In simple terms
A friendly intro before the formal notes — no formulas yet.
How Quickly Can Producers React to a Price Change?
Price elasticity of supply (PES) measures how much the quantity firms are willing and able to supply changes when the price changes. A high PES means producers can ramp output up or down quickly; a low PES means they are stuck, at least for now.
Think about two sellers on the day the price of their product jumps. A T-shirt printer with idle machines and blank stock in the back room can print and ship thousands more by tomorrow — highly responsive, high PES. An oak-furniture maker whose timber needs decades to grow and whose workshop is already full cannot make more this year no matter how tempting the price — very unresponsive, low PES. Same price rise, completely different ability to respond.
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Find the percentage change in price. Say the price of steel rises from 550 per tonne — a 10% rise.
- 2
Find the resulting percentage change in quantity supplied. Mills raise output by 5%.
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Apply the formula: PES = %ΔQs ÷ %ΔP = 5% ÷ 10% = 0.5.
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Interpret it. Because PES is below 1, supply is price inelastic — producers cannot expand output as fast as the price rose, since new capacity takes time to bring online.
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Defining and calculating PES
Price elasticity of supply measures the responsiveness of the quantity supplied of a good to a change in its own price. A large PES means firms can change output easily and quickly; a small PES means output is hard to change, at least in the time available. Because the responsiveness is expressed as a ratio of percentage changes, PES is a pure number with no units, which lets us compare very different goods on the same scale.
Each percentage change is measured against the ORIGINAL value: %ΔP = (new price − old price) ÷ old price × 100, and likewise for quantity. Getting that base right is what separates a correct answer from a near miss in an exam.
PES is the ratio of the percentage change in quantity supplied to the percentage change in price.
PES is always zero or positive, because price and quantity supplied move in the same direction (the law of supply).
A larger PES means greater responsiveness — producers can adjust output more freely.
Percentage changes are always taken relative to the starting (original) value, not the new value or the midpoint.
The spectrum of PES values
The PES number tells a story, and it is worth learning the five cases and how each looks as a supply curve. The two extreme cases — perfectly inelastic and perfectly elastic — are limiting benchmarks that make the middle three easier to picture.
Perfectly inelastic (PES = 0): quantity supplied is completely fixed; the supply curve is vertical. Example: seats in a fixed stadium on the night, or a Old Master painting.
Inelastic (0 < PES < 1): quantity supplied changes proportionally less than price; a relatively steep curve that, extended, cuts the quantity (horizontal) axis.
Unit elastic (PES = 1): quantity supplied changes in exactly the same proportion as price; ANY straight-line supply curve drawn through the origin, whatever its slope.
Elastic (1 < PES < ∞): quantity supplied changes proportionally more than price; a relatively flat curve that, extended, cuts the price (vertical) axis.
Perfectly elastic (PES = ∞): producers supply any amount at one price and nothing below it; a horizontal supply curve.
For diagram marks, remember the intercept rule. A straight-line supply curve through the ORIGIN is unit elastic wherever it sits. If it is extended and cuts the QUANTITY axis it is inelastic; if it cuts the PRICE axis it is elastic. Labelling that intercept is a fast, reliable way to prove your curve shows the elasticity you claim.
Determinants of PES
The elasticity of supply is not arbitrary — it follows from how production works. For Paper 3 you need to both state a determinant AND explain the direction in which it pushes PES.
Time period — the most important determinant. In the momentary (market) period, supply is often perfectly inelastic: nothing extra can be produced right now. In the short run at least one factor is fixed, but firms can work existing capacity harder, so supply becomes somewhat elastic. In the long run every factor is variable — new plant can be built, new workers trained — so supply is most elastic of all.
Spare (unused) capacity. Idle machinery and underemployed labour let a firm lift output quickly and cheaply when price rises, raising PES. A firm already at full capacity has little room to respond, lowering PES.
Ability to store stocks. If a good can be stockpiled cheaply (canned food, electronics, raw metals), firms can release inventory the instant price rises, so PES is higher. Perishable goods and services — which cannot be stored — have lower PES.
Mobility of factors of production. If land, labour and capital can be shifted easily between products, firms can redirect resources toward the good whose price has risen, raising PES. Highly specialised, immobile factors lower PES.
Ease of switching production. Related to mobility: if a producer can readily switch its production lines from one good to another (a farmer moving between similar crops, a factory retooling quickly), supply is more elastic. Where switching is slow or costly, PES is low.
Why primary commodities have low PES and manufactures higher PES
Bringing the determinants together explains a pattern examiners love to test. Primary commodities — agricultural crops, minerals, oil — tend to have LOW PES. Farming is bound by biological time lags: a crop cannot be grown faster than a season, and once harvested the quantity available is fixed until the next one. Mines, wells and plantations take years and large sunk investments to expand, and the specialised land and skills involved cannot easily be drawn in from elsewhere. So a price rise brings only a weak short-run supply response.
Manufactured goods tend to have HIGHER PES. Factories can add shifts, use spare capacity, hold and release inventories, and rely on general-purpose machinery and mobile labour that can be scaled up or switched between products comparatively quickly. The result is that when the price of a manufactured good rises, quantity supplied can respond more fully and faster. This contrast — inelastic primary supply versus more elastic manufactured supply — underlies much of the analysis of commodity price volatility later in the course.
Primary commodities: long/biological production lags, specialised and immobile factors, limited short-run capacity → LOW PES.
Manufactures: spare capacity, storable output, mobile general-purpose factors, easier switching → HIGHER PES.
Because inelastic supply cannot cushion demand shifts, low-PES commodity markets show larger price swings — a key link to price volatility.
Common mistakes examiners penalise
Inverting the formula. PES is %ΔQs ÷ %ΔP, with quantity on top. Writing %ΔP ÷ %ΔQs gives the reciprocal and loses the method mark.
Using the wrong percentage base. Divide each change by the ORIGINAL value, not the new value or the average — the most common source of a wrong PES.
Making PES negative. Unlike PED, PES is non-negative because price and quantity supplied move together. A negative PES signals an error.
Ignoring the time period. Calling a good's supply simply 'inelastic' misses marks — supply that is inelastic in the momentary period may be elastic in the long run. Always tie PES to a time horizon.
Assuming spare capacity makes supply LESS elastic. It is the opposite: spare capacity lets output rise quickly, so it RAISES PES.
Claiming primary and manufactured goods have the same PES. Primary commodities tend to be inelastic (production lags, immobile factors); manufactures tend to be more elastic. State the reason, not just the result.
Writing only the final number. On Paper 3, a bare answer with no working forfeits the method (M) marks and blocks follow-through — always show every step.
Where this leads
PES is not a stand-alone calculation — it drives real analysis. Because primary-commodity supply is inelastic, small shifts in demand cause large price swings, which motivates buffer-stock schemes and price-support policies. On Paper 3 you will combine PES with taxes, subsidies and price controls to work out who bears a burden and how quantity responds. Master the calculation and the determinants here, and those later questions become an application of one idea you already own.
Worked examples
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The market price of copper rises by 12% over a year. In response, global mines increase the quantity supplied by 3%. Calculate the PES for copper and interpret your answer.
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Step 1 — Apply the formula. PES = %ΔQs ÷ %ΔP = 3% ÷ 12%.
A furniture manufacturer sells tables at $200 each and supplies 8,000 tables a year while running its factory well below capacity. Demand rises and the price climbs to $230; using its idle capacity the firm raises output to 9,600 tables a year. Calculate the PES and comment on the result.
- 1
Step 1 — Percentage change in price. %ΔP = (230 − 200) ÷ 200 × 100 = 30 ÷ 200 × 100 = 15%.
Paper 3 quantitative: When price rises from 13, quantity supplied rises from 500 to 620 units. Calculate the PES and state whether supply is elastic or inelastic. [3]
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Model answer: Step 1 — Percentage change in quantity supplied. %ΔQs = (620 − 500) ÷ 500 × 100 = 120 ÷ 500 × 100 = 24%.
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Price elasticity of supply (PES)
A measure of the responsiveness of the quantity supplied of a good to a change in its own price. It captures how willing and able producers are to change output.
Key takeaways
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- ✓
PES is the ratio of the percentage change in quantity supplied to the percentage change in price.
- ✓
PES is always zero or positive, because price and quantity supplied move in the same direction (the law of supply).
- ✓
A larger PES means greater responsiveness — producers can adjust output more freely.
- ✓
Percentage changes are always taken relative to the starting (original) value, not the new value or the midpoint.
Practice — then mark it
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Get a Paper 3 PES calculation marked: show your working and see the M/A points awarded
Get a Paper 3 PES calculation marked: show your working and see the M/A points awarded
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