A café lowers its drink price by ten per cent and sells thirty per cent more drinks. That sounds successful, but how much revenue actually changes? A neighbouring shop increases a different product’s price and loses only a handful of customers; its revenue might rise. Economics uses price elasticity of demand to describe this responsiveness, rather than treating every price change as though buyers react in the same way.
Ministry of Education V3.0 Reference 291 follows Section 2.6, Price Elasticity of Demand (PED), of the 2027 Singapore–Cambridge SEC G3 Economics K343 syllabus. It explains how to calculate and interpret price responsiveness, distinguish elastic and inelastic demand, read special demand-curve cases, identify determinants, connect elasticity to consumer expenditure and firms’ revenue, and evaluate implications for producers, households, workers and governments. Twelve decision stages and a fictional neighbourhood market provide independently checkable examples.
This is an independent eduKateSG reference, not an official MOE/SEAB publication or pricing advice to any company. All prices, quantities, businesses, customer responses and revenue forecasts below are original hypothetical teaching data. A calculated elasticity describes a particular observed or assumed comparison; it is not proof that a real firm can safely change its prices without considering cost, competition, fairness and demand shifts.
1. Demand is not the same thing as a purchase
Demand is a relationship between quantities people are willing and able to buy and possible prices during a period, with other factors held constant. An observed purchase records an actual transaction, which may reflect stock availability and many conditions.
Before using a percentage change, make sure the quantities describe demand response to the product’s own price rather than an unrelated change in advertising, income or supply.
2. What K343 Section 2.6 requires
The 2027 SEC G3 Economics syllabus includes the definition and calculation of PED, the meanings of perfectly inelastic, inelastic, unitary, elastic and perfectly elastic demand, demand-curve diagrams, determinants, consumer expenditure and firms’ revenue, and policy or business significance.
This article stays with those specific learning outcomes. Price elasticity of supply (PES) appears separately in Section 2.7 and measures producers’ response to price, not buyers’ response. The two ideas are connected but must not be confused.
3. PED compares percentage changes
Price elasticity of demand measures the responsiveness of quantity demanded to a change in the product’s own price, all else held constant. A common formula is PED = percentage change in quantity demanded divided by percentage change in price.
Both changes are percentages so PED has no units. For a typical downward-sloping demand relationship, the price and quantity movements have opposite signs, producing a negative calculated value under the signed convention.
4. Why the magnitude matters
Many school diagrams and interpretations compare the absolute size of the coefficient. For example, a calculated PED of −2 has magnitude 2 and indicates elastic demand, while −0.4 has magnitude 0.4 and indicates inelastic demand.
State the sign convention if there could be ambiguity. A student who simply chooses −0.4 as ‘larger than −2’ in ordinary arithmetic may misclassify responsiveness. Economics is asking for relative percentage sensitivity.
5. Elastic demand: quantity responds proportionately more
Demand is price elastic when the absolute PED exceeds 1. The percentage change in quantity demanded is larger than the percentage change in price, for the specified comparison.
If a small price reduction leads to a relatively large increase in quantity demanded along the same demand curve, total revenue often rises. The conclusion is conditional on the demand response and other factors staying comparable.
6. Inelastic demand: quantity responds proportionately less
Demand is price inelastic when the absolute PED is below 1 but greater than zero. Buyers respond, yet quantity demanded changes by a smaller percentage than the price.
With inelastic demand, a price increase tends to increase total revenue for a sufficiently small change on the same demand relationship, because the fall in volume is proportionately smaller.
7. Unitary elastic demand: equal percentage responsiveness
A magnitude of 1 means the percentage change in quantity demanded matches the percentage change in price under the chosen calculation method. In the familiar marginal economic interpretation, total expenditure or revenue is unchanged by a sufficiently small price change.
For finite before-and-after data, exact multiplication can yield a small difference even when the initial-value percentage ratio equals one. Calculate the actual price × quantity revenue rather than promising perfect equality from a rounded coefficient.
8. Perfectly inelastic demand
In the limiting model, quantity demanded does not change when price changes, so PED is zero and a diagram shows a vertical demand curve. Real consumers often have alternatives or limits, so perfect inelasticity is an idealised case.
A necessity need not be perfectly inelastic. Calling an item ‘essential’ gives a reason for low sensitivity under some conditions, but not proof that buyers never respond to price.
9. Perfectly elastic demand
A perfectly elastic demand curve is horizontal in the idealised model: buyers would demand at the given price but are extremely sensitive to any upward change in that individual firm’s price. The magnitude of elasticity is treated as infinite.
Do not infer perfect elasticity merely because one price cut sold more units. It is a limiting curve condition with strong assumptions, often used to think about price-taking behaviour.
10. Demand-curve slope and elasticity are not the same number
Slope can describe the change in price or quantity per unit change of the other variable, while elasticity compares percentage changes. The same straight-line demand curve may exhibit different PED magnitudes at different points.
Therefore two diagrams with similar visual steepness cannot always be assigned elasticity values solely by their appearance when axes and reference quantities differ. Use the intended graph and formula carefully.
11. Read the starting values before changing them
Suppose price falls from $5 to $4.50 and quantity rises from 200 to 260. Using initial values, percentage price change is (4.50−5)/5 = −10%, and quantity change is (260−200)/200 = +30%. PED = +30% / −10% = −3.
The demand is elastic, magnitude 3. The calculation must state the base of the percentage changes; otherwise two students might compute different figures using different denominator conventions.
12. Initial-value versus midpoint percentage changes
Different introductory contexts may use initial-value percentages or a midpoint/arc convention for comparisons between two points. Those methods need not produce identical coefficients for finite changes. Follow the formula convention specified in the K343 examination materials and question.
Never change denominator method halfway through a calculation. All worked figures in this chapter use the stated initial-value convention unless explicitly described otherwise.
13. Revenue is simply price multiplied by quantity sold
Total revenue to the firm in the simple product example is P×Q. For the café case, revenue before the price cut is $5×200=$1,000 and after the cut is $4.50×260=$1,170. Revenue has increased by $170.
PED explains the responsiveness, but the revenue arithmetic is the direct check. The result does not tell us the shop’s net profit because ingredient, labour and other expenses are not included.
14. Consumer expenditure and seller revenue are two views of a transaction
In a simple market example with the same relevant quantity, buyers’ total spending on the product equals sellers’ total revenue from that product. Both are price times quantity, even though they are recorded from different viewpoints.
The amount spent by consumers is not a measure of their wellbeing or the producer’s profit. Economists distinguish spending, revenue, costs, consumer benefit and social effects.
15. A price increase under inelastic demand
A fictional product goes from $10 to $11, a 10% increase. Quantity demanded falls from 100 to 96, a 4% reduction. PED using the initial reference is −0.4, so demand is inelastic.
Revenue rises from $1,000 to $1,056. The percentage changes explain why the direction is plausible, while direct multiplication establishes the exact dollar outcome for this finite case.
16. A price increase under elastic demand
Suppose price rises from $5 to $6, a 20% increase, but quantity falls from 40 to 30, a 25% reduction. PED is −25%/+20%=−1.25, so demand is elastic.
Revenue falls from $200 to $180. A higher price per item is not enough to guarantee higher revenue when enough customers buy less.
17. What happens near unit elasticity?
Let a fictional item decrease from $20 to $19 while purchases increase from 100 to 105. The initial-value changes are −5% and +5%, giving PED −1. Revenue goes from $2,000 to $1,995.
This is a useful caution: the finite-change arithmetic differs slightly even though the computed initial-reference elasticity has magnitude 1. On the ideal marginal interpretation, unit elasticity corresponds to locally unchanged total revenue.
18. Many determinants shape responsiveness
Availability of close substitutes is important: if several nearly identical products are easily accessible, a price increase may prompt switching. If a product has no close substitute, demand is more likely to be inelastic, other things equal.
Do not give every product in a whole category one fixed elasticity. Actual choices depend on consumer preferences, brand loyalty, available information and circumstances.
19. Necessity and luxury are helpful but not absolute labels
Some necessities are relatively inelastic because households still require them, while discretionary purchases can be postponed or avoided. However, income level, substitution possibilities and the definition of the specific product all matter.
For example, ‘food’ as a broad category differs from demand for one expensive dessert sold by one shop. Define the market before predicting responsiveness.
20. Proportion of income influences choices
A price increase for a product that accounts for a large share of a consumer’s budget may provoke substantial adjustment. A small change in the price of an inexpensive everyday item might prompt less immediate response.
This is a tendency, not a guarantee. Consumers differ in income, needs and what alternatives they can access.
21. Time available for adjustment matters
Demand may be less elastic immediately after a price change because people need time to change suppliers, habits, travel routes or equipment. Over longer periods, the ability to substitute or reorganise spending can increase responsiveness.
A revenue prediction using only an immediate response may not hold over months or years. State the timeframe whenever it materially changes the interpretation.
22. Habit and brand loyalty can alter sensitivity
If a consumer strongly prefers one product or follows a habit, they may respond less to modest price changes. But loyalty is not immunity from budgets or competition; larger changes or better alternatives may eventually influence decisions.
A business should not assume that customers will accept any increase merely because they returned in the past.
23. Do not confuse movement along demand with a shift
A change in the product’s own price ordinarily produces movement along a given demand curve, while changes in income, tastes, substitute prices and other conditions can shift the entire demand curve. PED is defined using own-price responsiveness, holding other factors constant.
If a firm cuts its price during a viral marketing campaign and sales jump, the observed quantity change includes more than price response. A PED computed naively from those two points cannot isolate price causation without appropriate controls.
24. Complements and substitutes require careful wording
A substitute can replace another product for some consumers, while a complement is consumed together with another good. A change in the price of a different good may shift the demand for the product being studied.
That is a different relationship from the own-price elasticity of demand. Keep cross-price effects separate unless the question explicitly asks for them.
25. Consumer welfare is not identical to expenditure
A household may spend less after a price increase because it buys far fewer units. That does not automatically mean its welfare improved. Conversely, higher expenditure may reflect an improvement in availability or a worsening of affordability.
Economic evaluation asks what happened to choice, needs and outcomes, not only which total is higher.
26. Price changes affect workers as well as owners
A firm facing a sharp fall in revenue might adjust staffing, hours or investment. A firm with inelastic demand may have more pricing room in the short term, but higher costs and competitive changes can still limit profitability.
Do not present a single PED number as a reason a firm must raise prices or cut jobs. Decisions have trade-offs affecting consumers, workers and long-term trust.
27. Government tax decisions may depend on responsiveness
Where demand is relatively inelastic, a tax-related price increase can reduce quantity proportionately less, potentially supporting revenue objectives. But the distribution of the tax burden, effects on low-income households and wider policy goals matter.
A complete evaluation needs more than the sentence ‘inelastic goods are best to tax’. Consider equity, health, substitution, compliance and possible unintended consequences.
28. What elasticity cannot establish by itself
PED does not directly tell you profit, product quality, the firm’s cost structure or whether raising prices is ethically or commercially wise. It measures a particular aspect of buyer responsiveness.
Before advising a price change, seek evidence about production costs, substitutes, competitive reaction and whether other demand determinants changed.
29. A revenue estimate needs its period
Comparing daily sales before a promotion with monthly sales after it can produce a meaningless quantity-change percentage. Both price and quantity values should refer to compatible market definitions and time periods.
Record whether the figures are per day, per week or per year, and whether the quantity is demanded versus actually supplied and sold.
30. Twelve-stage price-elasticity decision route
- 1 — Identify the specific product and the relevant market.
- 2 — State that the question concerns own-price demand response rather than a shift caused by another factor.
- 3 — Record original price and original quantity demanded for one comparable period.
- 4 — Record the new price and new quantity, keeping the same units.
- 5 — Calculate the percentage change in price with the chosen reference convention.
- 6 — Calculate the percentage change in quantity demanded using the same convention.
- 7 — Divide quantity percentage change by price percentage change.
- 8 — State the signed result and classify demand by its absolute magnitude.
- 9 — Calculate initial and final revenue as P×Q to verify any proposed direction.
- 10 — Explain plausible determinants of responsiveness.
- 11 — Identify assumptions, confounders and the effects on consumers, firms, workers or government.
- 12 — Give a qualified decision or conclusion and explain which additional information could change it.
31. Original fictional case: the Meridian Neighbourhood Market
Meridian Market is an invented collection of shops with fabricated prices and demand records. It is not a survey of Singapore customers and provides no actual pricing recommendation. Every percentage below uses the original value as its reference unless stated otherwise.
A — An elastic café promotion
Fictional market record: A drink falls from $5 to $4.50, while weekly quantity rises from 200 to 260.
Calculation: Price −10%, quantity +30%, PED −3; revenue $1,000 to $1,170.
Interpretation: The change is elastic and the observed total revenue rises in the simple unchanged-demand model.
Try a change: What evidence would identify whether a marketing campaign also influenced sales?
B — An inelastic staple
Fictional market record: An item rises from $10 to $11, while quantity falls from 100 to 96.
Calculation: Price +10%, quantity −4%, PED −0.4; revenue $1,000 to $1,056.
Interpretation: Revenue rises, though that says nothing by itself about profit or affordability.
Try a change: What would happen if the firm also faced higher supplier costs?
C — A price rise on an elastic product
Fictional market record: An optional item rises from $5 to $6, while quantity falls from 40 to 30.
Calculation: Price +20%, quantity −25%, PED −1.25; revenue $200 to $180.
Interpretation: The loss of customers more than offsets the higher price per unit.
Try a change: Would brand loyalty make the same response certain at another shop?
D — Near unit elasticity
Fictional market record: Price changes from $20 to $19 and quantity from 100 to 105.
Calculation: Initial-value PED −1; revenue moves from $2,000 to $1,995.
Interpretation: For a finite change, calculate the exact revenue rather than claiming automatic equality.
Try a change: How would a small-change approximation differ from the exact calculation?
E — A mild elastic response
Fictional market record: Price falls from $8 to $7.60, a reduction of 5%, and quantity rises from 150 to 162, an increase of 8%.
Calculation: PED = +8%/−5%=−1.6; revenue $1,200 to $1,231.20.
Interpretation: Demand is elastic; total revenue increases under the stated record.
Try a change: How would costs affect a profit decision?
F — A mildly inelastic response
Fictional market record: Price rises from $8 to $8.40, a 5% increase, while quantity drops from 150 to 144, a 4% reduction.
Calculation: PED −0.8; revenue moves from $1,200 to $1,209.60.
Interpretation: The revenue increase is smaller than a simple 5% multiplication because volume is lower.
Try a change: Why should the result not be rounded before multiplying?
G — No quantity response
Fictional market record: A fictional vertical demand schedule shows buyers demanding 100 units at each price in a modelled range.
Calculation: Quantity percentage change is zero, so PED is zero.
Interpretation: This is perfectly inelastic in the ideal curve model.
Try a change: Would an actual product necessarily have this property for any price?
H — Perfectly elastic limiting case
Fictional market record: A fictional horizontal demand curve is shown at a stated market price.
Calculation: The model represents effectively infinite responsiveness to the individual firm’s price changes.
Interpretation: The diagram is a limiting theoretical case, not a finite two-point sales calculation.
Try a change: Why is a single promotion insufficient to prove perfect elasticity?
I — Price falls but advertising changes too
Fictional market record: A café changes price and launches a major publicity campaign in the same week.
Calculation: Simple price and quantity differences do not isolate own-price elasticity when tastes or awareness shift.
Interpretation: The demand curve itself may have changed.
Try a change: What controlled information would help separate the two effects?
J — A tax proposal on a necessity
Fictional market record: A fictional government considers a levy on a good thought to have inelastic demand.
Calculation: PED can help estimate the quantity response, but a responsible appraisal also considers equity and other market effects.
Interpretation: The figure alone cannot prove the policy is fair or effective.
Try a change: How might low-income households experience the tax differently?
K — An owner confuses revenue with profit
Fictional market record: A business’s revenue rises by $170 after a discount, while extra staffing and supplies cost $230.
Calculation: The operating impact cannot be inferred from revenue alone; an extra $230 cost would exceed the extra revenue before considering further factors.
Interpretation: Higher sales revenue need not mean higher profit.
Try a change: Which real cost information should the owner examine?
L — The shifted-curve mistake
Fictional market record: Prices and quantities differ between years, but the product’s income profile and competing offers also changed.
Calculation: The observed difference may reflect shifts in demand, not simply movement along one demand curve.
Interpretation: An elasticity estimate requires the ceteris paribus assumption or suitable methods to isolate own-price effect.
Try a change: What should an examiner-facing answer say about this limitation?
32. Draw curves with labels rather than decorative arrows
A demand diagram should identify price on the vertical axis and quantity demanded on the horizontal axis. Show the intended curve and label the price–quantity points correctly. A diagram of elastic demand can help communicate a proportionately larger quantity response than price response within the model.
But axis scales and starting values matter; visual steepness alone is not a universal coefficient. Use the diagram as an interpretation tool, not a replacement for the percentage formula.
33. A good explanation has cause and consequence
For a price rise under inelastic demand, say that quantity demanded falls by a smaller percentage than price rises, so total revenue tends to increase, other things equal. For an elastic response, quantity falls proportionately more, so revenue tends to decrease.
Merely writing ‘it is inelastic so revenue goes up’ may be too compressed when the question asks for economic analysis. Explain the percentage mechanism.
34. Evaluate with both sides of the argument
A price increase can raise revenue for a firm facing inelastic demand, but it may worsen affordability, provoke longer-term substitution or encourage competitors. A price reduction can attract customers in elastic markets but squeeze margins if unit costs are high.
The K343 syllabus evaluates reasoning and recognises uncertain outcomes. A balanced discussion should mention conditions and explain which side carries greater weight in the stated context.
35. A parent can check the calculation in two steps
First ask whether the learner converted both changes into percentages based on a consistent starting point. Second ask for the two P×Q totals, which reveal whether the claimed revenue effect fits the actual numbers.
A student who gets the elasticity right but the revenue direction wrong has probably memorised a classification without connecting it to multiplication.
36. The Fencing Method for price response
Begin with one price cut and a clear quantity increase. Then compare an elastic and an inelastic case. After classification becomes secure, introduce revenue, imperfect assumptions, shifts and policy effects. This keeps one conceptual boundary stable at a time.
The deeper skill is not just recognising a value above or below one. It is knowing what changed, why it changed and what can reasonably be inferred from the evidence.
37. Common mistakes and repairs
- Using absolute quantity changes rather than percentage changes — compare proportional responsiveness.
- Calling negative PED ‘less elastic’ merely because −2 is a smaller signed number — compare magnitude.
- Mixing initial-value and midpoint reference conventions — state one consistent formula.
- Treating own-price movement and whole-demand shifts as identical — identify the causal variable.
- Assuming revenue always rises with a higher price — calculate P×Q.
- Equating revenue with profit — include costs separately.
- Claiming unit elasticity guarantees exact equality for every finite two-point record — recompute totals.
- Reading a curve’s slope as its PED without reference values — use the percentage formula.
- Calling an essential good perfectly inelastic by definition — distinguish typical tendencies from limiting cases.
- Advising a tax purely from an inelastic coefficient — consider equity and broader effects.
38. Depth, Load and Transfer assessment
Depth: can the student explain why PED is unitless, how it differs from slope and why other determinants must be held constant? Load: can they work with price and quantity percentages, signed ratios, revenue calculations and an evaluation prompt without losing the causal route? Transfer: can they assess a fictional policy or pricing question with incomplete information and describe what would need to be measured?
If the pupil can calculate PED but cannot explain the revenue effect, teach P×Q explicitly. If they can give the correct sign but confuse demand shifts, revisit ceteris paribus.
39. Ten-question independent checkpoint
- 1. Define PED in terms of percentage changes.
- 2. If price drops 10% and quantity increases 30%, calculate PED.
- 3. Classify an absolute PED of 0.4.
- 4. Classify an absolute PED of 1.6.
- 5. What is the PED magnitude for perfectly inelastic demand?
- 6. What does a horizontal demand curve represent in the limiting model?
- 7. Calculate revenue before: 200 units at $5; after: 260 units at $4.50.
- 8. Why is a price change accompanied by an advertising campaign difficult to interpret?
- 9. Name two determinants of PED.
- 10. Explain why a revenue increase does not guarantee a profit increase.
Answers: (1) percentage quantity demanded change divided by percentage own-price change. (2) −3 using the signed convention; magnitude 3. (3) inelastic. (4) elastic. (5) zero. (6) perfectly elastic demand. (7) $1,000 before, $1,170 after. (8) demand may have shifted for reasons other than own price. (9) available substitutes, necessity, budget proportion or time horizon. (10) costs and other expenses may rise.
40. Questions families often ask
Why is PED usually negative?
On an ordinary downward-sloping demand curve, a higher price tends to reduce quantity demanded, so the two changes have opposite signs.
Why does a value of 2 count as more elastic than 0.5?
Quantity is twice as responsive proportionately to price in the first case, versus only half as responsive in the second.
Is demand for necessities always perfectly inelastic?
No. Necessities can have inelastic demand under some conditions without showing literally zero response to every price change.
Can a price cut raise revenue but lower profit?
Yes. Additional unit costs, discounts and other expenses can exceed the gain in revenue.
Why do economists draw diagrams as well as calculate?
Diagrams reveal relationships and comparative responses, while calculations measure the specific percentage changes.
Can a business use one elasticity number for every customer and year?
Not reliably. Responsiveness varies with market, substitutes, income, time horizon and the actual price range.
Is PED the same as price elasticity of supply?
No. PED concerns buyers’ quantity demanded; PES concerns producers’ quantity supplied.
41. Official syllabus, source integrity and next reading
The 2027 SEAB SEC G3 Economics K343 syllabus, Section 2.6 Price Elasticity of Demand specifies definitions, calculations, classification, diagrams, determinants, expenditure and revenue and implications for economic decisions. The official G3 2027 school-candidate directory identifies the examination-year source.
Previous: Reference 290 — Quadratic Inequalities. For introductory price and market foundations read Reference 68 — Markets, Prices and Supply–Demand Literacy; for everyday bill calculations compare Reference 237 — Financial Mathematics and Household Bills. Continue via the MOE V3.0 Reference Library and the immutable Clementi first-principles teaching reference.
Editorial review: 23 October 2026, Singapore. All Meridian Market observations are fictional. The lasting skill is to identify the price–quantity relationship, calculate responsiveness consistently, verify exact revenue and give a conclusion proportional to the evidence rather than the confidence of a slogan.