A price rise is an invitation to producers: make more and earn more. Some can accept it within hours by switching a machine on. Others cannot accept it for a year because the crop has to grow. Price elasticity of supply measures how quickly and how far producers can actually respond.
📚 What you need to know
PES measures how responsive quantity supplied is to a change in price.
PES = % change in quantity supplied ÷ % change in price.
PES is normally positive, because price and quantity supplied move the same way.
Values run from 0 (perfectly inelastic) to infinity (perfectly elastic), with 1 as the dividing line.
Five determinants: mobility of factors, how fast marginal costs rise, ability to store, spare capacity, and time.
Primary commodities tend to have low PES; manufactured goods tend to have higher PES.
Supply is always more elastic in the long run, because every factor of production can be changed given enough time.
The formula
Price elasticity of supply
PES = % change in quantity supplied ÷ % change in price
Same routine as the elasticities of demand. Two percentage changes, written down separately, then divide. There is no sign to worry about here, because a higher price almost always brings more supply. And as always, the answer is a plain number with no units and no percentage sign.
Whenever a question says PES, switch your head from consumer to producer. Half the marks lost on this topic come from students who answer with substitutes and necessities — which are demand-side ideas that have nothing to do with supply.
The five values of PES
Perfectly inelastic supply is a fixed number of seats in a theatre. Perfectly elastic supply means producers will make any amount at one price and nothing at all above it.
A shortcut most students never learn
For a straight-line supply curve you can work out whether PES is above or below 1 just by looking at where the line starts. No calculation needed.
Steepness on its own tells you nothing about PES. What matters is which axis the line hits when you extend it back.
Why the origin rule works. PES compares the percentage change in quantity with the percentage change in price. On a line through the origin, quantity and price are always in the same ratio, so a 10% rise in one is always matched by a 10% rise in the other. The percentages cancel, and PES is 1 no matter how steep the line looks.
What determines PES
Determinant
How it works
Example
Mobility of factors of production
If workers and machines can be switched to a different product quickly, supply is more elastic
A clothing factory can move a production line from trainers to boots in days
How fast marginal costs rise
If each extra unit costs much more than the last, firms stop expanding sooner, so supply is inelastic
Getting more output from an already busy factory means overtime pay
Ability to store the good
Stock can be released the moment price rises, which makes supply elastic. Perishables cannot be held back
Tinned food can be stockpiled; fresh strawberries cannot
Spare capacity
Idle machines and available workers mean output can rise immediately
A factory running one shift can add a second; one running flat out cannot
Time period
The longer producers have, the more factors they can change — so PES rises with time
A farmer cannot grow more wheat this month, but can plant more next season
Every one of those five is really the same question in different clothes: how quickly can this producer get hold of more output? If the answer is “within a week”, supply is elastic. If it is “in about three years”, it is not.
Why time changes everything
Time is the determinant examiners come back to most, because it changes the answer to the same question. In the short run at least one factor of production is fixed, so producers are stuck with the capacity they have. Given long enough, they can hire, buy machines, build a new site or plant a bigger crop — and supply becomes much more elastic.
Short run: big price jump to P2, small output gain to Q2. Long run: price settles back to P3 and output reaches Q3. Nothing changed except how much time producers had.
Primary commodities versus manufactured goods
This comparison comes up again and again, and the way to answer it is not to memorise a conclusion. Apply the five determinants one at a time and the conclusion falls out.
Determinant
Primary commodities (low PES)
Manufactured goods (higher PES)
Mobility of factors
Land planted with coffee cannot be switched to something else mid-season
A factory can retool between similar products fairly quickly
Marginal costs
Rise steeply — the best land and easiest deposits are used first
Rise slowly — extra units on an existing line are cheap to add
Storage
Many crops are perishable, so output cannot be held back and released
Finished goods keep for months in a warehouse
Spare capacity
Limited — production is tied to land and weather
Often plenty — add a shift or run machines longer
Time
Growing or extracting takes seasons or years
Manufacturing takes days or weeks
Why this matters beyond the definition. Because supply is so inelastic, primary commodity prices swing violently when demand or the harvest changes. That price volatility is a real problem for countries whose export earnings depend on one or two commodities — a link worth making in a development question.
Worked examples
WORKED EXAMPLE 1
The price of a box of eggs rises from $3.00 to $3.60. Weekly quantity supplied rises from 5,000 to 5,300. Calculate the PES and explain the value. [4]
Step 1: percentage change in quantity supplied(5300 − 5000) ÷ 5000 × 100 = +6%Step 2: percentage change in price(3.60 − 3.00) ÷ 3.00 × 100 = +20%Step 3: dividePES = 6 ÷ 20 = 0.3Step 4: explain it
Below 1, so supply is price inelastic. Producers cannot raise output quickly because hens take months to reach laying age and there is little spare capacity.
PES = 0.3, price inelastic supplyThe explanation must be a supply-side reason. Never write “few substitutes”.
WORKED EXAMPLE 2
A furniture maker has PES of 1.8 and currently supplies 4,000 units a month. Prices rise by 10%. Calculate the new quantity supplied. [3]
Step 1: rearrange
% change in QS = PES × % change in P
Step 2: substitute1.8 × 10 = +18%Step 3: apply4,000 × 1.18 = 4,7204,720 units a monthPES above 1 means output responds by proportionally more than price — a sign of spare capacity.
WORKED EXAMPLE 3
PES for a mineral is 0.5. By what percentage would price have to rise for quantity supplied to increase by 12%? [2]
Step 1: write the formula with X for the unknown0.5 = 12 ÷ XStep 2: solveX = 12 ÷ 0.5 = 24Price would have to rise by 24%Inelastic supply means a very large price rise is needed to squeeze out a modest increase in output.
💡 Exam tip
Think like a producer, not a consumer. Every PES explanation must be about capacity, costs, storage or time.
No percentage sign on the final answer, and no units.
Always name the time period. “In the short run PES is low, but in the long run it rises” is a complete evaluation in one sentence.
Use the origin rule if a question shows a straight supply curve and asks about elasticity.
For primary versus manufactured comparisons, go through the determinants one by one rather than asserting the conclusion.
Link PES to tax incidence: the more inelastic supply is, the more of an indirect tax the producer ends up bearing.
⚠️ Common mix-up
Answering a PES question with PED reasoning. Substitutes, necessities and addiction are demand-side and score nothing here.
Assuming a steeper supply curve is always more inelastic. Not if it passes through the origin — then PES is 1 whatever the slope.
Expecting a negative answer. PES is positive because supply slopes upward.
Writing the answer as a percentage. Elasticity has no units.
Saying supply is inelastic “because demand is inelastic”. The two are completely independent.
Forgetting spare capacity. Two firms in the same industry can have very different PES simply because one is already running flat out.
Up next: The Role of Government in Microeconomics — indirect taxes, subsidies and price controls. Everything you have just learned about PED and PES decides who really gains and who really pays.
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