Where demand meets supply

A competitive market is a market structure with numerous buyers and sellers, none of whom can determine the market price alone. The demand curve shows buyers' willingness and ability to purchase. The supply curve shows sellers' willingness and ability to produce.

A market equilibrium occurs when quantity demanded equals quantity supplied, leaving no pressure for the price to change. On a diagram, this is where the demand and supply curves intersect. The price at this point is the equilibrium price; the amount bought and sold is the equilibrium quantity.

On a market diagram, the vertical axis shows $P$, where $P$ is price per unit measured in currency units. The horizontal axis shows $Q$, where $Q$ is quantity traded measured in units per period. Label the downward-sloping demand curve $D$ and the upward-sloping supply curve $S$. Mark their intersection $E$, the equilibrium price $P_e$ and the equilibrium quantity $Q_e$, where $P_e$ is the equilibrium price in currency units per unit and $Q_e$ is the equilibrium quantity in units per period.

At $P_e$, buyers want to purchase exactly $Q_e$, while firms want to sell exactly $Q_e$. The market therefore clears: every unit offered by sellers at that price can find a buyer. Equilibrium doesn't mean everyone is satisfied, nor does it mean the outcome is necessarily fair. It simply describes a situation in which the plans of buyers and sellers are mutually consistent at the prevailing price.

Disequilibrium and price adjustment

An excess demand, or shortage, occurs when quantity demanded is greater than quantity supplied at the current price. When price sits below equilibrium, buyers compete for the limited output, pushing the price upwards. As it rises, quantity demanded contracts while quantity supplied extends. This continues until the two are equal again.

An excess supply, or surplus, occurs when quantity supplied is greater than quantity demanded at the current price. Above the equilibrium price, firms cannot sell all their output, so the price is pushed down. Quantity demanded then extends and quantity supplied contracts until the market returns to equilibrium.

Watch the terminology here. If the curves stay fixed, a price change causes movements along the demand and supply curves; neither curve shifts.

A shift in demand

A non-price determinant of demand changes the position of the entire demand curve. Suppose preferences shift towards a product. Demand moves right from $D_1$ to $D_2$: $D_1$ is the initial demand curve and $D_2$ is the new one. At the old equilibrium price, quantity demanded is now greater than quantity supplied, so excess demand appears. Price rises as a result, causing an extension of supply and a contraction of demand along the new curve. The new equilibrium has a higher price and a larger quantity.

For a decrease in demand, the chain works in reverse. A leftward shift creates excess supply at the old price, causing both equilibrium price and equilibrium quantity to fall.

A shift in supply

Suppose better production methods lower firms' unit costs. Supply shifts right from $S_1$ to $S_2$, with $S_1$ as the initial supply curve and $S_2$ as the new supply curve. At the old equilibrium price, quantity supplied is greater than quantity demanded. This excess supply pushes the price down. The new equilibrium has a lower price and a larger quantity.

A decrease in supply shifts the curve left. Excess demand then develops at the old price, raising equilibrium price and reducing equilibrium quantity.

For any applied change, work through the whole chain. Identify the relevant non-price determinant, then state which curve shifts and in which direction. Next, identify the imbalance at the old price and explain what happens to the new equilibrium price and quantity. On a market diagram, label both axes, every curve, both equilibria and the projected prices and quantities.

When both curves shift

When demand and supply change at the same time, consider their effects separately. An increase in demand and a decrease in supply both raise equilibrium price, but they have opposing effects on quantity. Without knowing the relative size of each shift, the change in quantity is indeterminate. A decrease in demand combined with an increase in supply definitely lowers price, while the change in quantity remains indeterminate. If both curves shift in the same direction, the change in quantity is clear, but the effect on price depends on which shift is larger.

Coordinating choices through prices

The price mechanism is the market process by which changes in demand and supply alter prices and guide the allocation of scarce resources. It coordinates decisions made by separate consumers and producers. Consumers don’t tell firms what to produce, but their purchases influence prices and profitability.

Resource allocation refers to how factors of production are distributed among competing uses. Through the price mechanism, land, labour and capital tend to shift towards markets where demand and potential returns have risen. They move away from markets where demand and returns have fallen.

Three connected functions drive this process:

• Signalling is the information function of a price change, communicating a change in relative scarcity or market conditions. A rising price may show producers that a good is more highly valued or less readily available. For consumers, it signals that obtaining the good now involves a greater sacrifice.
• Incentive is the motivational function through which a change in price encourages economic decision-makers to adjust their behaviour. A higher price can raise the prospective reward from production, prompting firms to expand output and attract additional resources. Consumers have an incentive to conserve the good or switch towards substitutes.
• Rationing is the allocation function through which price limits a scarce product to buyers who are both willing and able to pay. If demand exceeds available supply, a rising price lowers quantity demanded and allocates the limited output among the buyers who remain.

These functions work together. Suppose stronger demand raises the price. The new price signals an opportunity to producers and gives them an incentive to expand, while rationing the product for as long as output remains scarce. Firms then demand more factors of production, shifting resources towards that market. At the same time, the higher price changes consumer choices by encouraging substitution or reduced consumption.

Gains from market exchange

A demand curve shows the maximum price consumers will pay for each successive unit. Because some buyers would have paid more than the single market price, they gain from the exchange.

Consumer surplus is the welfare gain created by the difference between the highest price consumers are willing to pay and the price they actually pay. For one unit, it equals willingness to pay minus market price. Across the whole market, consumer surplus is the area below the demand curve but above the market price, up to the quantity traded. The entire area below demand does not count—only the part above the price line.

A supply curve shows the minimum price producers will accept for each successive unit. Some firms would have supplied their units at a price below the market price.

Producer surplus is the welfare gain created by the difference between the price producers receive and the minimum price they are willing to accept. Across the whole market, it is shown by the area above the supply curve and below the market price, up to the quantity traded.

Consumer surplus is the benefit buyers receive; producer surplus is the benefit sellers receive. Neither one is revenue. Producer revenue includes the amount needed to cover the minimum acceptable price shown by the supply curve, while producer surplus includes only the amount above it.

Combining the gains to buyers and sellers

Social surplus, also called community surplus, measures total market welfare. It equals consumer surplus plus producer surplus, so it captures the combined gains consumers and producers receive from the units exchanged:

$$\text{Social surplus} = \text{consumer surplus} + \text{producer surplus}$$

In a competitive market diagram, social surplus covers the whole area between the demand and supply curves, from zero output to the equilibrium quantity. The market price splits this area between consumers and producers. Moving the price line alone changes how the gains are distributed, not the combined area, as long as the same efficient quantity is traded.

Within this model, social surplus gives a specific measure of welfare. On its own, it doesn’t show whether the gains are distributed fairly. The diagram also includes only the benefits and costs shown by the demand and supply curves.

The quantity that maximizes social surplus

Allocative efficiency occurs when resources produce the combination and quantity of goods that gives society the greatest net benefit. In the competitive market model, this happens at the equilibrium quantity, where social surplus reaches its maximum.

Marginal benefit is the additional benefit society gains from consuming one more unit of a good. It is shown by $MB$, where $MB$ is marginal benefit measured in currency units per additional unit. Since consumers’ willingness to pay reflects the benefit of an extra unit, the demand curve can be read as the marginal-benefit curve.

Marginal cost is the additional opportunity cost to society of producing one more unit of a good. It is shown by $MC$, where $MC$ is marginal cost measured in currency units per additional unit. Firms require a price that covers the cost of supplying an additional unit, so the supply curve can be interpreted as the marginal-cost curve.

Allocative efficiency occurs where:

$$MB = MC$$

Below the equilibrium quantity, $MB > MC$. An additional unit provides more benefit than it costs to produce, so a rise in output increases social surplus. The unproduced units between the current quantity and equilibrium represent mutually beneficial exchanges that have been missed.

Above the equilibrium quantity, $MC > MB$. These additional units cost society more to produce than consumers value them, which reduces social surplus. The resources should instead move to uses where they provide greater benefits.

At the competitive equilibrium, demand intersects supply and $MB = MC$. The market produces every unit whose benefit is at least as great as its cost, but not units whose cost would exceed their benefit. Social surplus is therefore maximized at the equilibrium quantity. This isn’t because each stakeholder receives an equal share; rather, no further change in output can raise the model’s total net benefit.

Reading the relevant areas

Consumer and producer surplus can be worked out as geometric areas. When the demand and supply curves are straight, each surplus will usually form a triangle. Use:

For consumer surplus, the base usually extends from zero to the equilibrium quantity. Its height is the gap between the demand curve's price-axis intercept and the equilibrium price. Producer surplus also has a base that extends to the equilibrium quantity, but its height runs from the supply curve's price-axis intercept to the equilibrium price.

A rectangular shaded region is found by multiplying base by height. For a trapezium, use the trapezium-area formula or divide the shape into a rectangle and a triangle. First, though, identify the correct surplus region. Choose the area formula afterwards.

Check the axis scales carefully. If quantity is given in thousands of units, a reading of 40 on the diagram represents 40,000 units. Multiplying a price per unit by a quantity recorded in thousands gives thousands of currency units, unless the quantity is converted first. Since surplus measures value rather than physical quantity, the final answer needs a monetary unit.

To find social surplus, add the calculated consumer and producer surpluses. Where the two shaded regions make a single triangle between demand and supply, calculating the combined area directly produces the same result.