Price Elasticity of Demand

students, imagine you are choosing between two snacks at lunch 🍎🥤. If one becomes a little more expensive, would you still buy it, or switch to the other one? This is the big idea behind Price Elasticity of Demand (PED). It helps economists understand how strongly consumers react when a price changes. In this lesson, you will learn the meaning of PED, how to calculate it, how to interpret different values, and why it matters for firms, governments, and markets.

Learning objectives:

Explain the main ideas and terminology behind Price Elasticity of Demand.
Apply IB Economics SL reasoning and calculations related to Price Elasticity of Demand.
Connect Price Elasticity of Demand to consumer behaviour, market outcomes, and government policy.
Summarize how Price Elasticity of Demand fits into microeconomics.
Use evidence and examples to show how PED works in real life.

What is Price Elasticity of Demand?

Price Elasticity of Demand measures how much the quantity demanded of a good changes when its price changes. It tells us whether consumers are very responsive or only slightly responsive to price changes.

The standard formula is:

$$\text{PED} = \frac{\%\text{ change in quantity demanded}}{\%\text{ change in price}}$$

Because of the law of demand, price and quantity demanded usually move in opposite directions. So PED is often negative. In many IB Economics answers, economists focus on the absolute value of PED, which is written without the minus sign. For example, a PED of $-2$ is usually discussed as $2$.

A simple interpretation:

If price rises by $10\%$ and quantity demanded falls by $20\%$, then $\text{PED} = \frac{-20\%}{10\%} = -2$.
This means demand is elastic because consumers respond strongly.

PED is important because it shows how consumer behaviour changes in different markets. A student buying a notebook may react differently to a price rise than a family buying insulin. This difference affects total revenue, tax policy, and business strategy.

Elastic, inelastic, and unit elastic demand

The value of PED helps classify demand into three main types.

Elastic demand

If $|\text{PED}| > 1$, demand is elastic. This means the percentage change in quantity demanded is larger than the percentage change in price.

Example: If cinema ticket prices rise by $5\%$ and attendance falls by $15\%$, then $|\text{PED}| = 3$. Consumers are highly responsive 🎬.

Elastic demand is common when:

there are many substitutes,
the good is not a necessity,
a large share of income is spent on the good,
consumers have time to adjust their spending.

Inelastic demand

If $|\text{PED}| < 1$, demand is inelastic. Quantity demanded changes by a smaller percentage than price.

Example: If the price of salt rises by $10\%$ and quantity demanded falls by only $2\%$, then $|\text{PED}| = 0.2$.

Inelastic demand is common when:

there are few or no close substitutes,
the good is a necessity,
the good takes up a small part of income,
consumers cannot easily delay the purchase.

Unit elastic demand

If $|\text{PED}| = 1$, demand is unit elastic. The percentage change in quantity demanded equals the percentage change in price.

Example: If price rises by $8\%$ and quantity demanded falls by $8\%$, then total spending on the good stays the same.

How to calculate PED in IB Economics SL

In IB questions, you may be given two prices and two quantities. The main method is the midpoint method, which is useful because it avoids giving different answers depending on whether price rises or falls.

The midpoint formula is:

$$\text{PED} = \frac{\frac{Q_2-Q_1}{(Q_2+Q_1)/2}}{\frac{P_2-P_1}{(P_2+P_1)/2}}$$

Here, $Q_1$ and $P_1$ are the original quantity and price, while $Q_2$ and $P_2$ are the new quantity and price.

Worked example

Suppose the price of coffee rises from $4$ to $5$, and quantity demanded falls from $100$ cups to $80$ cups.

First, find the percentage change in quantity using the midpoint method:

$$\frac{80-100}{(80+100)/2} = \frac{-20}{90} \approx -0.2222$$

Then find the percentage change in price:

$$\frac{5-4}{(5+4)/2} = \frac{1}{4.5} \approx 0.2222$$

Now divide:

$$\text{PED} \approx \frac{-0.2222}{0.2222} = -1$$

So demand is unit elastic. This means the rise in price caused quantity demanded to fall by the same proportion.

A quick IB tip: always state whether demand is elastic, inelastic, or unit elastic, and then explain what that means for consumer response and total revenue.

Factors that affect Price Elasticity of Demand

PED is not random. It depends on several important factors.

1. Availability of substitutes

The more substitutes a good has, the more elastic demand is. If the price of one brand of bottled water rises, consumers can switch to another brand easily.

2. Necessity versus luxury

Necessities tend to have inelastic demand because people need them regardless of price. Luxuries are more elastic because consumers can postpone or avoid buying them.

3. Proportion of income spent

Goods that take a large share of income often have more elastic demand. A small increase in the price of a car is a bigger decision than a small increase in the price of a pencil.

4. Time period

Demand is usually more elastic in the long run than in the short run. People need time to find substitutes and change habits. For example, after a petrol price increase, drivers may not change immediately, but over time they may buy fuel-efficient cars or use public transport.

5. Brand loyalty and habit

If consumers are very loyal to a brand, demand may be more inelastic. This is common in fashion, technology, and everyday habit-based purchases.

Why PED matters for total revenue and business decisions

Total revenue is:

$$\text{Total Revenue} = P \times Q$$

PED helps firms predict what happens to total revenue when price changes.

If demand is elastic, a price increase causes a proportionally larger fall in quantity demanded, so total revenue falls.
If demand is inelastic, a price increase causes only a small fall in quantity demanded, so total revenue rises.
If demand is unit elastic, total revenue stays the same.

Real-world example

A bus company may notice that passengers have few alternatives in a rural area. If it raises fares slightly, most people still travel, so demand may be inelastic. This can increase revenue. But if a fast-food chain raises prices too much in a city full of substitutes, customers may switch elsewhere, lowering revenue.

This is why firms study PED before changing prices. It helps them make pricing decisions, plan sales, and estimate the effect of discounts. 📈

PED and government intervention

PED is also important for government policy, especially taxes and subsidies.

When the government places a tax on a good, buyers and sellers share the burden of the tax depending on elasticity.

If demand is inelastic, consumers pay a larger share of the tax because they cannot easily avoid buying the good.
If demand is elastic, consumers can reduce consumption more easily, so producers may bear more of the burden through lower sales.

This is why governments often tax goods with inelastic demand, such as tobacco or alcohol. The tax can raise revenue and reduce consumption with less risk of a huge drop in demand.

Example

If a government taxes cigarettes, demand may be relatively inelastic because addicted consumers are less able to cut back quickly. As a result, tax revenue can be high and smoking may fall only gradually.

PED also affects subsidy policy. If demand is inelastic, a subsidy may not increase consumption very much. Governments use elasticity information to judge whether a policy will be effective.

PED and market failure

PED helps explain some types of market failure and policy design. Market failure happens when the market allocates resources inefficiently.

One example is demerit goods, such as cigarettes or sugary drinks, which create negative externalities and can reduce welfare. If demand is inelastic, taxes may reduce consumption only a little, so governments may combine taxes with advertising bans, warning labels, or education campaigns.

PED also matters in cases where consumers cannot quickly adjust behavior, such as medicine or electricity. If demand is very inelastic, price changes can cause hardship. Governments may step in with price controls, subsidies, or support for low-income households.

students, this is where microeconomics becomes practical: PED helps explain not just consumer choice, but also how markets respond to rules, taxes, and real-life pressures.

Conclusion

Price Elasticity of Demand is a key concept in microeconomics because it shows how sensitive consumers are to price changes. You should know how to calculate it, interpret whether demand is elastic or inelastic, and explain the role of substitutes, necessity, income share, and time. PED helps firms set prices, helps governments design taxes, and helps economists understand market outcomes. In IB Economics SL, strong answers use correct terminology, a clear calculation, and a link to real-world examples. If you can explain PED confidently, you are building a strong foundation for the rest of microeconomics.

Study Notes

$\text{PED} = \frac{\%\text{ change in quantity demanded}}{\%\text{ change in price}}$
Demand is usually negative because price and quantity demanded move in opposite directions.
Use the absolute value of PED when saying whether demand is elastic or inelastic.
If $|\text{PED}| > 1$, demand is elastic.
If $|\text{PED}| < 1$, demand is inelastic.
If $|\text{PED}| = 1$, demand is unit elastic.
The midpoint formula is often used in calculations to avoid inconsistent answers.
The more substitutes a good has, the more elastic demand tends to be.
Necessities usually have more inelastic demand than luxuries.
Demand is usually more elastic in the long run than in the short run.
Total revenue is $P \times Q$ and changes depending on elasticity.
Taxes on goods with inelastic demand often generate more revenue.
PED helps explain consumer behaviour, firm pricing, and government policy in microeconomics.