Define and calculate price elasticity of supply, interpret its value, and explain its determinants

What this dot point is asking

SEAB wants you to define and calculate the price elasticity of supply (PES), interpret its value, and explain the determinants that make supply more or less responsive. The central insight is that PES is mostly about how easily and quickly producers can change output, which depends above all on spare capacity and the time available.

The answer

Defining and calculating PES

PES is normally positive, because the law of supply makes quantity supplied rise with price. As with PED, compute the two percentage changes first, then divide.

Interpreting the value

• $PES > 1$: supply is elastic (quantity responds more than proportionately to price).
• $PES < 1$: supply is inelastic (quantity responds less than proportionately).
• $PES = 1$: unit elastic.
• $PES = 0$: perfectly inelastic (a vertical supply curve); quantity cannot change at all, as with a fixed stock such as seats in a stadium.
• $PES = \infty$: perfectly elastic (a horizontal supply curve); firms supply any amount at the going price.

Determinants of PES

What makes supply responsive:

• Spare (unused) capacity. With idle machines and workers, firms can raise output quickly, so supply is elastic. Near full capacity, supply is inelastic.
• Level of stocks. If firms hold inventories, they can release them in response to a higher price, raising elasticity.
• Mobility of factors of production. The more easily land, labour and capital can be switched into producing the good, the more elastic supply.
• Time period. The most important determinant: supply is more elastic the longer the time frame, because more factors become variable.
• Ease of producing more. Goods that can be made quickly and cheaply have more elastic supply than those needing long lead times (mined minerals, mature trees).

The role of time

Economists distinguish three time frames:

• Momentary (market period): supply is fixed (perfectly inelastic); output cannot change at all.
• Short run: at least one factor is fixed, so supply can change somewhat but is constrained.
• Long run: all factors are variable and new firms can enter, so supply is most elastic.

Examples in context

Example 1. Housing supply in Singapore. The supply of housing is highly inelastic in the short run because building takes years, so a surge in demand pushes prices up sharply before any new supply appears. Over the long run, public and private construction makes supply more elastic, which is why the government plans housing supply years ahead to dampen price volatility.

Example 2. Manufacturing versus mining. A consumer-electronics maker with spare assembly capacity can ramp output quickly, giving elastic supply, while a copper mine facing a higher price cannot raise extraction fast because of fixed geology and long development times, giving inelastic supply. The contrast shows how production constraints set PES.

Try this

Q1. Price rises 5 percent and quantity supplied rises 2 percent. Calculate PES and classify it. [2 marks]

• Cue. $PES = \dfrac{+2\%}{+5\%} = +0.4$; the magnitude is below 1, so supply is inelastic.

Q2. Explain why supply is more elastic in the long run. [3 marks]

• Cue. Over time, fixed factors such as capital become variable and new firms can enter, so output can respond much more fully to a price change than when capacity is fixed.

Q3. Give one example of a good with perfectly inelastic supply and explain why. [2 marks]

• Cue. Seats at a fixed-capacity stadium: the quantity cannot change whatever the price, so the supply curve is vertical and PES is zero.