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Supply and demand is the model economists use to explain how prices and quantities are set in a market. Demand is the relationship between price and the amount buyers are willing and able to purchase. Supply is the matching relationship on the seller side. The price at which the two quantities are equal is the equilibrium price, and the amount traded at that price is the equilibrium quantity.
Demand and supply are relationships, not single amounts
Demand and supply describe how buyers and sellers would respond across a range of prices. They do not describe what happens at one price alone. That distinction drives everything else in the model.
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Demand
Demand is the amount consumers are willing and able to purchase at each price. The word “able” matters: a desire to buy without the money to pay does not count toward effective demand. A demand schedule lists the quantity demanded at several prices, and plotting that schedule produces the demand curve.
Quantity demanded is narrower. It is one point on the demand relationship: a specific price paired with the specific amount buyers will purchase at that price.
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Supply
Supply is the amount producers are willing to offer at each price. A supply schedule or supply curve expresses that price-quantity relationship, while quantity supplied refers to one point on it.
The law of supply describes the usual pattern: with other relevant factors held constant, a higher price is associated with a larger quantity supplied. That is why supply curves slope upward, while the law of demand gives demand curves their downward slope.
How to draw and label the model
The standard diagram puts price on the vertical axis and quantity on the horizontal axis. The demand curve slopes downward, the supply curve slopes upward, and they cross at the equilibrium.
Before using the diagram, define three things so the picture means something specific:
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- The market: for example, gasoline sold in one country, or a specific grade of a product, not “goods” in general.
- The unit: gallons, kilograms, or units sold, so price and quantity can be read consistently.
- The time period: per day, per month, or per year. Quantities without a period cannot be compared across studies.
The simple diagram leaves out many real-world influences. It is a tool for organizing reasoning about one market, not a complete description of why any particular price is what it is.
Finding equilibrium
Equilibrium is the price at which quantity demanded equals quantity supplied. At that price, buyers’ plans and sellers’ plans match, so neither side has a reason within the model to change the amount it trades.
Reading equilibrium from a schedule
- List the prices in the schedule and, for each price, write down the quantity demanded and the quantity supplied.
- Compare the two quantities in each row.
- Find the row where they are equal. That price is the equilibrium price, and the shared quantity is the equilibrium quantity.
- If no row matches exactly, the equilibrium lies between the two prices where the excess demand changes sign. Read the schedule more finely or use the graph to estimate it.
OpenStax’s introductory macroeconomics text (Principles of Macroeconomics 3e, published December 14, 2022) uses a gasoline schedule to illustrate this. The figures below are textbook illustrations chosen to teach the method. They are not current gasoline prices or sales data.
| Price per gallon | Quantity demanded (million gallons) | Quantity supplied (million gallons) | Condition |
|---|---|---|---|
| $1.20 | 700 | 550 | Excess demand of 150 million gallons (shortage) |
| $1.40 | 600 | 600 | Equilibrium: quantities match |
| $1.80 | 500 | 680 | Excess supply of 180 million gallons (surplus) |
Reading equilibrium from a graph
On a graph, equilibrium is the point where the demand curve and supply curve intersect. Draw a line from that point to the price axis to read the equilibrium price, and to the quantity axis to read the equilibrium quantity. This is the graphical equivalent of finding the matching row in a schedule.
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The Federal Reserve Bank of St. Louis describes the relationship in its Economic Lowdown educational resource on equilibrium: “Like the two blades of a scissors, supply and demand work together to determine price.” The quoted passage does not identify an individual speaker.
Shortages and surpluses away from equilibrium
When the price sits away from equilibrium, the two quantities no longer match, and the schedule shows which side is short.
- Price above equilibrium (surplus): quantity supplied exceeds quantity demanded. In the gasoline illustration at $1.80, producers offer 680 million gallons but buyers take only 500 million, leaving 180 million gallons of excess supply.
- Price below equilibrium (shortage): quantity demanded exceeds quantity supplied. At $1.20, buyers want 700 million gallons but sellers offer 550 million, leaving 150 million gallons of excess demand.
In the model, these gaps are what push the price back toward the level where the two quantities match.
Movement along a curve versus a shift of the curve
The most common error in applying the model is treating every change as the same kind of event. The model distinguishes two cases.
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| Type of change | What causes it | What happens on the graph | Term used |
|---|---|---|---|
| A change in the good’s own price | The price itself moves, other influences held constant | The market moves to a different point on the existing curve | Change in quantity demanded or quantity supplied |
| A change in another determinant | Examples include income, prices of related goods, or input costs for producers | The whole curve moves to a new position | Change in demand or change in supply |
OpenStax’s summary of this material recommends a four-step check for any event:
- Decide whether the event changes the good’s own price or another factor.
- If it is another factor, identify whether it affects demand or supply.
- Determine the direction of the shift (increase or decrease).
- Compare the new equilibrium price and quantity with the original.
Price elasticity: measuring how much quantity responds
Elasticity converts the response of buyers or sellers into a single number, so two markets can be compared on the same scale. The formula for price elasticity of demand or supply is:
Price elasticity = percentage change in quantity demanded (or supplied) ÷ percentage change in price
The OpenStax chapter 5 summary in Principles of Economics 2e uses this formula and sorts the result into three categories by absolute magnitude:
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1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minute| Category | Absolute value | Meaning |
|---|---|---|
| Elastic | Greater than 1 | Quantity responds by a larger percentage than price |
| Unit elastic | Equal to 1 | Quantity responds by the same percentage as price |
| Inelastic | Less than 1 | Quantity responds by a smaller percentage than price |
Demand elasticity is usually negative when signed, because a higher price tends to reduce quantity demanded. Many writers report the absolute value when labeling a good as elastic or inelastic. If you do this, say so explicitly so readers do not compare a negative number to a positive one.
Worked calculation with hypothetical numbers
The figures below are invented to show the arithmetic. They are not observed market data. The percentage changes use the starting value as the base.
- Case A: price rises from $2.00 to $2.20. That is a change of +$0.20, or +10% of $2.00. Quantity demanded falls from 500 to 450 units, a change of −50, or −10% of 500. Elasticity = −10% ÷ +10% = −1.0. The absolute value is 1.0, so this demand is unit elastic.
- Case B: price rises by the same 10%, but quantity demanded falls from 400 to 300 units, a change of −25%. Elasticity = −25% ÷ +10% = −2.5. The absolute value is 2.5, so this demand is elastic.
Using the midpoint of each range instead of the starting value gives slightly different percentages for large changes, so state which method you used if you publish a figure.
Why this matters for everyday spending
The model is useful for household budgeting because it gives you a structured way to read price changes.
- Ask which side moved. A price increase caused by rising demand is a different situation from one caused by higher production costs, even though both show up as a higher price on the shelf.
- Check elasticity before assuming behavior. Goods with few close substitutes tend to be less responsive to price, so buyers absorb more of an increase. Goods with many alternatives tend to be more responsive, so buyers are more likely to switch.
- Treat the equilibrium as a model result, not a forecast. It tells you where a market would settle if the stated assumptions held, not what a specific price will be next month.
Sources
- OpenStax, Principles of Macroeconomics 3e, section 3.1, published December 14, 2022: definitions, equilibrium, the gasoline schedule, and shortages and surpluses.
- OpenStax, Principles of Microeconomics 2e, section 3.1: the distinction between supply and quantity supplied, and the law of supply.
- OpenStax, Principles of Economics 2e, chapter 5 summary: the elasticity formula and response categories.
- Federal Reserve Bank of St. Louis, Economic Lowdown educational resource on equilibrium: the quoted passage.
Frequently Asked Questions
Is the equilibrium price the fair price?
Not in the model’s terms. Equilibrium is defined only by the point where quantity demanded equals quantity supplied. Whether that price is fair is a value judgment that the model does not address.
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