Price elasticity of demand and supply (PED & PES)

Q: What does Price Elasticity of Demand (PED) measure?

PED measures the responsiveness of the quantity demanded of a product to a change in its price.

Q: What is the difference between elastic and inelastic demand?

Elastic demand means consumers are very responsive to price changes (large change in demand), whereas inelastic demand means they are not very responsive (small change in demand).

Q: How do we interpret PED values of greater than 1 and less than 1?

A PED value greater than 1 indicates elastic demand, while a value less than 1 indicates inelastic demand.

Q: What are unitary elastic, perfectly inelastic, and perfectly elastic demand?

Unitary = 1 (equal changes); Perfectly Inelastic = 0 (no change in demand); Perfectly Elastic = infinite (demand drops to zero if price rises).

Q: How can you identify elasticity by looking at the slope of a demand curve?

A relatively flat demand curve represents elastic demand, while a steep demand curve represents inelastic demand.

Q: How does the availability of substitutes affect the PED of a product?

The more close substitutes a product has, the more elastic its demand will be.

Q: How does the proportion of income spent on a good affect its PED?

Goods that take up a large proportion of a consumer's income tend to have elastic demand, while inexpensive goods tend to be inelastic.

Q: Why do necessities and luxuries have different price elasticities?

Necessities have inelastic demand because they are essential for survival, while luxuries have elastic demand because consumers can easily go without them.

Q: How do addictiveness and time influence the elasticity of a product?

Addictive products are inelastic; similarly, demand is inelastic in the short term but becomes more elastic over time.

Economics (2286) · GCE O-Level · 11 min read · Updated: 22 Oct 2026

Welcome to the World of Market "Bounciness"!

Ever wondered why a small increase in the price of a chocolate bar might make you stop buying it, but a huge jump in the price of electricity doesn't change how much you use? That "bounciness" of consumer reaction is what Economists call Price Elasticity of Demand (PED). Understanding this helps us see how resources are allocated in a market and why some businesses are more successful at changing prices than others. Don't worry if it sounds technical—by the end of these notes, you'll be a pro!

1. What is Price Elasticity of Demand (PED)?

PED measures how much the quantity demanded of a product changes when its price changes. It tells us how sensitive or "responsive" consumers are to a price change.

The Core Idea:
If the price goes up, we know demand usually goes down (the Law of Demand). But PED asks: "By how much?"

Elastic Demand: Consumers are very sensitive. A small price change leads to a BIG change in demand. (Think of a very stretchy rubber band!)
Inelastic Demand: Consumers are not very sensitive. A big price change leads to only a small change in demand. (Think of a stiff piece of string!)

Key Takeaway: PED is all about responsiveness. It’s the "how much" of the Law of Demand.

2. How to Calculate PED

To find the PED, we use a simple formula. Don't let the math scare you—it's just comparing two percentages!

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

Step-by-Step Calculation:

1. Find the % change in quantity: $ \frac{\text{New Q} - \text{Old Q}}{\text{Old Q}} \times 100 $
2. Find the % change in price: $ \frac{\text{New P} - \text{Old P}}{\text{Old P}} \times 100 $
3. Divide the result of step 1 by the result of step 2.

Note for O-Level: The result is usually a negative number because price and demand move in opposite directions. However, Economists often look at the "absolute value" (ignoring the minus sign) to keep things simple.

Interpreting the Result:

If PED is greater than 1 (> 1): Demand is Elastic.
If PED is less than 1 (< 1): Demand is Inelastic.
If PED is exactly 1: Demand is Unitary Elastic (the percentage changes are identical).
If PED is 0: Demand is Perfectly Inelastic (price change has zero effect on demand).
If PED is infinite ($\infty$): Demand is Perfectly Elastic (any price increase causes demand to drop to zero).

Quick Review: High number (>1) = Very Responsive (Elastic). Low number (<1) = Low Response (Inelastic).

3. Visualizing PED: The Demand Curves

The "slope" or steepness of a demand curve gives us a clue about its elasticity.

Flat Demand Curve: This represents Elastic demand. Since the line is flatter, you can see that a small move on the Price (vertical) axis causes a wide move on the Quantity (horizontal) axis.
Steep Demand Curve: This represents Inelastic demand. Even a huge jump in Price only results in a tiny shift in Quantity.

Common Mistake to Avoid: Don't assume a straight line has the same PED all the way down. PED actually changes at different points along a straight-line demand curve, but for O-Level, focusing on the general "flatness" or "steepness" is a great starting point!

4. What Determines PED? (The "SPLAT" Mnemonic)

Why are some things elastic and others inelastic? Use the SPLAT trick to remember the factors!

S – Substitutes: This is the most important one! If a product has many close substitutes (like different brands of bottled water), demand is Elastic. If there are no substitutes (like a unique life-saving medicine), demand is Inelastic.

P – Proportion of Income: If a good is very cheap (like a box of matches), a 10% price increase is only a few cents. People don't care, so it's Inelastic. If it’s expensive (like a car), a 10% increase is thousands of dollars, making it Elastic.

L – Luxury or Necessity: Necessities (milk, bread, electricity) are Inelastic because we need them to survive. Luxuries (jewelry, vacations) are Elastic because we can easily say "no" if the price rises.

A – Addictiveness/Habit: Products like cigarettes or coffee tend to be Inelastic because once consumers are hooked, they will keep buying even if prices go up.

T – Time: In the short term, demand is often Inelastic because it takes time to find alternatives. In the long term, demand becomes more Elastic as people switch to different products.

Key Takeaway: More choices (substitutes) = More Elasticity. No choice = Inelasticity.

5. PED and Total Spending (Revenue)

This is a favorite topic in exams! Total Revenue (TR) is the total amount of money a shop receives: $ \text{Price} \times \text{Quantity} $.

The Elasticity Relationship:

If Demand is Elastic: Price and Total Revenue move in opposite directions.
Example: If you raise the price, you lose so many customers that your total money goes DOWN.
If Demand is Inelastic: Price and Total Revenue move in the same direction.
Example: If you raise the price, people keep buying anyway, so your total money goes UP.

Memory Aid:
- Inelastic = "In-sync" (Price and Revenue move together).
- Elastic = "Enemies" (Price and Revenue move in opposite directions).

6. Why is PED Significant?

PED isn't just for textbooks; it helps people make big decisions!

For Producers (Businesses):

Businesses use PED to decide on their pricing strategy. If they know their product is inelastic, they might raise prices to increase profit. If it's elastic, they might have a "sale" to attract a massive number of new customers.

For Governments:

Governments use PED to decide which goods to tax (indirect taxes). They usually tax inelastic goods like cigarettes, alcohol, or petrol. Why? Because they know demand won't drop much, so they can collect a lot of tax revenue!

For Consumers:

Knowing PED helps consumers understand why certain prices fluctuate more than others and helps them make better choices about spending when prices change.

Did you know? Governments also use PED to see if a subsidy (financial help) will actually encourage people to buy more of a "good" product, like solar panels or healthy food.

Quick Review Box

- PED > 1: Elastic (Sensitive to price).
- PED < 1: Inelastic (Not sensitive).
- Revenue Rule: To increase revenue, raise price for inelastic goods, but lower price for elastic goods.
- Main Determinant: Availability of Substitutes.

Welcome to Price Elasticity of Supply (PES)!

In our previous chapters, we learned that when the price of a product goes up, firms usually want to supply more of it. But here is the big question: How much more?

Some businesses can double their production overnight if the price rises, while others might take years to increase their output even a little bit. Price Elasticity of Supply (PES) is the tool economists use to measure this "stretchiness" or responsiveness of supply. Don't worry if it sounds technical—think of it as measuring how quickly a business can "react" to a change in price!

Section Context: This chapter is part of The Allocation of Resources, helping us understand how producers decide where to put their time, land, and money.

1. What is Price Elasticity of Supply (PES)?

PES measures the responsiveness of the quantity supplied of a good to a change in its price.

In simple terms: If the price goes up by 10%, will the business supply 50% more (very responsive), or only 2% more (not very responsive)?

The Formula

To find the PES, we use this simple calculation:

$ PES = \frac{\% \text{ change in quantity supplied}}{\% \text{ change in price}} $

Quick Review: How to calculate % change?
If you've forgotten, just use this: $ \frac{\text{New Value} - \text{Old Value}}{\text{Old Value}} \times 100 $

Memory Aid: Remember "Q over P". Quantity always goes on top, and Price always goes at the bottom. Just like in the alphabet, Q comes before P in this calculation!

Key Takeaway: PES tells us how much supply "stretches" when the price moves.

2. Understanding the Values (The "Results")

When you calculate PES, your answer will tell you what kind of supply the product has. Unlike Demand (PED), PES is almost always positive because price and supply move in the same direction.

Elastic Supply (PES > 1)

Supply is elastic when the percentage change in quantity supplied is greater than the percentage change in price. This means the producer is very responsive to price changes.

Example: A taxi driver can easily work more hours if the fare price increases.

Inelastic Supply (PES < 1)

Supply is inelastic when the percentage change in quantity supplied is less than the percentage change in price. The producer finds it hard to change output quickly.

Example: A farmer cannot suddenly grow more coconuts just because the price went up today; it takes years for trees to grow!

Unitary Elasticity (PES = 1)

Quantity supplied changes by the exact same percentage as the price.

The "Extreme" Cases

1. Perfectly Inelastic (PES = 0): The supply is fixed. No matter how much the price rises, the supply stays exactly the same. The supply curve is a vertical line.
Example: The number of seats in a football stadium for a specific match.

2. Perfectly Elastic (PES = $\infty$): At a specific price, supply is infinite, but if the price drops even a tiny bit, supply falls to zero. The supply curve is a horizontal line.

Quick review box:
- Vertical line = Perfectly Inelastic (PES = 0)
- Steep line = Inelastic (PES < 1)
- Gentle/Flat slope = Elastic (PES > 1)

3. Determinants of PES (What makes supply "stretchy"?)

Why are some goods elastic and others inelastic? Here are the main factors:

A. Time Period

This is the most important factor! In the short run, supply is usually inelastic because firms can't easily change their factory size or find new land. In the long run, all factors of production can be changed, making supply more elastic.

B. Availability of Stocks (Inventories)

If a firm has a warehouse full of finished goods (stocks), it can respond to a price rise instantly by shipping them out. This makes supply elastic. If the goods are perishable (like fresh strawberries) and cannot be stored, supply is inelastic.

C. Spare Capacity

If a factory is only running at 50% capacity (some machines are turned off), it is easy to turn them on and produce more if the price rises. This makes supply elastic. If the factory is already working 24/7 at 100% capacity, supply is inelastic.

D. Flexibility of Resources (Mobility of Factors)

Can the workers and machines be easily switched from making one product to another? If a company can switch from making pens to making pencils easily, the supply for pencils is elastic.

Analogy: Imagine you are baking cookies. If you already have the dough ready in the fridge (Stock) and the oven is already hot (Spare Capacity), you can "supply" cookies very quickly if someone offers you a high price! If you haven't even bought the ingredients yet, your supply is inelastic.

Key Takeaway: Supply is more elastic when a firm has time, extra space, and stored goods.

4. Why is PES significant?

Knowing the PES is vital for different groups in the economy:

For Producers (Firms)

Firms want their supply to be as elastic as possible. Why? Because if the price of their product jumps up, they want to be able to produce a lot more to make huge profits! If their supply is inelastic, they miss out on that extra money.

For the Government

Governments look at PES when planning taxes or subsidies. If they want to encourage the production of houses, they need to know if the supply of houses is elastic enough to respond to a government grant.

For Consumers

If supply is inelastic (like petrol or certain foods), consumers should be prepared for huge price swings. If there is a small increase in demand and supply can't catch up quickly, the price will skyrocket!

Did you know? Agricultural products (crops) usually have very inelastic supply because of the growing time and weather dependence. This is why food prices often change so much from month to month!

5. Common Mistakes to Avoid

1. Mixing up PES and PED: Students often confuse these. PED is about consumers (demand); PES is about producers (supply). Always check which one the question is asking for!

2. Forgetting the %: In the formula, it is the percentage change, not just the change in units. If the price goes up by $2, you must convert that to a percentage of the original price first.

3. Drawing the wrong slope: Remember that an elastic supply curve is flatter (more horizontal), while an inelastic curve is steeper (more vertical).

Don't worry if this seems tricky at first! Just remember that PES is simply a way of asking: "How easy is it for this business to make more of this stuff right now?" If it's easy, it's elastic. If it's hard, it's inelastic.

Quick Summary Checklist

- [ ] I can define PES.
- [ ] I can calculate PES using the formula $ \frac{\% \Delta QS}{\% \Delta P} $.
- [ ] I know that PES > 1 is elastic and PES < 1 is inelastic.
- [ ] I can list at least three factors that make supply elastic (Time, Stocks, Spare Capacity).
- [ ] I understand why PES is important for firms and governments.

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What does Price Elasticity of Demand (PED) measure?

PED measures the responsiveness of the quantity demanded of a product to a change in its price.

What is the difference between elastic and inelastic demand?

Elastic demand means consumers are very responsive to price changes (large change in demand), whereas inelastic demand means they are not very responsive (small change in demand).

What is the formula for calculating Price Elasticity of Demand?

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

How do we interpret PED values of greater than 1 and less than 1?

A PED value greater than 1 indicates elastic demand, while a value less than 1 indicates inelastic demand.

What are unitary elastic, perfectly inelastic, and perfectly elastic demand?

Unitary = 1 (equal changes); Perfectly Inelastic = 0 (no change in demand); Perfectly Elastic = infinite (demand drops to zero if price rises).

How can you identify elasticity by looking at the slope of a demand curve?

A relatively flat demand curve represents elastic demand, while a steep demand curve represents inelastic demand.

How does the availability of substitutes affect the PED of a product?

The more close substitutes a product has, the more elastic its demand will be.

How does the proportion of income spent on a good affect its PED?

Goods that take up a large proportion of a consumer's income tend to have elastic demand, while inexpensive goods tend to be inelastic.

Why do necessities and luxuries have different price elasticities?

Necessities have inelastic demand because they are essential for survival, while luxuries have elastic demand because consumers can easily go without them.

How do addictiveness and time influence the elasticity of a product?

Addictive products are inelastic; similarly, demand is inelastic in the short term but becomes more elastic over time.

What is the relationship between Total Revenue and inelastic demand?

For inelastic goods, price and total revenue move in the same direction (e.g., raising price increases revenue).

What is the relationship between Total Revenue and elastic demand?

For elastic goods, price and total revenue move in opposite directions (e.g., raising price decreases revenue).

How do businesses use PED to determine their pricing strategies?

Firms raise prices for inelastic goods to increase revenue and lower prices for elastic goods to attract more customers and increase revenue.

Why does the government use PED when deciding which goods to tax?

Governments tax inelastic goods to ensure high tax revenue and minimize the drop in consumption.

What is Price Elasticity of Supply (PES)?

PES measures the responsiveness of the quantity supplied of a good to a change in its price.

What is the formula for calculating Price Elasticity of Supply (PES)?

The formula is: $ PES = \frac{\% \text{ change in quantity supplied}}{\% \text{ change in price}} $

When is supply considered elastic?

Supply is elastic when the percentage change in quantity supplied is greater than the percentage change in price, resulting in a $ PES > 1 $.

When is supply considered inelastic?

Supply is inelastic when the percentage change in quantity supplied is less than the percentage change in price, resulting in a $ PES < 1 $.

What is Unitary Elasticity of supply?

Unitary elasticity occurs when the percentage change in quantity supplied is exactly equal to the percentage change in price, meaning $ PES = 1 $.

What does it mean if supply is perfectly inelastic?

Supply is perfectly inelastic when $ PES = 0 $, meaning the quantity supplied remains constant regardless of the price; the supply curve is a vertical line.

What does it mean if supply is perfectly elastic?

Supply is perfectly elastic when $ PES = \infty $, meaning producers will supply any amount at a specific price, but supply falls to zero if the price drops; the curve is a horizontal line.

How does the time period affect PES?

Supply is generally inelastic in the short run but becomes more elastic in the long run as firms have more time to adjust their factors of production.

How does the availability of stocks (inventories) influence PES?

Supply is more elastic when a firm holds high levels of stock, as they can quickly release items to the market if prices rise.

What is spare capacity and how does it affect PES?

Spare capacity refers to unused production resources; supply is more elastic when a firm has spare capacity because it can easily increase output.

What is mobility of factors of production?

It refers to how easily resources like labor and capital can be switched from making one product to another; higher mobility leads to more elastic supply.

Why is PES significant for producers (firms)?

Firms want elastic supply so they can increase production quickly when prices rise to maximize their profits.

How can students avoid confusing PES with PED?

Remember that PED focuses on the consumer (Demand), while PES focuses on the producer (Supply).

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