How do I calculate price elasticity of demand and explain the result?

Price elasticity of demand measures how responsive quantity demanded is to a price change. Students often lose marks because they stop at the number. In an assignment, the interpretation matters: is demand elastic or inelastic, what does that imply for total revenue, and why might customers respond that way?

What the question is asking

The question is asking for calculation and economic interpretation. A marker expects the formula, substitution of figures, classification of elasticity, and a short explanation of the business implication. If the question says "comment" or "advise", the answer should go beyond arithmetic.

Key concepts to cover

• Percentage change in quantity demanded
• Percentage change in price
• Absolute value of elasticity
• Elastic demand: elasticity greater than 1
• Inelastic demand: elasticity less than 1
• Unit elastic demand: elasticity equal to 1
• Total revenue: price multiplied by quantity
• Determinants of elasticity, such as substitutes, necessity and time period

Suggested answer structure

1. 1Write the formula.
2. 2Calculate percentage change in quantity demanded.
3. 3Calculate percentage change in price.
4. 4Divide the quantity change by the price change.
5. 5Use the absolute value to classify elasticity.
6. 6Explain the total revenue effect.
7. 7Add a reason why demand may be elastic or inelastic in this market.

Mini worked example

Suppose a coffee shop raises the price of a drink from 4.00 to 4.40. Weekly quantity demanded falls from 500 cups to 430 cups.

The price increased by 10 percent:

• Price change: 0.40 / 4.00 = 10 percent

Quantity demanded fell by 14 percent:

• Quantity change: -70 / 500 = -14 percent

Price elasticity of demand is:

• -14 percent / 10 percent = -1.4

Economists often discuss the absolute value, so demand has elasticity of 1.4. Because this is greater than 1, demand is elastic. That means quantity demanded changed proportionately more than price. Total revenue would fall after the price rise because the loss of customers is larger than the gain from the higher price per cup.

Common mistakes

• Forgetting to convert changes into percentages
• Treating the negative sign as meaning "inelastic"
• Saying "elasticity is 1.4 percent" when elasticity itself is a ratio, not a percent
• Calculating the number but not explaining revenue
• Ignoring context, such as substitutes or whether the product is a necessity
• Using final quantity as the denominator when the module expects the midpoint method

How to make the answer stronger

If your module uses the midpoint method, use average price and average quantity as the base. That avoids different answers depending on whether price rises or falls. If the brief does not specify a method, state which one you are using.

For evaluation marks, explain why elasticity may differ in the short run and long run. Customers may keep buying immediately after a price rise, but over time they can find substitutes, change habits or switch supplier. That makes demand more elastic over a longer period.

Related questions

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• How do I interpret a regression coefficient when a variable is in logs?
• How do I analyse a tax or subsidy using demand and supply diagrams?
• How do I explain monopoly pricing compared with perfect competition?

Academic use note

This guide is for economics assignment support. Use it to understand the calculation and interpretation, then apply your own data and diagram requirements.

Sources and further reading

• OpenStax: Price elasticity of demand and supply
• OpenStax: Elasticity and pricing