quantity demanded
Quantity demanded is how much of something people actually want to buy at a particular price, at a particular moment, not what they wish they could buy.
In plain English
Mama Ngozi sells tomatoes at Mile 12 market and she has watched this thing happen every single week for years without fail. When tomatoes cost ₦500 a basket after the rains are good, customers buy plenty, sometimes twenty baskets before noon. When the dry season hits and she has to raise the price to ₦1,500 a basket because supply is scarce, the same customers suddenly only want two or three baskets, or they switch to pepper stew without tomatoes and grumble about it.
That switch in her customers' behaviour, that exact number of baskets they choose to buy at each price, is what economists call quantity demanded. It is not about how much people love tomatoes in general. It is about how much they are willing and able to buy at one specific price, right now, today.
This is different from just saying demand for tomatoes is high or low. Demand is the whole relationship, the full list of how much people would buy at every possible price. Quantity demanded is just one point on that list, tied to one price. If the price changes, the quantity demanded changes too, but the underlying demand, people's overall appetite for tomatoes, may not have shifted at all.
A salary earner in Lagos feels this with fuel. At ₦620 a litre, he fills his tank weekly without much thought. When it jumps to ₦900, he still needs fuel to get to work, but he starts trimming unnecessary trips, and the quantity he demands each month drops, even though his need for transport has not gone away.
Someone in Leeds sending money home sees a version of this too. When the exchange rate makes sending ₦100,000 cheaper in pounds, they might send that amount more often. When the rate worsens, they send less each time, and not since they love their family less, but because the price of sending has changed what quantity they are willing to commit at that moment.
At ₦500 per basket, Mama Ngozi sells 20 baskets in a morning. At ₦1,500 per basket, she sells only 4. The quantity demanded fell as the price rose, while her tomatoes stayed exactly the same tomatoes.
Questions people ask
Is quantity demanded the same thing as demand?
No. Demand is the whole consider, the full relationship between price and how much people would buy at each price level. Quantity demanded is just one specific amount at one specific price, a single point taken from that bigger consider.
Why does quantity demanded change even when people still want the product?
Because wanting something and being able or willing to pay for it at a given price are two different things. A price rise can leave desire untouched while still shrinking the amount someone actually buys, simply because their budget has limits.
What makes quantity demanded rise or fall, apart from price?
Price is the main driver for quantity demanded specifically, since it moves along a fixed demand relationship. Other things like income, taste, or the price of alternatives shift the whole demand relationship itself, which is a related but separate idea.
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