At the competitive equilibrium, consumer surplus plus producer surplus is as large as possible, and any tax or price control creates a deadweight loss of trades that no longer happen. That efficiency result depends on conditions that often fail: market power lets a seller restrict output to raise the price; pollution imposes costs on people outside the trade; and when sellers know much more than buyers, as in insurance or used cars, the market can shrink or collapse. In each case the supply-and-demand diagram still describes the market, but the equilibrium it predicts is no longer the efficient one.

The takeaway

Supply and demand is a theory of adjustment, not just a crossing point. Prices move until quantities match, how far they move depends on elasticities, those elasticities grow over time, and they decide who bears every tax and every shock.

Further reading

• Understanding Inflation — internal
• Understanding GDP — internal
• Council Regulation (EU) 2022/1369 on coordinated demand-reduction measures for gas — external reference