Price elasticity of supply: how strongly producers react to price changes
Price elasticity of supply, usually written as PES, measures how responsive quantity supplied is when price changes. In simple words, PES asks: if the price rises, can producers increase output by a lot, a little, or not at all?
Key idea: Supply is elastic when producers can quickly increase or decrease output. Supply is inelastic when producers find it difficult to change output, even if price changes.
1. How to calculate PES
The formula compares the percentage change in quantity supplied with the percentage change in price:
PES = percentage change in quantity supplied ÷ percentage change in price
PES is usually positive because price and quantity supplied normally move in the same direction. If price rises, producers usually want to supply more. If price falls, producers usually supply less.
In the example above, price rises by 10% and quantity supplied rises by 5%. Therefore, PES = 5 ÷ 10 = 0.5. This means supply is price inelastic because quantity supplied changes by a smaller percentage than price.
2. Price elastic supply
Supply is price elastic when a change in price causes a larger percentage change in quantity supplied. This usually happens when firms have spare capacity, available workers, enough raw materials and the ability to produce more quickly.
Easy rule: A flatter supply curve usually means supply is more elastic. Producers can change quantity supplied by a lot when price changes.
3. Price inelastic supply
Supply is price inelastic when a change in price causes a smaller percentage change in quantity supplied. This happens when it is difficult or slow to increase production. For example, fresh fruit and vegetables take time to grow, so farmers cannot instantly supply much more even if price rises.
Easy rule: A steeper supply curve usually means supply is more inelastic. Price changes, but quantity supplied changes only a little.
4. Special cases of PES
Some supply curves show extreme cases. These are useful because they make the meaning of elasticity easier to remember.
Perfectly price inelastic supply
Supply is perfectly price inelastic when quantity supplied does not change at all, even if price changes. The PES value is 0. This can happen when the total quantity is fixed, such as seats in a concert hall or land in a particular location.
Perfectly price elastic supply
Supply is perfectly price elastic when producers are willing to supply any quantity at one particular price, but not below it. This is shown by a horizontal supply curve. It is rare in real life, but it helps students understand the idea of an unlimited response at one price.
Unitary price elastic supply
Supply has unitary price elasticity when the percentage change in quantity supplied is exactly equal to the percentage change in price. The PES value is 1. For example, if price rises by 10% and quantity supplied also rises by 10%, PES is 1.
5. What affects price elasticity of supply?
| Factor | Effect on PES | Simple explanation |
|---|---|---|
| Spare capacity | More spare capacity = more elastic supply | Firms can produce more without building new factories or buying major new equipment. |
| Stocks/inventories | More stocks = more elastic supply | If firms already have goods stored, they can supply more quickly when price rises. |
| Time period | Long run = more elastic supply | Over time, firms can hire workers, buy machines and expand production. |
| Availability of resources | Easy access to resources = more elastic supply | If labour and raw materials are easy to obtain, output can rise quickly. |
| Nature of the product | Perishable or slow-to-produce goods = more inelastic supply | Fresh crops, handmade goods and specialist products cannot be increased instantly. |
6. PES categories to remember
| PES value | Type of supply | Meaning |
|---|---|---|
| PES > 1 | Elastic supply | Quantity supplied changes by a larger percentage than price. |
| PES < 1 | Inelastic supply | Quantity supplied changes by a smaller percentage than price. |
| PES = 1 | Unitary elastic supply | Quantity supplied and price change by the same percentage. |
| PES = 0 | Perfectly inelastic supply | Quantity supplied does not change when price changes. |
| PES = ∞ | Perfectly elastic supply | Any quantity can be supplied at one price. |
7. Exam method for PES questions
- Write the formula: PES = % change in quantity supplied ÷ % change in price.
- Calculate the percentage changes: use the change divided by the original value, then multiply by 100.
- Divide carefully: put the percentage change in quantity supplied on top and the percentage change in price underneath.
- Classify the result: greater than 1 = elastic; less than 1 = inelastic; equal to 1 = unitary.
- Explain the reason: link your answer to spare capacity, stocks, time period, resources or the nature of the product.
Quick check
- If PES is 2.0, supply is elastic.
- If PES is 0.5, supply is inelastic.
- If PES is 1, supply is unitary elastic.
- If PES is 0, quantity supplied is fixed.
- Supply is usually more elastic in the long run than in the short run.
