What is a demand curve and how do you draw it?

In one line

The rule

The demand curve is a graphical representation showing the relationship between the price of a good and the quantity demanded by consumers, with other factors held constant.

There is no single formula, but the basic equation often used is: Q_d = a - bP Where:

• Q_d = quantity demanded
• P = price
• a and b are constants (numbers that do not change)

Let's understand it simply

Imagine you are buying mangoes at the market. When the price is high, say ₹100 per kg, you may buy only 1 kg. If the price drops to ₹50 per kg, you might buy 2 kg. If it falls to ₹20 per kg, maybe you buy 5 kg. This is true for most people: the lower the price, the more they want to buy. If we write down these price and quantity pairs and plot them on a graph, we get a line that slopes down. That line is the demand curve.

Think of it like a cricket match ticket. If tickets are ₹5000, only a few fans will buy. If tickets are ₹200, the stadium will be full. The demand curve shows this pattern for any good.

How it works

1. Choose the good: Decide what product you want to study (e.g., mangoes).
2. List prices: Write down different possible prices (e.g., ₹100, ₹80, ₹60, ₹40, ₹20).
3. Find quantity demanded: For each price, estimate or find out how much people would buy at that price.
4. Make a table: Create a table with two columns — one for price, one for quantity demanded.
5. Draw axes: On graph paper, draw two lines: the vertical (y-axis) for price, and the horizontal (x-axis) for quantity.
6. Plot points: For each price and quantity pair, put a dot on the graph.
7. Join the dots: Connect the dots smoothly. The line will slope downwards from left to right.

The story behind it

The idea of the demand curve comes from early economists like Alfred Marshall in the late 1800s in England. He wanted to explain why prices change in markets. Marshall noticed that as prices fall, people buy more, and as prices rise, they buy less. He drew this as a downward-sloping curve, which became a basic tool in economics. The demand curve helps us understand and predict how people react to price changes.

Where you see it in real life

• Vegetable market: When onions become expensive in the monsoon, people buy less, or switch to potatoes. The demand curve shows this fall in quantity as price rises.
• Mobile recharge offers: If a recharge pack is cheaper, more people buy it. The demand curve explains this increase.
• Movie tickets: Lower ticket prices on weekdays fill more seats. The demand curve shows higher demand at lower prices.
• Diwali crackers: When prices go up before Diwali, families buy fewer crackers. The demand curve captures this.
• Cycle sales: Shops offer discounts in the off-season, and more cycles are sold. The demand curve predicts this jump.
• Bus fares: If bus fares rise, fewer people travel by bus, and more walk or take cycles. The demand curve shows the drop in demand.

Why it matters

• Helps businesses set prices: Shops and companies use the demand curve to decide the best price to charge.
• Government planning: The government uses demand curves to predict how people will react to tax or subsidy changes.
• Exam questions: Many economics questions ask you to draw or explain the demand curve.
• Foundation for other models: The demand curve is used with the supply curve to find the market price and quantity.

Worked example

Let's draw a demand curve for mangoes:

Given: Prices (₹ per kg): 100, 80, 60, 40, 20
        Quantity demanded (kg): 1, 2, 3, 4, 5
Step 1: Draw axes — y-axis = price, x-axis = quantity
Step 2: Mark points (100,1), (80,2), (60,3), (40,4), (20,5)
Step 3: Plot these points on the graph
Step 4: Join the points with a smooth line
Step 5: The line slopes down from left to right — this is the demand curve

Common mistakes

• Drawing the curve upwards: The demand curve always slopes downwards, not upwards. As price goes up, quantity demanded goes down.
• Mixing up axes: Price is always on the vertical (y) axis, quantity on the horizontal (x) axis.
• Forgetting 'other things constant': The demand curve only shows the effect of price, assuming other things (like income, tastes) do not change.
• Plotting supply instead of demand: The supply curve slopes up, but the demand curve slopes down.

Remember it

Demand curve = Downward slope: as price falls, people buy more.