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Business Economics · Theory of Demand and Supply

Elasticity of Supply for CA Foundation Business Economics

Updated 1 October 2026 · Fact-checked

Price elasticity of supply measures how much quantity supplied changes when price changes. Es = % change in quantity supplied ÷ % change in price. Compute both percentage changes, divide, and compare the result with 1 to classify supply as elastic, inelastic, unitary, perfectly elastic or perfectly inelastic.

Understand Elasticity of Supply

The law of supply says that when price rises, producers supply more. It does not say how much more. Elasticity of supply answers that. It tells you how responsive quantity supplied is to a change in price.

Think of two sellers. A vegetable vendor can bring more stock the next day if prices rise. A power plant cannot add capacity quickly. The vendor has high elasticity of supply. The power plant has low elasticity of supply.

We measure it as a ratio of percentage changes. If price rises 10% and quantity supplied rises 20%, supply is elastic because the quantity moved more than the price. If quantity rises only 5%, supply is inelastic.

There are five types. Elastic supply has Es > 1. Inelastic supply has Es < 1. Unitary elastic supply has Es = 1. Perfectly elastic supply has Es = ∞ (the supply curve is horizontal). Perfectly inelastic supply has Es = 0 (the supply curve is vertical).

A straight-line supply curve that starts from the origin has Es = 1 at every point, whatever its slope. If it cuts the price axis (positive intercept on the price axis), Es > 1. If it cuts the quantity axis (positive intercept on the quantity axis), Es < 1.

The main factors are: time period, availability of inputs, nature of the good (perishable or durable), spare capacity, ease of storage, technology, and how easily producers can shift between products. More time and more spare capacity make supply more elastic.

Key formulas to remember

Percentage method
Es = (% change in Qs) ÷ (% change in P)
Es is normally positive because price and quantity supplied move in the same direction.
Change method
Es = (ΔQ ÷ Q) ÷ (ΔP ÷ P) = (ΔQ ÷ ΔP) × (P ÷ Q)
Use the original P and Q as the base unless the question asks for arc or midpoint elasticity.
Arc (midpoint) elasticity
Es = (ΔQ ÷ ΔP) × [(P1 + P2) ÷ (Q1 + Q2)]
Use when the question asks for elasticity between two points using average values.
Classification
Es > 1 elastic; Es < 1 inelastic; Es = 1 unitary; Es = 0 perfectly inelastic; Es = ∞ perfectly elastic
Compare your answer with 1 to name the type.
Straight-line supply curve rule
Through origin: Es = 1; cuts price axis: Es > 1; cuts quantity axis: Es < 1
Applies to a linear curve at every point, regardless of slope.

How to solve Elasticity of Supply questions

Use this method for any numerical or conceptual question on elasticity of supply.

  1. 1Write down the initial price (P1), new price (P2), initial quantity (Q1) and new quantity (Q2).
  2. 2Find ΔP = P2 − P1 and ΔQ = Q2 − Q1.
  3. 3Calculate the percentage change in price: ΔP ÷ P1 × 100.
  4. 4Calculate the percentage change in quantity supplied: ΔQ ÷ Q1 × 100.
  5. 5Divide the quantity percentage by the price percentage to get Es.
  6. 6If the question says arc or midpoint, use the average price and average quantity as the base instead.
  7. 7Compare Es with 1 and name the type: elastic, inelastic or unitary.
  8. 8For conceptual questions, check the time period and spare capacity. Longer time and more spare capacity mean more elastic supply.

Quickest way: Ratio shortcut with option elimination

When to use it: Use for numerical MCQs where prices and quantities are round numbers.

  1. Skip the percentage symbols. Compute ΔQ ÷ Q1 and ΔP ÷ P1 as simple fractions.
  2. Divide the first fraction by the second. For example, (1/5) ÷ (1/10) = 2.
  3. Before calculating, estimate: if quantity changes by a bigger fraction than price, Es > 1. Remove options that are below 1.
  4. For diagram questions, look at where the straight line meets the axes. Origin means 1, price axis means above 1, quantity axis means below 1.
  5. For definition questions, remember: vertical is 0, horizontal is ∞.
  6. If the base is unclear and options differ only by base, re-read for the word 'arc' or 'average' before answering. Skip if still unsure, since wrong answers cost 0.25.

Common mistakes in Elasticity of Supply

  • Using the new price and quantity as the base instead of the original ones.

    Students divide the change by the larger or latest figure by habit.

    Fix: Unless the question says arc or midpoint, always divide changes by the initial P1 and Q1.

  • Mixing up perfectly elastic and perfectly inelastic.

    The words sound alike and students forget which curve is which.

    Fix: Vertical line: quantity does not respond at all, so Es = 0. Horizontal line: quantity responds infinitely, so Es = ∞.

  • Thinking a steeper straight supply curve always means lower elasticity.

    Slope and elasticity are confused.

    Fix: Elasticity depends on where the curve meets the axes. Any straight line through the origin has Es = 1, steep or flat.

  • Dividing price change by quantity change (inverting the ratio).

    Students rush and reverse the formula.

    Fix: Quantity is on top, price is at the bottom. Say it as 'quantity responds to price'.

  • Ignoring the time period when asked about factors.

    Students list only inputs and technology.

    Fix: Remember that supply is more elastic in the long run because firms can expand capacity. In the very short run it is least elastic.

  • Treating perishable goods as highly elastic.

    Students assume cheap goods adjust quickly.

    Fix: Perishable goods cannot be stored, so supply is relatively inelastic in the short run.

Worked examples

Example 1

When the price of a good rises from ₹20 to ₹25, quantity supplied rises from 100 units to 130 units. The price elasticity of supply is: (a) 0.8 (b) 1.2 (c) 1.5 (d) 2.0

Show the solution
  1. P1 = 20, P2 = 25, so ΔP = 5.
  2. Q1 = 100, Q2 = 130, so ΔQ = 30.
  3. % change in price = 5 ÷ 20 × 100 = 25%.
  4. % change in quantity = 30 ÷ 100 × 100 = 30%.
  5. Es = 30 ÷ 25 = 1.2.
  6. Since 1.2 > 1, supply is elastic.

Answer: (b) 1.2

Example 2

A straight-line supply curve cuts the quantity axis to the right of the origin. At any point on it, supply is: (a) elastic (b) inelastic (c) unitary elastic (d) perfectly elastic

Show the solution
  1. Recall the rule for linear supply curves.
  2. A curve through the origin has Es = 1.
  3. A curve that cuts the price axis has Es > 1.
  4. A curve that cuts the quantity axis has Es < 1.
  5. So this curve shows inelastic supply (Es < 1) at every point.

Answer: (b) inelastic

Example 3

The price of a commodity rises by 10% and the quantity supplied rises by 4%. Which statement is correct? (a) Es = 0.4, supply is inelastic (b) Es = 2.5, supply is elastic (c) Es = 1.4, supply is elastic (d) Es = 6, supply is elastic

Show the solution
  1. % change in quantity supplied = 4.
  2. % change in price = 10.
  3. Es = 4 ÷ 10 = 0.4.
  4. Since 0.4 < 1, supply is inelastic.
  5. Option (b) is the inverted ratio, a common trap.

Answer: (a) Es = 0.4, supply is inelastic

Exam tips

  • Most questions are either a one-step calculation or a type-identification question. Practise both until each takes under a minute.
  • Memorise the three straight-line rules (origin, price axis, quantity axis). Diagram questions are easy marks.
  • Watch for trap options that are the inverted ratio of the correct answer.
  • Link factors to time. Questions often ask which period gives the most elastic supply, and the answer is the long run.
  • If a question mentions arc or average values, switch to the midpoint formula.

Practice questions from Theory of Demand and Supply

Elasticity of Supply: frequently asked questions

What is the formula for elasticity of supply?

Es = % change in quantity supplied ÷ % change in price. It can also be written as (ΔQ ÷ ΔP) × (P ÷ Q). Use the original price and quantity as the base unless told otherwise.

Can elasticity of supply be negative?

For normal goods under the law of supply, it is positive because price and quantity supplied move together. Negative values are not tested in the usual CA Foundation questions.

What are the five types of elasticity of supply?

They are elastic (Es > 1), inelastic (Es < 1), unitary (Es = 1), perfectly elastic (Es = ∞) and perfectly inelastic (Es = 0). The last two are shown by a horizontal and a vertical supply curve.

Why is supply more elastic in the long run?

In the long run firms can build new capacity, hire more workers and change technology. In the short run some inputs are fixed, so output cannot increase much when price rises.