Quantity Demanded vs Quantity Supplied
Quantity Demanded and Quantity Supplied are two Supply & Demand concepts in AP Economics that students often mix up. Quantity demanded is the amount of a good or service consumers are willing and able to purchase at a given price. Quantity supplied is the amount of a good or service producers are willing and able to offer for sale at a given price. Here is how they compare side by side.
The quantity demanded is determined by the market price, holding all else constant. As price rises, quantity demanded falls. Producers use the concept to determine output levels and pricing strategies. It is graphically represented by the demand curve.
The quantity supplied is determined by the market price, holding all else constant. As price rises, quantity supplied also rises. Producers use the concept to determine output levels and pricing strategies. It is graphically represented by the supply curve.
Quantity Demanded vs Quantity Supplied: Reading Two Numbers at One Price
| Quantity Demanded | Quantity Supplied | |
|---|---|---|
| What the number counts | Units buyers will purchase at a stated price | Units sellers will offer at a stated price |
| Curve it is read from | A single point on the demand curve | A single point on the supply curve |
| Response to a price increase | It falls, by the law of demand | It rises, by the law of supply |
| Its role at a price below equilibrium | The larger number, so some buyers go unserved | The smaller number, so it caps units traded |
| Its role at a price above equilibrium | The smaller number, and it limits actual sales | The larger number, so stock goes unsold |
| Usual textbook symbol | Qd | Qs |
| Sign on the price term in its equation | Negative coefficient | Positive coefficient |
Neither number exists until you name a price
Ask how much of a good is demanded and the honest reply is a question back: at what price? That is the whole reason a change in demand is not the same statement as a change in quantity demanded, and the reason the two quantities must be compared at the same price before the comparison means anything. Work an illustrative market where quantity demanded equals 120 minus 4P and quantity supplied equals 2P. At a price of 15, quantity demanded is 120 minus 60, which is 60, while quantity supplied is 30, leaving a gap of 30 units. At a price of 25, quantity demanded is 20 and quantity supplied is 50, a gap of 30 units the other way. Equilibrium sits where 120 minus 4P equals 2P, so 120 equals 6P, the price is 20 and both quantities are 40. Two conclusions follow. The two numbers are equal at exactly one price and unequal at every other price. And whichever number is smaller decides how much actually trades, because a completed sale needs a willing buyer and a willing seller at the same time. You can drill this arithmetic at /calculate/equilibrium-price-and-quantity.
Only the good's own price moves these two numbers
A change in either quantity is a movement, never a shift. If the price of coffee rises, quantity demanded of coffee falls and quantity supplied of coffee rises, and both are slides along curves that never moved. If instead the price of tea rises, the coffee demand curve itself travels right, and the quantity demanded at the old coffee price is now read off a different curve. The trigger separates the cases: the good's own price moves you along, anything else moves the curve. Graders take points off for the wrong verb here. Writing that a tax on sellers reduced demand loses the mark, because buyers' willingness to pay never changed; supply shifted left, the price buyers pay rose, and quantity demanded fell in response. The mirror error is just as common, where a rise in income gets described as a rise in quantity demanded when it moves the entire curve. A test that holds up under exam pressure: write down the good's own price before and after. If that price is the only thing that changed, the answer is quantity demanded or quantity supplied. If something else changed first, name the curve that moved and see /glossary/determinants-of-demand.
Frequently asked questions
What is the difference between quantity demanded and quantity supplied?
Quantity demanded is how many units buyers will purchase at one stated price, and quantity supplied is how many units sellers will offer at that same price. Both are single numbers read off a curve rather than curves themselves, and they are equal only at the equilibrium price. At any price below equilibrium the demanded figure is bigger, and at any price above it the supplied figure is bigger.
Can quantity demanded and quantity supplied rise at the same time?
Yes, but only when a curve shifts rather than the price simply moving on its own. An increase in demand raises the equilibrium price, which pulls quantity supplied up along the unchanged supply curve, so both numbers end up higher. A pure price change with no shift always sends the two numbers in opposite directions.
How do you find quantity supplied at a given price?
Substitute that price into the supply equation, or read straight up from the price on the vertical axis to the supply curve and across to the quantity axis. If supply is Qs equals 2P, a price of 18 gives a quantity supplied of 36. Nothing from the demand side enters the calculation.
Live Supply and Demand graph. Drag the curves, or open the full version.
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