Price Elasticity of Supply Explained: Formula, Midpoint Method, and Examples
Elastic supply bends when price moves; inelastic supply barely moves at all. One ratio tells you which you are looking at — this guide shows how to compute it, read it, and write it up.
Price elasticity of supply measures how much the quantity supplied of a good changes, in percentage terms, when its price changes by 1%. It is the percentage change in quantity supplied divided by the percentage change in price. The number has no units, which is the point: it lets you compare the responsiveness of oil wells, concert tickets, and haircuts on the same scale. This guide covers the formula, the midpoint method that fixes its one awkward asymmetry, how to read the result, and why some supply is elastic and some is not.
- Elasticity = (% change in quantity supplied) ÷ (% change in price). Above 1 is elastic, below 1 is inelastic, exactly 1 is unit elastic, 0 is perfectly inelastic, and infinity is perfectly elastic.
- The midpoint method divides each change by the average of the start and end values, so the elasticity is the same whether price rises or falls. On the worked example it gives 1.43 where the simple method gives 1.50.
- Elasticity is a local measure at one point on one supply curve. It says nothing about demand, nothing about welfare, and nothing about what will happen if costs, technology, or the time horizon change.
What price elasticity of supply is
When the price of a good rises, suppliers usually offer more of it. Elasticity asks how much more, proportionally. A 10% price rise that brings 30% more quantity to market is elastic supply; the same rise bringing only 2% more is inelastic. The ratio strips out the units — tonnes, litres, tickets — so a steel mill and a lemonade stand can be compared directly. What it does not do is explain why the quantity moved. That is the supply curve’s slope, the time horizon, and the production technology, which the number summarizes but does not replace.
Two cautions belong with the definition. First, elasticity is measured at a point, or over one interval. The same good can be inelastic over a week and elastic over a year, because producers need time to build capacity, hire, and retool. Second, the sign is normally positive for supply: price and quantity move together, unlike demand, where they move in opposite directions. A negative supply elasticity usually means something else is moving at the same time — costs, expectations, or a data error — and it should be investigated, not quoted.
The formula, and the midpoint method
The simple formula is PES = (%ΔQs) ÷ (%ΔP), where %ΔQs = (Q₂ − Q₁) ÷ Q₁ × 100 and %ΔP = (P₂ − P₁) ÷ P₁ × 100. Work it through: price rises from $10 to $12, a 20% rise, and quantity supplied rises from 100 to 130 units, a 30% rise. PES = 30 ÷ 20 = 1.50. Supply is elastic: quantity moves one and a half times as much, proportionally, as price.
The simple method has one flaw. Measured from the other direction — price falling from $12 to $10 — the percentage changes are different: quantity falls 23.08% from 130, price falls 16.67% from 12, and PES = 1.38. The same interval gives two answers depending on which end you start from. The midpoint method fixes this by dividing each change by the average of the two values: %ΔQs = (Q₂ − Q₁) ÷ ((Q₁ + Q₂) ÷ 2) and %ΔP = (P₂ − P₁) ÷ ((P₁ + P₂) ÷ 2). On the same example: %ΔQs = 30 ÷ 115 = 26.09%, %ΔP = 2 ÷ 11 = 18.18%, and PES = 26.09 ÷ 18.18 = 1.43, the same in either direction.
Use the method your course or exam specifies, and say which one you used. The midpoint method is the standard in most textbooks and in the Price Elasticity of Supply Calculator, which reports both the simple and midpoint figures so the difference is visible. Mixing methods inside one answer produces numbers that cannot be compared with each other — the commonest avoidable error in elasticity write-ups.
| Reading | PES value | Meaning | Example |
|---|---|---|---|
| Perfectly inelastic | 0 | Quantity does not respond at all | A fixed number of seats at tonight’s show |
| Inelastic | Between 0 and 1 | Quantity moves less than proportionally | Fresh fish landed today; the catch is already in |
| Unit elastic | 1 | Quantity moves in exact proportion | A textbook case; rare in practice |
| Elastic | Above 1 | Quantity moves more than proportionally | A factory with spare capacity and stored inputs |
| Perfectly elastic | Infinity | Any quantity at one price; none above it | A small seller in a world market at the world price |
Worked examples, step by step
Example 1 — elastic. A bakery sells 200 loaves a day at $3.00. At $3.30, it sells 260. Simple method: %ΔQ = 60 ÷ 200 = 30%, %ΔP = 0.30 ÷ 3.00 = 10%, PES = 3.00. Midpoint: %ΔQ = 60 ÷ 230 = 26.09%, %ΔP = 0.30 ÷ 3.15 = 9.52%, PES = 2.74. Both say elastic; the midpoint figure is the one to quote when direction should not matter. The story behind it: the bakery has spare oven capacity and can buy more flour at short notice.
Example 2 — inelastic. A vineyard produces 10,000 bottles a year at $20. At $24, it still produces 10,200, because the vines were planted years ago and the harvest is fixed. Simple: %ΔQ = 2%, %ΔP = 20%, PES = 0.10. Midpoint: %ΔQ = 200 ÷ 10,100 = 1.98%, %ΔP = 4 ÷ 22 = 18.18%, PES = 0.11. The near-zero elasticity is not a failure of the market; it is the biology of grapevines. Given five years, new planting would make the same vineyard far more elastic.
Example 3 — the zero-change case. If price does not change at all, the percentage change in price is zero and the ratio is undefined — you cannot divide by zero. That is not a finding of infinite elasticity; it is the absence of an experiment. Elasticity needs a price change to be measured. Likewise, if quantity does not change when price moves, PES is 0, which is a real finding: perfectly inelastic over that interval.
Why supply is elastic in some cases and inelastic in others
Time is the biggest determinant. In the immediate run, supply is nearly fixed: the fish are caught, the concert is tonight, the crop is in the ground. Over months, producers adjust inputs, overtime, and inventories. Over years, they build plants, plant orchards, and train workers. The same industry is inelastic this week and elastic this decade, which is why every elasticity figure should carry its time horizon in the same sentence.
Spare capacity and storability come next. A factory running at 60% can raise output quickly; one at 100% cannot without new investment. A good that stores well — grain, metals — can be released from inventory when price rises, making short-run supply look elastic even when production is fixed. A perishable good cannot. Mobility of inputs matters too: labour and materials that can be shifted between products make each product’s supply more elastic, because producers can reallocate toward the higher price.
Costs set the limit. If expanding output needs sharply rising marginal cost — overtime premiums, scarce skilled labour, depleting mines — quantity responds weakly even to large price rises. If marginal cost is flat over the relevant range, quantity responds strongly. The shape of the cost curve is the economics underneath the elasticity number, and a write-up that names the cost reason behind its figure is worth more than one that only quotes the figure.
What this number cannot tell you
Elasticity does not say whether a price rise is good or bad, for producers or consumers. A tax on an inelastically supplied good raises revenue with little change in quantity; a tax on an elastically supplied good shrinks the market. Those are policy conclusions drawn from elasticity plus a welfare framework, not from the number alone. Elasticity also does not predict the effect of a cost shock: a rise in input costs shifts the whole supply curve, and the new quantity at the new price is governed by both supply and demand elasticities together.
It does not compare across goods without care either. An elasticity of 1.50 for wheat and 1.50 for concert tickets means the same proportional responsiveness, but the quantities, the time horizons, and the reasons differ entirely. And it is silent about the long run: a figure measured over one season should never be used to forecast a decade of adjustment. Quote the interval, the method, and the horizon, and the number earns its keep; quoted bare, it invites exactly the over-reading it is meant to prevent.
Who pays a tax: elasticity decides the split
When a tax is placed on a good, the legal liability — who hands the money to the government — is not the economic burden. The burden splits between buyers and sellers according to their relative elasticities: the side that can adjust less bears more. If supply is elastic and demand is inelastic, buyers bear most of the tax, because they cannot easily reduce their purchases while sellers can easily reduce their supply. If supply is inelastic and demand is elastic, sellers bear most of it, because they cannot easily leave the market while buyers can easily walk away.
The split has a formula. The consumer’s share of a per-unit tax is Es ÷ (Es + |Ed|), where Es is the price elasticity of supply and Ed the price elasticity of demand, and the producer’s share is the remainder. With Es = 1.50 and Ed = −0.50, the consumer’s share is 1.50 ÷ (1.50 + 0.50) = 75%, and the producer’s share is 25%. A $1.00 tax then raises the price buyers pay by about $0.75 and lowers the price sellers keep by about $0.25. The formula is an approximation for small taxes, but the direction is exact: the inelastic side pays, whatever the statute says.
The same logic governs subsidies in reverse and minimum wages in a related form. A subsidy to producers of an inelastically supplied good mostly reaches buyers as lower prices; a subsidy to a market with inelastic demand mostly stays with sellers. And a policy aimed at helping one side of a market will largely help the other side if the elasticities point that way. Elasticity is therefore not only a description of a market; it is a prediction about who any intervention will actually affect, and a policy designed without it is a transfer aimed at the wrong pocket.
Frequently asked questions
What is price elasticity of supply?
Price elasticity of supply measures how much the quantity supplied of a good changes, in percentage terms, when its price changes by 1%. It is the percentage change in quantity supplied divided by the percentage change in price.
How do you calculate price elasticity of supply?
Divide the percentage change in quantity supplied by the percentage change in price. With the midpoint method, each percentage change is measured against the average of the start and end values, so the answer is the same whether price rises or falls.
What does an elasticity of 1.50 mean?
A 1% rise in price is linked to a 1.50% rise in quantity supplied. Above 1, quantity moves more than proportionally, which is elastic: at 1.50, a 1% price rise is linked to a 1.50% rise in quantity supplied.
What is the difference between elastic and inelastic supply?
Elastic supply (PES above 1) responds more than proportionally to a price change; inelastic supply (PES below 1) responds less than proportionally. Perfectly inelastic supply (PES 0) does not respond at all, and perfectly elastic supply responds without limit at one price.
What if the price does not change at all?
The elasticity is undefined: the percentage change in price is zero and division by zero is not allowed. That is the absence of an experiment, not a finding. Elasticity needs a price change to be measured.
Can price elasticity of supply be negative?
Normally no: price and quantity supplied move together, so PES is zero or positive. A negative figure usually means something else moved at the same time — costs, expectations, or a data error — and should be investigated before it is quoted.
Try it on the calculator
Run your own figures through the calculator and see the answer in numbers.
Open the Price Elasticity of Supply Calculator →- 1Price Elasticity of Supply Calculator
Compute simple and midpoint elasticity from your own prices and quantities.
- 2Markup Calculator
Price, cost, and margin in one place.
- 3Discount Calculator
Sale prices and savings, with the percentage change shown.
- 4Percentage Calculator
Parts, wholes, and changes — the arithmetic under every elasticity.