Supply, Demand, and How Prices Are Determined

Every day, without anyone in charge, millions of prices get set. Nobody decides that a cup of coffee should cost what it costs. Yet somehow there is enough coffee on the shelf, and it sells. How? The answer is the single most powerful idea in economics: supply and demand. Once you truly understand it, you will start to see it everywhere — in rents, wages, oil, even in the price of a concert ticket. This chapter builds that machinery from the ground up.

Demand: what buyers want, at each price

Demand is the relationship between the price of a thing and how much of it buyers are willing and able to buy in a given period — say, per week. "Willing and able" matters: wanting a yacht is not demand unless you can pay.

The law of demand says: when the price of something rises, the quantity people want to buy falls — and vice versa. This is why a demand curve slopes downward. Two forces cause it. The substitution effect: when coffee gets pricey, some people switch to tea. The income effect: a higher price makes your money buy less, so you feel a bit poorer and cut back.

One technical convention to note now, because it confuses beginners: economists put price on the vertical axis and quantity on the horizontal axis. This is a quirk we inherited from Alfred Marshall's Principles of Economics (1890). It is technically "backwards" — but it is the universal language, so we keep it.

Supply: what sellers offer, at each price

Supply is the mirror image: the relationship between price and how much sellers are willing to produce and sell per period. The law of supply says: when price rises, quantity supplied rises — an upward-sloping curve. Why? A higher price covers the higher cost of squeezing out extra units (the marginal cost, the cost of making one more), and it tempts new sellers to enter the business.

Equilibrium: where the two curves meet

Now lay the two curves on the same chart. They cross at one point. That crossing is the equilibrium — the price at which the quantity buyers want exactly equals the quantity sellers offer. We call it the market-clearing price, because at that price the market "clears": nothing is left unsold, and no buyer is turned away. Once reached, it has no built-in reason to move.

 Price
   ^
   |\                         /
   | \  Demand (D)           /  Supply (S)
   |  \                     /
P2 |---\-------------------/----    (a price set above P*)
   |    \                 /
P* |-----\------ E ------/-------    Equilibrium: Qd = Qs
   |      \     /\      /
   |       \   /  \    /
   |        \ /    \  /
   |         X      \/
   |        / \     /\
   |       /   \   /  \
   +-----+-------+------+---------> Quantity
              Q*

  E = equilibrium (price P*, quantity Q*).
  Price above P*  -> Qs > Qd -> SURPLUS  (sellers cut price).
  Price below P*  -> Qd > Qs -> SHORTAGE (buyers bid price up).

Shortages and surpluses: how the market self-corrects

What if the price is "wrong"? The market fixes it automatically.

• If price sits above equilibrium, sellers offer more than buyers want. That is a surplus (excess supply). Unsold stock piles up, so sellers cut prices — pushing price back down toward equilibrium.
• If price sits below equilibrium, buyers want more than sellers offer. That is a shortage (excess demand). Shelves empty, queues form, and buyers bid the price up — back toward equilibrium.

Movements vs. shifts: the #1 source of confusion

This distinction trips up almost everyone, so read it twice.

• A movement along a curve happens only when the good's own price changes. We call this a change in "quantity demanded," not in "demand." You slide up or down the same curve.
• A shift of the whole curve happens when something other than the good's own price changes. This changes "demand" or "supply" itself — the entire curve jumps left or right.

What shifts demand? A memory aid is TRIBE: Tastes/preferences, Related goods (the price of substitutes — rivals like tea — or complements — partners like milk), Income, Buyers (their number), and Expectations about the future. What shifts supply? Input prices, technology, the number of sellers, expectations, taxes or subsidies, and weather for crops.

Elasticity: how sensitive is the response?

Knowing that demand falls when price rises is not enough. We need to know how much. That is elasticity — the responsiveness of one thing to another, measured as a ratio of percentage changes.

Price elasticity of demand (PED) = (% change in quantity demanded) ÷ (% change in price). It is always negative (price up, quantity down), so we usually quote the absolute value. If it is greater than 1, demand is elastic (buyers react a lot). If less than 1, inelastic (buyers barely budge). Exactly 1 is unit elastic.

What makes demand elastic? Mainly the availability of substitutes (the biggest factor), whether the good is a luxury or a necessity, how big a share of your budget it takes, and — crucially — the time horizon. Longer time means more elastic, because people find ways to adapt.

Elasticity has a powerful business punchline, the total-revenue test. If demand is inelastic, raising the price raises total revenue (you lose few customers but charge each more). This is why utilities, addictive products, and life-saving drugs can hike prices and still earn more. If demand is elastic, raising the price lowers revenue — you scare off too many buyers.

Income elasticity of demand (YED) = (% change in quantity) ÷ (% change in income). For normal goods it is positive (richer, you buy more). For luxuries it exceeds 1 (buying grows faster than income). For inferior goods it is negative — demand falls as income rises, like bus travel, instant noodles, or store-brand staples. A related historical pattern is Engel's Law (Ernst Engel, 1857): as income rises, the share of it spent on food falls, even though total food spending may rise.

Why prices matter so much: signals, incentives, rationing

Step back and see the deeper magic. A price does three jobs at once. It is a signal (a high price screams "we need more of this!"). It is an incentive (that high price pays producers to make more). And it is a rationing device (it decides who gets the scarce good — those who value it most).

The economist Friedrich Hayek called this the answer to the knowledge problem (The Use of Knowledge in Society, 1945). No central planner could ever gather all the scattered facts — every shortage, every preference, every cost — that millions of people hold. But the price quietly aggregates all of it into one number everyone can act on. This is what Adam Smith (The Wealth of Nations, 1776) meant by the invisible hand: order without a commander.

When governments override the price: ceilings and floors

Because prices ration scarce goods, they sometimes feel cruel. Governments try to fix this by capping or propping prices. The supply-and-demand model predicts exactly what goes wrong.

A price ceiling is a legal maximum price. To bite, it must sit below equilibrium — and that guarantees a persistent shortage (look at the diagram: below P*, Qd > Qs, and the price can't rise to close the gap).

A price floor is a legal minimum. To bite, it must sit above equilibrium — which guarantees a persistent surplus (Qs > Qd).

The textbook floor is the minimum wage — a price floor on labor. In the simple model, setting wages above equilibrium creates a labor surplus, which is unemployment. We will study this in depth later, so here is just a preview. The U.S. federal minimum wage has been frozen at $7.25/hour since July 2009 — the longest gap since the Fair Labor Standards Act created it in 1938, and inflation has eroded roughly 30% of its real value. Meanwhile, 30 states plus D.C. now exceed it, with several states reaching $15 for the first time around 2026.

But here is where good economics gets honest. The simple model says minimum-wage hikes cost jobs. The evidence is genuinely mixed. A University of Washington team studying Seattle's increase toward $13 (2017) found low-wage hours fell about 9%. Yet a UC Berkeley team found pay rose with no job loss, and the famous Card & Krueger New Jersey fast-food study (1994) found modest hikes often have small or negligible employment effects — possibly because real labor markets aren't perfectly competitive. Hold both ideas: the model gives you the default prediction; the data tells you where reality bends it.

Key Takeaways

• Demand slopes down, supply slopes up; they meet at the market-clearing price where quantity demanded equals quantity supplied.
• Above equilibrium you get a surplus (price falls); below it a shortage (price rises). Markets self-correct toward equilibrium.
• A change in the good's own price moves you along a curve; anything else shifts the whole curve — don't confuse the two.
• Elasticity measures responsiveness: inelastic goods (gas, insulin) barely react to price; the total-revenue test explains who can profitably raise prices.
• Income elasticity sorts goods into normal, luxury, and inferior — and Engel's Law shows food's budget share falling as incomes rise.
• Prices act as signals, incentives, and rationing devices all at once, coordinating an economy no planner could direct (Hayek, Smith).
• Price ceilings cause shortages (rent control, 1970s gas lines); price floors cause surpluses (crop mountains, the minimum-wage debate) — though real-world data adds nuance to the simple model.