Foundationsmicroeconomics
Shortages and Surpluses: Reading Market Disequilibrium
Overview
Shortages and surpluses are the two forms of market disequilibrium: at the going price, buyers' and sellers' plans do not match. A shortage (excess demand) occurs when quantity demanded exceeds quantity supplied; a surplus (excess supply) is the reverse. This lesson teaches how to identify and measure each on a graph and in a table, how markets naturally eliminate them through price adjustment, and when they persist -- under price ceilings, price floors, or sticky prices.
Learning objectives
- Define shortage and surplus and compute each from a schedule.
- Locate shortages and surpluses on a supply-and-demand graph.
- Explain how price adjustment eliminates each in a free market.
- Explain why each persists under price controls.
Definitions and measurement
At any price, compare quantity demanded with quantity supplied. If Qd > Qs, the difference is a shortage (excess demand): buyers collectively want more than sellers offer. If Qs > Qd, the difference is a surplus (excess supply): sellers offer more than buyers want. The size is simply the gap -- at $3, if Qd = 120 and Qs = 80, the shortage is 40 units. Note the terminology: shortages and surpluses describe quantities at a price, never shifts of curves.
Qd > Qs: shortage of (Qd - Qs). Qs > Qd: surplus of (Qs - Qd). Qd = Qs: equilibrium.
Seeing them on the graph
On a supply-and-demand diagram, pick any price and read horizontally: the demand curve gives Qd, the supply curve gives Qs. Above equilibrium, the supply curve lies to the right of demand -- the horizontal gap between the curves is the surplus. Below equilibrium, demand lies to the right of supply -- the gap is the shortage. The gap is largest far from equilibrium and shrinks to zero exactly at the intersection. This horizontal-gap reading is the fastest way to answer disequilibrium questions.
How free markets resolve them
Shortages resolve upward: frustrated buyers bid against each other, sellers notice they can raise prices and still sell out, so price rises -- quantity demanded falls and quantity supplied rises until the gap closes. Surpluses resolve downward: sellers stuck with inventory cut prices to move stock, bargain-hunting buyers buy more, so price falls until the gap closes. In both cases no planner is needed; the imbalance itself creates the incentives that eliminate it. Speed varies -- gasoline prices adjust daily, apartment rents take months.
When they persist: blocked or sticky prices
Disequilibrium persists when price cannot do its job. A binding price ceiling blocks the upward adjustment, freezing a shortage in place (rent control). A binding price floor blocks the downward adjustment, freezing a surplus (minimum wage, farm supports). Prices can also be 'sticky' for institutional reasons -- long-term contracts, menu costs of changing prices, or regulations -- so shortages and surpluses linger even without formal controls. Persistent disequilibrium is therefore a clue: look for the price rigidity causing it.
Temporary shortage/surplus: market is adjusting. Persistent shortage/surplus: some rigidity -- a control, contract, or cost -- is blocking price adjustment.
Shortages vs. scarcity (again)
Students often conflate these. Scarcity is the permanent background condition (unlimited wants, limited resources); a shortage is a specific, measurable gap at a particular price, and it can be eliminated by letting price rise. Saying 'there is a shortage of affordable housing' usually describes scarcity-plus-high-prices, not a true Qd > Qs gap. Use 'shortage' only when you can point to the price and the unfilled quantity demanded.
Key points
- Shortage: Qd > Qs (excess demand). Surplus: Qs > Qd (excess supply). Size = the gap.
- On a graph, each is the horizontal distance between the curves at the given price.
- Free markets eliminate shortages via rising prices and surpluses via falling prices.
- Persistent disequilibrium signals blocked adjustment: price ceilings, floors, or sticky prices.
- Shortage is a measurable gap at a price; scarcity is the permanent background condition.
Common misconceptions
✓ Truth: A shortage is defined at a price, and in free markets it is self-correcting: rising prices bring forth more supply and cool demand. Permanent disappearance would require the supply curve itself to vanish, which is a different (and rarer) situation.
✓ Truth: They are signals, not verdicts. A surplus of last year's phones means bargains for buyers; a 'shortage' of concert tickets at face value just means the face price is below market value. What matters is whether the price is free to adjust.
✓ Truth: If prices rose enough to balance Qd and Qs, there is indeed no shortage -- just a high price. Empty shelves at the old price with no price increase is the true shortage. The distinction matters for diagnosing whether price is adjusting.
Practice questions
Q1. At $10, quantity demanded is 200 and quantity supplied is 140. There is:
- A surplus of 60.
- A shortage of 60.
- Equilibrium.
- A surplus of 340.
Qd (200) exceeds Qs (140), so buyers want 60 more units than sellers offer -- a shortage of 60 (200 - 140). In a free market, this shortage would put upward pressure on price.
Q2. On a supply-and-demand graph, the surplus at a price above equilibrium is shown by:
- The vertical distance between the curves.
- The horizontal distance between the supply curve (right) and demand curve (left) at that price.
- The area under the demand curve.
- The intersection point of the curves.
At a fixed price, read quantities horizontally: Qs on the supply curve minus Qd on the demand curve is the surplus -- the horizontal gap between the curves. (Vertical distances at a fixed quantity show price differences, a different concept.)
Q3. A persistent, years-long shortage of apartments in a city most likely indicates:
- That builders are lazy.
- A binding price ceiling (rent control) or similar rigidity blocking upward price adjustment.
- That demand curves slope upward.
- That the city has no scarcity.
Temporary shortages self-correct through rising prices. A shortage lasting years means the correction is blocked -- the classic cause is a binding rent ceiling, though long-term contracts or zoning can also freeze adjustment. Persistent disequilibrium is a clue to look for the rigidity.
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Frequently asked questions
How do I calculate a shortage from a table?
At the given price, subtract quantity supplied from quantity demanded (Qd - Qs). If the result is positive, that is the shortage. If negative, its absolute value is the surplus. At the price where they are equal, the market is in equilibrium.
Why do economists say a shortage 'puts upward pressure on price'?
Because frustrated buyers compete for limited goods -- offering more to secure them -- while sellers realize they can charge more and still sell everything. Both behaviors push the transaction price up, which is the market's self-correction mechanism.
Can a surplus exist if the good is perishable?
Yes, and it is especially costly: unsold fish or fresh flowers cannot be stored, so sellers slash prices aggressively. Perishability speeds up the downward price adjustment -- which is why produce markets clear daily while housing markets take months.
What is the difference between a shortage and excess demand?
Nothing -- they are synonyms. 'Excess demand' is the formal term (Qd > Qs); 'shortage' is the everyday word. Likewise 'excess supply' and 'surplus' are the same thing. Exams may use either term.