The price of a rare metal doubles and mining output barely moves for three years, because opening a mine takes three years. The price of printed t-shirts doubles and supply floods in within a fortnight. Same price signal, completely different response, and the difference decides who actually gains from a price rise. Price elasticity of supply measures how quickly producers can react, and it explains why some markets absorb shocks smoothly while others lurch.
Price elasticity of supply
A measure of the responsiveness of the quantity supplied of a good to a change in its price.
PES = percentage change in quantity supplied / percentage change in pricePES is normally positive, because supply curves slope upward and price and quantity move together. This is the main arithmetical difference from PED, which is normally negative. Formulae are not printed in any 9708 paper, so recall it.
The value is interpreted against one. Supply is elastic when PES is greater than 1, meaning quantity responds proportionately more than price, and producers can react easily. It is inelastic when PES is between 0 and 1, so quantity responds proportionately less than price and producers are constrained. Unit elastic supply gives exactly 1. Two extreme cases matter: perfectly inelastic supply, PES of 0, where quantity cannot change at all, as with seats in a stadium on the night; and perfectly elastic supply, infinite PES, where any amount can be supplied at the going price and none at all above it.
Calculating and interpreting PES
The price of tomatoes rises from $2.00 to $2.50 per kilo. Quantity supplied rises from 8,000 to 9,200 kilos per week. Calculate PES, classify it, and state what would happen to the value if the same price rise were sustained for two years.
The determinants all reduce to how easily producers can change output. Time is the most important: supply is more elastic the longer the period allowed, since factors of production become adjustable. Spare capacity makes supply elastic, because a firm with idle machinery can increase output immediately. The availability of stocks matters, since a producer holding inventory can release it at once. Ease of switching production between goods raises elasticity, which is why a factory making one style of shirt can quickly make another. And the mobility of factors of production matters: where skilled labour or specialised equipment is hard to obtain, output cannot rise quickly whatever the price.
Almost every PES question can be answered through time and capacity, so start there. Agricultural goods are supply inelastic in the short run because crops take a season to grow, and manufactured goods with spare capacity are elastic because output can be raised at once. Naming the specific constraint in the scenario, rather than asserting that supply is inelastic, is what earns the explanation marks.
Draw three supply curves on separate diagrams: one perfectly inelastic (vertical), one perfectly elastic (horizontal) and one relatively inelastic (steep). On each, show a rise in demand and mark the resulting change in price and quantity.
The comparison is the point. With perfectly inelastic supply the entire increase in demand goes into price and none into quantity. With perfectly elastic supply it goes entirely into quantity and none into price. Real markets fall between, and where they fall determines whether a demand shock raises prices or output. This is the same reasoning used for tax incidence.
Which good is most likely to have the lowest price elasticity of supply in the short run?
Price rises by 20% and quantity supplied rises by 30%. PES is:
Why elasticity of supply matters
At AS the concept earns its keep through applications rather than calculation. Price volatility depends directly on PES: where supply is inelastic, any shift in demand produces a large price change and a small quantity change, which is why commodity and housing markets are volatile while manufactured goods prices are relatively stable. Agricultural markets combine inelastic supply with inelastic demand, which is why farm prices swing violently and why governments intervene with buffer stocks and price supports so often.
The distinction between the short run and the long run is formalised here. In the momentary period supply is effectively fixed and PES is close to zero, since nothing can be changed. In the short run at least one factor is fixed, so output can be varied only by working existing capacity harder, giving low but positive PES. In the long run all factors are variable and firms can enter or leave the industry, so PES is much higher. Any statement about elasticity of supply is therefore incomplete without specifying the period being considered.
Why inelastic supply concentrates the effect of a demand shock on price
- Demand for a good rises, shifting the demand curve right.
- At the original price, quantity demanded now exceeds quantity supplied, so a shortage appears.
- Price begins to rise, rationing the available quantity among buyers.
- Producers would like to supply more at the higher price, since it is more profitable.
- But supply is inelastic: capacity is fixed, stocks are exhausted and new production takes time, so quantity supplied rises only slightly.
- Because quantity cannot adjust, the shortage is eliminated almost entirely by price rising rather than by output expanding.
- The equilibrium price ends up substantially higher while equilibrium quantity is little changed, so producers of the existing output enjoy a windfall gain.
Step 7 explains the distributional consequence, which is where evaluation marks sit. In markets with inelastic supply, such as urban land and housing, a demand increase transfers income to existing owners rather than generating additional output, which is the analytical basis for land value taxation.
Practise this chain →PES also determines who bears an indirect tax, working alongside PED. The side of the market that is less able to respond bears more of the burden. Where supply is inelastic and demand elastic, producers cannot reduce output much and consumers can easily switch away, so producers absorb most of the tax. Where supply is elastic and demand inelastic, the reverse holds and consumers pay most of it. This is why taxes on goods with inelastic demand and elastic supply, such as tobacco, fall almost entirely on consumers, and it is the same logic that makes such taxes regressive.
Draw two markets with the same rightward shift of demand: one with a steep, inelastic supply curve and one with a shallow, elastic supply curve. Mark the change in price and quantity in each. Then, on a third diagram, show a specific tax shifting supply upwards where supply is inelastic and demand elastic, and shade the producer and consumer shares of the tax.
Keep the size of the demand shift identical across the first two diagrams so the comparison isolates the elasticity. On the tax diagram, the consumer share is the vertical distance from the old price up to the new price, and the producer share is the distance from the old price down to what the seller now receives after tax. Candidates routinely shade one and forget the other.
Housing supply in a major city is highly inelastic. A rise in demand from population growth will most likely:
Applied context
Semiconductor supply during the shortages of 2021 and 2022 illustrates extreme short-run inelasticity. A fabrication plant costs billions and takes several years to build and commission, so when demand from vehicles and electronics rose sharply, output could not respond and prices, lead times and allocation rationing did the adjusting instead. Firms in downstream industries curtailed production for want of chips. The eventual supply response arrived only as new capacity came online years later, by which point demand conditions had changed again, which is the classic pattern of markets with long production lags.
This example also introduces the cobweb problem: where supply responds with a long lag, producers make decisions based on prices that no longer apply by the time output arrives, which can generate cycles of over and under supply rather than convergence to equilibrium.
PES works alongside PED in almost every application, so the two notes are best read together. Tax incidence depends on both elasticities and appears in the government intervention topic. Buffer stock schemes and agricultural price support exist because agricultural supply is inelastic and subject to unpredictable shifts. In the theory of the firm, the shape of the short-run supply curve is the marginal cost curve above average variable cost, so PES is really a statement about how quickly marginal cost rises. And the short run and long run distinction defined here is the same one used throughout the analysis of costs.
9708-2.2 · 9708-2.4 · 9708-3.2 · 9708-7.5 · 9708-8.1
How reliable is PES as a guide for policy?
- Its core insight is robust and repeatedly confirmed: markets where supply cannot adjust quickly show volatile prices, and this predicts behaviour in housing, agriculture and commodities well.
- Against that, PES is difficult to measure with any precision. It requires observing quantity responses to price changes while everything else is held constant, and in practice several determinants move at once, so published estimates vary widely for the same market.
- The value is not a fixed property of a good, since it rises continuously with the time period considered. A single figure is therefore meaningless without a stated horizon, and policy based on a short-run estimate may badly misjudge the long-run response.
- The formula assumes producers respond to price alone. Expectations matter as much: producers deciding whether to invest respond to the price they expect to prevail when output arrives, not to today's price, which is why long-lag markets can cycle rather than settle.
- Elasticity also assumes a smooth relationship, whereas real supply often changes in large discrete steps, since a new plant either opens or does not. Below the threshold nothing happens and above it a great deal does.
- The strongest answers use PES qualitatively, to reason about whether a shock will show up in prices or in quantities, rather than treating any particular numerical estimate as reliable.
Three habits protect the marks. Divide quantity by price and not the other way round, since inverting is the commonest error and produces a plausible-looking wrong answer. Expect a positive sign, and if you get a negative one check your arithmetic rather than writing it down. And always follow the number with the classification and one sentence of interpretation, because the calculation mark and the interpretation mark are usually separate.