What Is the Law of Supply and Demand?

Search Fintrev

Search our guides, reviews and glossary.

The law of supply and demand is the economic principle that the price of a good or service settles where the quantity buyers want to purchase equals the quantity sellers want to offer. It combines two separate rules. Under the law of demand, buyers purchase less as the price rises. Under the law of supply, sellers offer more as the price rises. When both relationships are drawn on one graph, the point where the curves cross is the equilibrium, or market-clearing, price. Both rules assume “all else being equal” (ceteris paribus): only the price changes, while income, tastes, costs and every other influence stay fixed.

The law of demand

The law of demand holds that, with everything else unchanged, the quantity of a product buyers want moves in the opposite direction to its price. Buyers have limited money, so a higher price means each dollar buys less of the product, and some buyers switch to alternatives or go without. A lower price makes the product affordable to more people and more attractive relative to substitutes.

Two effects explain the response. The income effect is the change in purchases caused by the price shifting how far a buyer’s income stretches. The substitution effect is the change caused by the product becoming cheaper or dearer relative to other goods. Plotted with price on the vertical axis and quantity on the horizontal axis, the result is a demand curve that slopes downward from left to right.

Diminishing marginal utility reinforces the pattern: each additional unit of a good tends to add less satisfaction than the one before, so buyers take on more units only at lower prices.

The law of supply

The law of supply runs the other way. With everything else unchanged, a higher price encourages producers to offer more, and a lower price encourages them to offer less. A higher price raises the potential profit on each unit, as long as production costs are not climbing just as fast, which justifies extra output and draws in new sellers. A lower price squeezes margins, and where firms will not sell at a loss, supply tends toward zero once the price drops below the cost of production.

The supply curve therefore slopes upward. How steeply it rises depends on how quickly producers can change output. A factory with idle capacity can respond fast. A farmer whose crop is already planted, or a mine that takes years to open, cannot.

Equilibrium price

The equilibrium price is the price at which the quantity demanded equals the quantity supplied. At that price every buyer willing to pay it finds a seller, and every seller willing to accept it finds a buyer. Above equilibrium, sellers offer more than buyers want, creating a surplus that pushes prices down. Below equilibrium, buyers want more than sellers offer, creating a shortage that pushes prices up. This process of trial and adjustment is called price discovery.

Worked example

Hypothetical market (not real data): suppose demand and supply for a product are described by these linear equations, where P is price in dollars and Q is units per week:

• Demand: Qd = 100 − 2P
• Supply: Qs = 20 + 2P

At equilibrium, Qd = Qs:

100 − 2P = 20 + 2P 80 = 4P P = $20

Substituting back gives Q = 100 − 2(20) = 60 units. The market clears at $20 with 60 units traded.

Now test prices away from equilibrium:

• At $25, buyers want 50 units and sellers offer 70. The 20-unit surplus gives sellers a reason to cut prices.
• At $15, buyers want 70 units and sellers offer 50. The 20-unit shortage lets sellers raise prices.

Movement along a curve vs. a shift of the curve

Readers often confuse two kinds of change. A change in quantity demanded (or supplied) is a movement along an existing curve caused only by a change in the product’s own price. A change in demand (or supply) is a shift of the entire curve caused by something other than price.

Continuing the hypothetical example, suppose rising incomes mean buyers want 20 more units at every price. The demand function becomes Qd = 120 − 2P. Setting it equal to supply:

120 − 2P = 20 + 2P P = $25, Q = 70

Demand increased, so both price and quantity rose. The outward shift caused the higher price, not the reverse, which is why price and quantity demanded can rise together in real data without contradicting the law of demand. Likewise, when demand decreases (the curve shifts left), equilibrium price and quantity both fall, assuming supply is unchanged.

Factors that shift the demand curve

• Income. For normal goods, higher income raises demand. For inferior goods, such as low-cost staples, higher income can lower demand as people trade up.
• Preferences. Changes in taste, fashion or information move demand. A swing toward a product shifts the curve right, raising the equilibrium price and quantity.
• Prices of related goods. A rise in the price of a substitute increases demand for the product. A rise in the price of a complement (for example, fuel for cars) reduces it.
• Expectations. Buyers who expect higher prices later may buy more now.
• Number of buyers. Population growth or a new market expands demand.

Factors that shift the supply curve

• Input costs. Cheaper labor, materials or energy, the four factors of production, shift supply right.
• Technology. Productivity gains let firms supply more at each price.
• Number of sellers and capacity. More producers, or more capacity, increase supply.
• Taxes, subsidies and regulation. Taxes and compliance costs reduce supply. Subsidies increase it.
• Market power and competition. Concentrated industries may restrict output to hold prices up.
• Supply shocks. Weather, conflict or disruptions can suddenly cut supply, sometimes producing large price swings for essential commodities.

Price elasticity

Supply and demand do not respond to price in proportion. Price elasticity of demand measures the percentage change in quantity demanded divided by the percentage change in price:

Elasticity = % change in quantity ÷ % change in price

A product is called elastic when the absolute value exceeds 1 and inelastic when it is below 1. Hypothetical example: if a 10% price increase cuts quantity demanded by 20%, elasticity is −2 (elastic). If it cuts quantity by 3%, elasticity is −0.3 (inelastic).

Necessities such as basic food, medicine and household fuel tend to be inelastic because people cannot easily do without them. Products with many close substitutes tend to be elastic. Supply has its own elasticity: when output cannot be expanded quickly, a jump in demand shows up mostly as a higher price rather than more units.

Exceptions to the law of demand

Two well-known cases break the downward slope.

Giffen goods are inferior goods, typically cheap staples, for which a price rise increases quantity demanded. When a staple that takes up a large share of a poor household’s budget gets more expensive, the household can no longer afford pricier foods, so it buys more of the staple to fill the gap. The income effect outweighs the substitution effect.

Veblen goods sit at the other end of the wealth scale: luxury goods whose appeal rises with price because a high price signals status. Thorstein Veblen called this “conspicuous consumption” in The Theory of the Leisure Class (1899).

Why the law of supply and demand matters

The model gives businesses, investors, economists and policymakers a common way to reason about prices. A company considering a price increase expects to sell fewer units and will estimate elasticity and the pull of substitutes to judge whether higher revenue per unit outweighs the lost volume. Investors use it to gauge how a supply disruption or a demand slump may affect a commodity or an industry’s profits. Policymakers use it to predict the effects of taxes, subsidies, price ceilings (which tend to cause shortages when set below equilibrium) and price floors (which tend to cause surpluses when set above it).

In a market economy, prices act as signals: a rising price tells producers to make more and buyers to economize, without any central plan. That signaling role is central to capitalism as an economic system.

Examples of supply and demand

Winter coats. In the fall, demand is strong and shoppers who need a coat pay full price. As the season ends, fewer people want coats, so stores mark down remaining stock and stop reordering. Prices fall until the coats buyers want match the coats left on the racks.

Gasoline in 2020. When the COVID-19 pandemic began, travel collapsed and gasoline consumption fell sharply. With storage filling up, prices dropped quickly, signaling refiners and producers to cut output. As demand recovered and crude oil prices rose in 2022, the higher prices gave producers an incentive to increase output again.

Limitations of the law of supply and demand

The model is a simplification, and its conclusions depend on assumptions that real markets may not meet.

• Free choice is assumed. Price discovery requires buyers and sellers who can freely decide whether to trade at a given price. Price controls, rationing and monopolies interfere with that.
• Ceteris paribus rarely holds. In practice, many factors change at once, so separating the effect of price from shifts in income, costs or expectations is difficult.
• Adjustment takes time. Supply often cannot respond quickly, so markets can sit away from equilibrium for long periods.
• Speculation can reverse the slope. In speculative buying, rising prices attract more buyers rather than fewer.
• Information is imperfect. Buyers and sellers may not know the prices or quality available elsewhere.

FAQs

The Bottom Line

Businesses, investors and policymakers rely on supply and demand to predict how prices respond to price changes, supply disruptions, demand slumps, taxes and price controls. The model works best as a starting point. Real markets often break its assumptions through price controls, slow adjustment, speculation and imperfect information, and Giffen and Veblen goods break the downward slope of demand.