Shortage (Excess Demand) vs Surplus (Excess Supply)
Shortage (Excess Demand) and Surplus (Excess Supply) are two Supply & Demand concepts in AP Economics that students often mix up. A shortage occurs when quantity demanded exceeds quantity supplied at a given price. A surplus occurs when quantity supplied exceeds quantity demanded at a given price. Here is how they compare side by side.
A shortage, or excess demand, happens when consumers are willing to buy more than producers are willing to sell at the current price. This puts upward pressure on the price, as consumers compete to buy the scarce goods. The shortage will be eliminated as the price rises to the equilibrium level.
A surplus, or excess supply, happens when producers are willing to sell more than consumers are willing to buy at the current price. This puts downward pressure on the price, as producers compete to sell their excess goods. The surplus will be eliminated as the price falls to the equilibrium level.
Shortage vs Surplus: How the Two Gaps Differ
| Shortage (Excess Demand) | Surplus (Excess Supply) | |
|---|---|---|
| Price relative to equilibrium | Current price sits below equilibrium | Current price sits above equilibrium |
| Which quantity is larger | Quantity demanded exceeds quantity supplied | Quantity supplied exceeds quantity demanded |
| Pressure on price | Upward, as buyers bid the price up | Downward, as sellers cut the price |
| Quantity actually traded | Capped by the smaller quantity supplied | Capped by the smaller quantity demanded |
| Who is left unsatisfied | Buyers who cannot find the good | Sellers holding unsold stock |
| Policy that makes it persist | A binding price ceiling, such as rent control | A binding price floor, such as a minimum wage |
Both are defined at a price, not for the good
Neither word describes a good in general, and neither says anything about how much of it exists in the world. Each describes what happens in one market at one specific price. Take a market where quantity demanded is 100 minus 2 times the price and quantity supplied is 20 plus 2 times the price. Equilibrium sits at a price of 20 and a quantity of 60. Set the price at 15 and quantity demanded is 70 while quantity supplied is 50, a shortage of 20 units. Set the price at 25 instead and quantity demanded is 50 while quantity supplied is 70, a surplus of 20 units. Same market, same curves, same good, and the only thing that changed was the price. On the diagram at /micro/supply-and-demand the size of either gap is just the horizontal distance between the two curves measured at whatever price you are standing on.
How the market closes the gap, and when it cannot
In an unregulated market the gap is self-correcting, and the correction is a movement along both curves rather than a shift in either one. During a shortage, buyers who cannot get the good bid the price up, and as the price rises quantity demanded slides back down the demand curve while quantity supplied climbs up the supply curve, so the gap closes from both sides at once. A surplus runs the same process in reverse, with sellers cutting the price until the two quantities meet. Calling this adjustment a shift in demand or supply is one of the most common free-response errors, because nothing about buyers' or sellers' underlying willingness has changed. The gap persists only when something stops the price from reaching equilibrium. A binding price control is the case the exam tests, though prices can also adjust slowly for contractual or menu-cost reasons. See /blog/price-controls-ceilings-and-floors for how a ceiling or floor locks a market away from equilibrium.
The two confusions worth clearing up
The first is the word surplus, which does double duty in economics. Here it means excess supply, a quantity gap in a market whose price is too high. In welfare analysis, consumer surplus, producer surplus and total surplus instead measure the dollar value that buyers and sellers gain from trading. A competitive market with no externalities sitting exactly at equilibrium has no excess supply at all and yet achieves the largest possible total surplus, so the two meanings can point in opposite directions. The second confusion is between shortage and scarcity. Scarcity is the permanent condition that wants exceed available resources, and it holds at every price in every economy. A shortage is a temporary disequilibrium at one price that disappears once the price is free to rise to equilibrium. A shortage does not mean the good is running out; it means not enough of it is being offered at the price currently posted.
Frequently asked questions
What is the difference between a shortage and a surplus?
A shortage exists when quantity demanded exceeds quantity supplied at the current price, and a surplus exists when quantity supplied exceeds quantity demanded at the current price. Shortages appear at prices below equilibrium and push the price up, while surpluses appear at prices above equilibrium and push it down.
Does a price ceiling cause a shortage or a surplus?
A binding price ceiling causes a shortage, because a ceiling is a legal maximum price that only has an effect when it is set below the equilibrium price. At that lower price quantity demanded rises and quantity supplied falls, so buyers want more than sellers are willing to provide.
Is a shortage the same as scarcity?
No, a shortage is a temporary gap between quantity demanded and quantity supplied that exists only when the price sits below equilibrium, while scarcity is the permanent condition that wants exceed available resources at every price in every economy. A shortage disappears once the price is free to rise to equilibrium, and scarcity never disappears.
What happens to the price when there is a surplus?
The price falls, because a surplus means quantity supplied exceeds quantity demanded and sellers left holding unsold goods compete by cutting the price. As the price falls, quantity demanded rises along the demand curve and quantity supplied falls along the supply curve until the surplus is gone and the market reaches equilibrium.
Want the long version? Shortage vs Surplus: The Two States of Disequilibrium walks through the same comparison as a full guide, with worked examples and the exam traps. This page is the quick side-by-side.
Live Supply and Demand graph. Drag the curves, or open the full version.
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