Shortage vs Surplus: The Two States of Disequilibrium

A shortage and a surplus are the two ways a market can be out of balance. A shortage (also called excess demand) occurs when the quantity demanded exceeds the quantity supplied at the current price, which happens when the price sits below equilibrium. A surplus (excess supply) occurs when the quantity supplied exceeds the quantity demanded, which happens when the price sits above equilibrium. In both cases the market is in disequilibrium, and unless something stops it, the price will move to erase the gap. This guide works through both with real numbers, shows exactly how prices adjust back to balance, and explains why price controls can freeze a shortage or surplus in place. See shortage (excess demand) and surplus (excess supply).

Start from equilibrium

A market is in equilibrium when the quantity demanded equals the quantity supplied, so there is no pressure on the price to change. The price where that happens is the equilibrium price, and the quantity is the equilibrium quantity. See market equilibrium. Shortages and surpluses are simply what you get when the actual price is not the equilibrium price.

Here is a demand and supply schedule for a market in coffee mugs. Quantity demanded falls as price rises, following the law of demand, and quantity supplied rises as price rises, following the law of supply.

Equilibrium is at $6, where both quantity demanded and quantity supplied equal 60. Every other price produces a gap.

Working out a shortage

Look at the $4 row. Buyers want 80 units but sellers only offer 40, so quantity demanded exceeds quantity supplied by:

Shortage = Quantity demanded - Quantity supplied = 80 - 40 = 40 units

At this low price the good is a bargain, so lots of people want it, but few sellers find it worth supplying. Shelves empty, waiting lists form, and some willing buyers go home empty-handed. That frustrated demand is the signal of a shortage.

Drop the price further, to $2, and the shortage grows to 100 - 20 = 80 units. The lower the price below equilibrium, the bigger the shortage. This is why a shortage is a low-price problem, not a sign that the good is scarce in some absolute sense.

Working out a surplus

Now look at the $8 row. Sellers offer 80 units but buyers only want 40, so quantity supplied exceeds quantity demanded by:

Surplus = Quantity supplied - Quantity demanded = 80 - 40 = 40 units

At this high price sellers are eager to produce, but the steep price scares off buyers. Unsold inventory piles up in warehouses. Raise the price to $10 and the surplus widens to 100 - 20 = 80 units. A surplus is a high-price problem: the further price sits above equilibrium, the more goes unsold.

How prices adjust back to equilibrium

In a free market, a shortage or surplus does not last, because the imbalance itself pushes the price toward equilibrium.

A shortage pushes the price up. When buyers compete for too few goods at $4, some offer to pay more, and sellers happily raise prices. As the price rises from $4 toward $6, two things happen at once: quantity demanded falls (from 80 toward 60) because the good is now pricier, and quantity supplied rises (from 40 toward 60) because production becomes more profitable. The gap shrinks from both sides until it closes at $6.

A surplus pushes the price down. When sellers are stuck with unsold mugs at $8, they cut prices to move inventory. As the price falls from $8 toward $6, quantity demanded rises (from 40 toward 60) and quantity supplied falls (from 80 toward 60). Again the gap closes at equilibrium.

This self-correction is the reason economists treat equilibrium as the market's resting point. You can watch it happen by dragging the curves in the supply and demand sandbox, and the full model is covered in our supply and demand guide.

When the adjustment is blocked: price controls

The self-correcting story assumes the price is free to move. Price controls are government limits that stop it, and they are the main reason a shortage or surplus can persist instead of vanishing.

A price ceiling causes a lasting shortage. A price ceiling is a legal maximum price. To have any effect it must be set below equilibrium, which is called binding. Suppose the government caps mug prices at $4. From the table, quantity demanded is 80 and quantity supplied is 40, a shortage of 40 that cannot self-correct, because the price is legally forbidden from rising to $6. The shortage is now permanent. Rent control is the classic real-world example: capped rents below the market rate produce a lasting shortage of apartments.

A price floor causes a lasting surplus. A price floor is a legal minimum price, and to bind it must sit above equilibrium. Suppose mugs get a floor of $8. Quantity supplied is 80 and quantity demanded is 40, a surplus of 40 that cannot clear, because the price is forbidden from falling to $6. The minimum wage is a price floor in the labor market: set above the equilibrium wage, it produces a surplus of labor, which is unemployment. Agricultural price supports create surpluses of crops the government then has to buy or store.

For the full treatment of how ceilings and floors work, including their effect on total surplus and deadweight loss, see our guide on price controls: ceilings, floors, shortages, and surpluses.

Do not confuse a shortage with scarcity

Two quick clarifications the exam rewards. A shortage is not the same as scarcity. Scarcity is the permanent condition that resources are limited relative to wants, and it applies to every good all the time. A shortage is a temporary, price-specific mismatch that disappears when the price rises to equilibrium. A market at equilibrium still faces scarcity but has no shortage.

Similarly, a market surplus is not the same as a firm's producer surplus or a wasteful oversupply in a moral sense. A market surplus is just quantity supplied exceeding quantity demanded at a too-high price, and it clears when the price falls.

Summary

A shortage is excess demand at a price below equilibrium; a surplus is excess supply at a price above equilibrium. Compute each as the simple difference between quantity demanded and quantity supplied at the going price. In a free market the price rises to erase a shortage and falls to erase a surplus, both pushing back toward the equilibrium where quantity demanded equals quantity supplied. The exception is price controls: a binding price ceiling locks in a shortage and a binding price floor locks in a surplus. Practice the adjustment on the supply and demand sandbox and lock in the vocabulary in the glossary.