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Just as price elasticity of demand measures how responsive buyers are to a price change, price elasticity of supply (PES) measures how responsive sellers are. It answers a deceptively important question: when the price of a good rises, by how much — and how quickly — can producers expand the quantity they offer? The answer shapes how sharply prices move when demand shifts, who bears the burden of a tax, and why some markets (housing, oil, commodities) are so much more volatile than others. PES is rooted in the cost and capacity constraints that producers face, and above all in the time period over which they can adjust. This lesson develops the formula, the full range of PES values with diagrams, the determinants, and the short-run/long-run distinction that runs through the whole topic.
This lesson maps to AQA 7136 section 4.1.3 — Individual economic decision-making and how markets work, specifically the calculation, interpretation and determinants of price elasticity of supply. It is examined in Paper 1 (Markets and market failure) through multiple-choice, calculation-based data response and extended evaluation. The content is synoptic: PES is decisive for tax and subsidy incidence (the next lesson but one) and for price volatility in commodity and housing markets; it also connects to short-run and long-run aggregate supply in Paper 2, since the responsiveness of industry supply mirrors the responsiveness of aggregate supply. All four assessment objectives apply: AO1 for the formula and the range of values, AO2 for applying determinants to named markets, AO3 for chains linking elasticity to price stability and tax incidence, and AO4 for evaluating the reliability and the time-dependence of PES estimates.
Key Definition: Price elasticity of supply (PES) measures the responsiveness of the quantity supplied of a good to a change in its own price, ceteris paribus.
PES=%ΔPrice%ΔQuantity supplied
Because the law of supply establishes a positive relationship between price and quantity supplied, PES is normally positive (or, in the limiting case, zero). Unlike price elasticity of demand — where the negative sign is taken for granted and the focus is on magnitude — with PES there is no sign ambiguity to worry about. The whole interpretation rests on the magnitude: is the value above one (elastic), below one (inelastic), or exactly one (unit elastic)?
Suppose the price of strawberries rises from £2.00 to £2.40 a punnet — a 20% increase — and over the same period growers raise the quantity supplied from 10,000 to 11,000 punnets a week, a 10% increase. Then:
PES=+20%+10%=+0.5
A value of 0.5 lies between zero and one, so supply is price-inelastic: the quantity supplied rose proportionately less than the price. This is exactly what we would expect for an agricultural good in the short run, where growers cannot easily expand output mid-season. Now suppose that for a mass-produced manufactured good with spare capacity, a 20% price rise calls forth a 40% rise in quantity supplied:
PES=+20%+40%=+2.0
A value of 2.0 exceeds one, so supply is price-elastic — producers respond more than proportionately, drawing on idle capacity to expand output rapidly.
Exam Tip: Do not waste words debating the sign of PES — it is positive. Spend them instead justifying why the magnitude is high or low (time period, spare capacity, stocks, factor mobility), which is where the AO3 marks lie.
It is worth recalling why a higher price calls forth a greater quantity supplied, because it explains what makes PES large or small. As established in the supply lesson, the supply curve is, in effect, the producers' marginal cost curve: each unit is offered for sale only if its price covers the cost of producing it. When the price rises, units that were previously unprofitable to make now become worth making, so quantity supplied rises. The responsiveness of that quantity — the PES — therefore depends on how easily and cheaply producers can bring extra units into production. If extra output can be produced at little additional cost (spare capacity, mobile factors, plenty of time), a small price rise unlocks a large quantity response and supply is elastic. If extra output is very costly or slow to produce (full capacity, immobile specialised factors, a binding growing season), even a large price rise unlocks little extra quantity and supply is inelastic. Seen this way, every determinant of PES below is really a statement about how steeply marginal cost rises as output expands.
Suppose two markets each experience the same 25% price rise. In the first, a craft producer of a hand-made good — limited skilled labour, no spare capacity — raises quantity supplied from 200 to 210 units, a 5% increase:
PES=+25%+5%=+0.2
Supply is highly inelastic (0.2): the specialised, capacity-constrained producer can barely respond. In the second market, a factory with idle machines and a flexible workforce raises quantity supplied from 1,000 to 1,750 units, a 75% increase:
PES=+25%+75%=+3.0
Supply is highly elastic (3.0): the same price signal unlocks a far larger quantity response because the extra units are cheap and quick to make. The contrast — identical price stimulus, wildly different quantity responses — is entirely explained by the cost and capacity differences between the two producers, and it is exactly the reasoning an exam answer should reproduce.
A small but examinable subtlety concerns how the percentage changes are calculated. The simple formula above uses the initial values as the base for each percentage change, which is fine for small movements but gives a slightly different answer depending on whether you measure from the old point or the new one. Economists sometimes prefer the midpoint (arc) method, which uses the average of the old and new values as the base, so that the elasticity between two points is the same whichever direction you travel. At A-Level you will almost always use the simple initial-value method, but it is worth understanding why the answer can be sensitive to the base: elasticity is a local property that can differ at every point on a curve, so a single PES figure is really an approximation over the range you have chosen. This is the same reason the textbook assumption of one constant PES for a whole market is a simplification — and noticing that is exactly the kind of awareness that distinguishes a confident answer from a mechanical one.
It also follows that PES, like PED, is a unit-free ratio of proportionate changes — it compares percentage with percentage, not pounds with tonnes. That is what allows us to compare the responsiveness of utterly different markets (strawberries, electronics, oil) on a single scale, and it is why the sign and size of the number, not the raw quantities, carry the economic meaning.
| PES Value | Description | Meaning | Curve shape |
|---|---|---|---|
| PES = 0 | Perfectly inelastic | Quantity supplied cannot change at all | Vertical |
| 0 < PES < 1 | Inelastic | Quantity supplied changes less than proportionately | Steep |
| PES = 1 | Unit elastic | Quantity supplied changes proportionately | Straight line through the origin |
| PES > 1 | Elastic | Quantity supplied changes more than proportionately | Shallow |
| PES = ∞ | Perfectly elastic | Any quantity can be supplied at the going price | Horizontal |
The two extreme cases are the clearest to picture: a perfectly inelastic supply curve is vertical (quantity fixed whatever the price), and a perfectly elastic supply curve is horizontal (unlimited quantity at the going price, none below it).
Between these extremes lie the cases that matter most in practice: a steep curve (inelastic, 0 < PES < 1) and a shallow curve (elastic, PES > 1). A subtle but heavily examined point concerns the unit-elastic case.
Any straight-line supply curve drawn through the origin has PES = 1, whatever its gradient. This surprises many students, because a steep line "looks" inelastic. The reason is that along a line through the origin, the proportionate change in quantity always exactly equals the proportionate change in price (price and quantity are in fixed proportion), so the ratio is one. By contrast, a straight-line supply curve that hits the price axis above the origin is elastic throughout (PES > 1), and one that hits the quantity axis to the right of the origin is inelastic throughout (PES < 1).
Exam Tip: Never judge PES from slope alone. The test is where the line would intercept the axes: through the origin → unit elastic; intercepts the price axis → elastic; intercepts the quantity axis → inelastic. This is a favourite multiple-choice trap.
Five determinants decide whether supply is elastic or inelastic. The first four can all be understood as different aspects of how easily and quickly producers can expand output.
The longer the time producers have to adjust, the more elastic supply becomes. Economists distinguish three periods:
| Time period | Typical PES | Why |
|---|---|---|
| Momentary / very short run | Perfectly inelastic (PES = 0) | Output is fixed; nothing can be changed in time |
| Short run | Inelastic (0 < PES < 1) | At least one factor is fixed; firms can add variable inputs but hit diminishing returns |
| Long run | Elastic (PES > 1) | All factors are variable; firms can build new capacity and new firms can enter |
In the momentary period a fishmonger with a fixed catch on the quay cannot supply more however high the price climbs. In the short run a factory can run extra shifts and hire more labour, but it is constrained by its fixed plant. In the long run the firm can build new plant, and new entrants can entirely reshape industry supply — so supply becomes far more elastic.
If firms have unused capacity — idle machines, under-employed workers, empty floor space — they can raise output quickly and cheaply in response to a higher price, so supply is elastic. Firms already at full capacity can do little in the short run, so supply is inelastic. This is why supply tends to be more elastic in a downturn (when spare capacity is plentiful) than at the peak of a boom (when the economy is straining against its limits) — a useful synoptic bridge to the aggregate-supply curve. The same point explains why a single firm or industry often has a PES that varies along its supply curve: at low levels of output, when plenty of spare capacity remains, a price rise can be met cheaply and supply is relatively elastic; as the firm approaches full capacity, each extra unit requires increasingly expensive measures (overtime premia, less-efficient older machines, paying to attract scarce labour), marginal cost rises steeply, and supply becomes progressively more inelastic. Elasticity, in other words, is not necessarily a single fixed number for a market but can change as the market expands towards its capacity ceiling — which is precisely why the same industry can look elastic in a slump and inelastic at the peak of a boom.
A firm holding large stocks of finished goods can meet a surge in demand instantly by running down inventories, making supply elastic. Where the product is perishable or stocks are thin, this cushion is absent and supply is inelastic. Fresh produce and many services (which cannot be stored at all) therefore tend to have less elastic supply than durable manufactured goods that can be warehoused.
If the factors of production can be switched easily from making other goods, supply is elastic; if factors are occupationally or geographically immobile, supply is inelastic. A clothing factory can re-tool from one garment to another fairly quickly (mobile capital and labour, elastic supply). Highly specialised capital and skilled labour — a power station, a surgeon — cannot be redeployed quickly, so the supply of their output is inelastic.
| Type of good | Typical PES | Reason |
|---|---|---|
| Agricultural goods | Inelastic (especially short run) | Growing seasons; weather; biological time lags |
| Extracted resources (oil, minerals) | Inelastic (short–medium run) | Long lead times for exploration and extraction |
| Manufactured goods | Elastic (if spare capacity) | Output adjusts via shifts, hiring, stocks |
| Services | Varies | Depends on the availability of skilled labour and capacity |
Primary commodities are typically supply-inelastic because nature, not the firm, sets the pace: a grower cannot bring forward a harvest, and an oil company cannot conjure a new field overnight. Manufactured goods, by contrast, can usually be scaled up far more quickly.
Exam Tip: Whatever determinant you discuss, anchor it to time. "In the short run, agricultural supply is inelastic because growers cannot raise output mid-season; in the long run it becomes more elastic as they can plant more land, invest in irrigation, and adopt higher-yielding varieties." That short-run/long-run framing is exactly what lifts a determinant from AO1 description to AO3 analysis.
A subtle but examinable point is that PES is not necessarily the same for a price rise as for a price fall. Expanding output is often easier than contracting it. When prices rise, a firm with spare capacity can quickly hire, run extra shifts and draw on stocks. But when prices fall, the firm may be reluctant — or unable — to cut output quickly: it has fixed costs and sunk costs already committed, contractual obligations to workers and suppliers, and capital that cannot simply be unmade. The result is an asymmetric supply response, in which quantity supplied rises readily but falls only sluggishly. This matters for real markets: it helps explain, for example, why retail prices sometimes "rocket and feather" — rising quickly when costs rise but falling only slowly when costs fall — and it is a reminder that the textbook assumption of a single, symmetric PES is itself a simplification. Noting this asymmetry, where relevant, is a strong evaluation point.
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