Price Elasticity of Supply (PES)

Economics· 2.8· 18 min read

1. 1. Definition and Formula of PES★★☆☆☆⏱ 4 min

📘 Definition

Price Elasticity of Supply (PES)

PES = \frac{\%\Delta Q_s}{\%\Delta P}

A measure of how much the quantity supplied of a good responds to a change in its market price.

PES values are always positive because of the law of supply: as price rises, producers increase output, so price and quantity supplied move in the same direction. No negative sign is ever required for IGCSE PES answers.

\%\Delta Q_s = \frac{Q_{s2} - Q_{s1}}{Q_{s1}} \times 100

\%\Delta P = \frac{P_2 - P_1}{P_1} \times 100

📐 Worked Example

When the price of bottled water rises from $1 to $1.20, the quantity supplied by a manufacturer increases from 50,000 units to 65,000 units. Calculate the PES of bottled water.

1
Calculate percentage change in quantity supplied:
$\%\Delta Q_s = \frac{65000 - 50000}{50000} \times 100 = 30\%$
2
Calculate percentage change in price:
$\%\Delta P = \frac{1.20 - 1.00}{1.00} \times 100 = 20\%$
3
Divide to get final PES value:
$PES = \frac{30\%}{20\%} = 1.5$

Exam tip:

Show all 3 calculation steps for PES questions: you earn method marks even if your final answer is wrong.

2. 2. Interpreting PES Values★★★☆☆⏱ 5 min

All PES values fall into 5 core categories, which you will need to link to supply curve shapes and real-world contexts for exams:

PES Value	Classification	Supply Curve Shape	Description
0	Perfectly Inelastic	Vertical straight line	Quantity supplied does not change when price changes
0 < PES < 1	Inelastic	Steep upward slope	Quantity supplied changes proportionally less than price
1	Unitary Elastic	Line passing through origin	Quantity supplied changes by the same proportion as price
PES > 1	Elastic	Shallow upward slope	Quantity supplied changes proportionally more than price
∞	Perfectly Elastic	Horizontal straight line	Producers supply any amount at a fixed price, zero at lower prices

📐 Worked Example

A small bakery sells custom cakes, with a PES value of 0.6. Classify the PES of the cakes, and explain how the bakery will respond to a 10% increase in cake prices.

1
Classify the PES: 0.6 is between 0 and 1, so supply is price inelastic.
2
Calculate the expected change in quantity supplied:
$\%\Delta Q_s = 0.6 \times 10\% = 6\%$
3
Interpret the result: A 10% price rise will lead to a 6% increase in cake supply, as the bakery has limited capacity to bake more custom cakes quickly.

Exam tip:

For diagram questions: perfectly inelastic supply = vertical line, perfectly elastic supply = horizontal line, inelastic = steep slope, elastic = shallow slope.

3. 3. Factors Affecting PES★★★☆☆⏱ 5 min

PES depends on how easily producers can adjust output when price changes. The 4 key factors you need to memorize for IGCSE exams are:

Time period: Supply is more elastic in the long run, as producers have time to expand capacity, buy inputs, or hire more workers. Supply is often inelastic in the short run.
Spare production capacity: If firms have unused machinery, labour, or raw materials, they can increase output quickly when price rises, making supply more elastic.
Ease of storage: Non-perishable goods that can be stored (e.g. canned food) have higher PES, as firms can release stored stock when prices rise. Perishable goods have lower PES.
Factor mobility: If factors of production can be easily switched between producing different goods, supply is more elastic.

📐 Worked Example

Explain why the PES for fresh tomatoes is lower than the PES for plastic water bottles.

1
Identify relevant PES factors: perishability, storage ease, production time.
2
Apply to tomatoes: Fresh tomatoes are perishable and cannot be stored long. They also take months to grow, so farmers cannot increase supply quickly if prices rise, making PES low.
3
Apply to water bottles: Plastic bottles are non-perishable, easy to store, and factories often have spare capacity to increase production quickly, making PES high.

Exam tip:

Generic factor statements only get 1 mark each. Always link factors to the specific good in the question to earn full marks.

4. 4. Exam-Focused PES Applications★★★★☆⏱ 4 min

PES is frequently tested in context-based questions about agricultural markets, government subsidies, and price controls. Understanding the command terms used for PES questions will help you meet examiner expectations:

📐 Worked Example

A government introduces a subsidy for rice production to increase output. Using PES theory, explain why output rises only slightly in the first year, but rises significantly after 5 years.

1
Link to time period factor: PES becomes more elastic the longer producers have to adjust output.
2
Short run explanation: In the first year, farmers have already planted their rice crops, and cannot easily increase land, labour, or inputs used for rice. PES is inelastic, so the higher price from the subsidy only leads to a small increase in supply.
3
Long run explanation: After 5 years, farmers can switch more land from other crops to rice, buy more machinery, and hire more workers. PES is elastic, so the same price rise leads to a much larger increase in supply.

5. Common Pitfalls

Wrong move:

Adding a negative sign to PES values

Why:

PES is always positive because price and quantity supplied move in the same direction per the law of supply. Examiners deduct marks for incorrect negative signs.

Correct move:

Always write PES values as positive numbers, no sign is required.

Wrong move:

Swapping %ΔP and %ΔQs in the PES formula

Why:

This reverses the calculation and gives an incorrect elasticity value, leading to wrong classification.

Correct move:

Remember: quantity supplied always goes on the top of the formula, price on the bottom.

Wrong move:

Stating generic PES factors without context application

Why:

Generic answers only get 1 mark per factor, even if factually correct, as exam questions require context application.

Correct move:

Link every factor to the specific good in the question, e.g. "fresh fish is perishable so cannot be stored, making PES low".

Wrong move:

Mixing up perfectly elastic and perfectly inelastic supply curve shapes

Why:

Confusing vertical and horizontal curves leads to lost marks on diagram and interpretation questions.

Correct move:

Use the mnemonic: Inelastic = Infinite slope (vertical), Elastic = Even flat slope (horizontal).

Wrong move:

Assuming unitary elastic supply is always a 45-degree line

Why:

Any upward sloping supply line that passes through the origin has unitary PES, regardless of its angle.

Correct move:

When drawing unitary elastic supply, start the line at the intersection of the x and y axes, at any upward slope.

6. Quick Reference Cheatsheet

Topic	Key Information	Exam Reminder
PES Formula	PES = %ΔQs / %ΔP	Show all working steps, no negative signs needed
PES Classifications	0=perfectly inelastic, <1=inelastic, 1=unitary, >1=elastic, ∞=perfectly elastic	Link classification directly to supply curve shape for diagram questions
PES Factors	Time period, spare capacity, storage ease, factor mobility	Always apply factors to the specific context given in the question
Long Run vs Short Run	PES is always more elastic in the long run	Use this to explain slow supply responses for agricultural goods

7. Frequently Asked

Do I need to add a negative sign to PES values?

No, PES is always positive because price and quantity supplied move in the same direction per the law of supply. Examiners deduct marks for unnecessary negative signs.

How do I get full marks for PES factor explanation questions?

Do not just list generic factors. Always link each factor directly to the specific good or scenario given in the question, for example: "Fresh fish is perishable so cannot be stored, making its PES low".

Going deeper

study_guideSupply Curves and Law of Supply
practice_packElasticity Calculation Practice

What's Next

Now that you have mastered PES, you are ready to apply this concept to more complex market analysis topics in CIE IGCSE Economics 0455. PES is a core tool for understanding how producers respond to changes in market conditions, including price controls, subsidies, and shifts in demand. You will encounter PES frequently in questions about agricultural markets, government intervention, and supply-side policies in later units. Make sure you practice PES calculations and interpretation regularly, as this topic is tested in almost every exam series, often as part of longer 6-mark or 8-mark essay questions in Paper 2.