Price Elasticity of Supply: How Fast Output Reacts to Price
Definition
Price Elasticity of Supply
A measure of the responsiveness of quantity supplied when there is a change in price.
Price elasticity of supply (PES) measures how much the quantity supplied of a good responds to a change in its price, calculated as $PES = \frac{\%\Delta Q_s}{\%\Delta P}$.
It is the supply-side twin of PED but tracks producers, and because both parts are percentages it has no units, so a PES of 2 means quantity supplied changes twice as fast as price.
PES is almost always positive, because the law of supply moves quantity supplied and price in the same direction, the opposite of PED.
Example
Chocolate bars are priced at 50 and producers supply 15; when the price rises to 100 they supply 25.
Price doubled, a 100% rise, while quantity rose from 15 to 25, about a 66.7% rise.
PES is 66.7 divided by 100, about 0.67, so supply here is inelastic.
Degrees of PES: Reading Elastic and Inelastic Supply
When $0 < PES < 1$, supply is price inelastic: quantity supplied changes less than price, like seasonal fruit that cannot grow faster within a season.
When $PES > 1$, supply is price elastic: quantity supplied changes more than price, like taxi rides where a small fare rise quickly draws in extra drivers.
Steepness carries the same message on shared axes: a flatter supply curve is more elastic and a steeper one more inelastic.
The Special Cases: PES of Zero, One, and Infinity
When $PES = 1$, supply is unit elastic: any straight-line supply curve through the origin is unit elastic, whatever its steepness.
When $PES = 0$, supply is perfectly inelastic, a vertical line: quantity supplied is fixed, like the seats for a single cinema screening.
When $PES = \infty$, supply is perfectly elastic, a horizontal line: producers supply any quantity at one price, but none below it.
Determinants of PES: What Makes Supply Flexible
Time period matters most: in the momentary period output cannot change, but given time producers build capacity and hire, so supply grows more elastic.
Spare capacity works alongside time: a firm with idle machines and workers can raise output cheaply, while one at full capacity cannot.
Factor mobility makes supply elastic when land, labour, and capital switch easily between uses, and inelastic when they are specialised.
The ability to store stock lets producers hold goods back and release them as price changes, so durable goods are more elastic than perishable ones.
Ease of entry makes supply elastic when new producers can join quickly, and inelastic when barriers are high.
Ethical or cultural pressures can override the profit response: a pharmaceutical firm may cap the supply of an essential medicine in a shortage, and a luxury brand may keep supply scarce to protect status.
Theory of Knowledge
When producers could raise supply but choose not to, like a drugmaker capping an essential medicine in a shortage, is holding output back a responsible act or a failure to meet need?
Why Time Is the Master Factor
Every other determinant works through time: given long enough, firms build capacity, factors retrain, and new rivals enter, so supply that is inelastic today becomes elastic later.
A fresh crop shows it: in one season supply is nearly fixed, but over several seasons farmers plant more and PES rises.
So PES is not one fixed number for a good; it rises as the time horizon lengthens.
Active recall
State the PES formula and explain why PES is normally positive.
A 10% rise in price raises quantity supplied by 4%. Calculate PES and classify the supply.
Describe the supply curve for each of the three special PES values: 0, 1, and infinity.
Name the five determinants of PES and give one that makes supply more elastic.
Explain why the PES of a fresh crop rises as the time period lengthens.
