Business and Technology · Microeconomic factors
Costs, Economies of Scale and Firm Size Explained
Updated 11 October 2026 · Fact-checked
Economies of scale are falling average (unit) costs as a firm increases its output in the long run. Diseconomies of scale are rising average costs when a firm grows too large. To answer questions, identify the cost type or the source of the saving, then say whether average cost falls or rises with size.
Understand Costs, Economies of Scale and Firm Size
Every business has costs. Fixed costs do not change with output in the short run, such as rent, insurance and management salaries. Variable costs change in line with output, such as raw materials and piece-rate wages. Total cost is fixed plus variable cost. Average cost is total cost divided by units produced.
When output rises, fixed costs are spread over more units. This is why average cost often falls at first. Many costs are also semi-variable, with a fixed part and a variable part. In economics, the short run is a period where at least one input, such as factory size, is fixed. The long run is a period where all inputs can be changed.
Economies of scale are the long-run cost advantages a firm gets as it grows, so average cost falls. Internal economies come from the firm's own growth. Examples are:
- Technical: larger, more efficient machinery and mass production.
- Purchasing (bulk buying): bigger discounts from suppliers.
- Financial: cheaper loans because large firms are seen as lower risk.
- Managerial: specialist managers and division of labour.
- Marketing: advertising costs spread over more units.
- Risk-bearing: diversifying products or markets.
External economies come from the growth of the whole industry or area, not one firm. Examples are a skilled labour pool, specialist suppliers nearby, better transport links and shared training colleges. Any firm in the area benefits, whatever its own size.
Diseconomies of scale appear when a firm becomes too big and average cost rises. Causes include poor communication, slow decisions, weak coordination, low staff morale and more bureaucracy. External diseconomies can occur too, such as traffic congestion or higher wages in a crowded industrial area.
Economies of scale help explain why firms grow. Other reasons are higher profit, more market power, spreading risk, reaching new markets and the objectives of managers or owners. Growth can be organic (internal expansion) or external (merger or takeover). Large firms can often undercut smaller rivals, which is why small firms may struggle or find a niche.
Key formulas to remember
- Total cost
- Total cost = Fixed costs + Variable costs
- Fixed costs stay the same in the short run; variable costs rise with output.
- Average (unit) cost
- Average cost = Total cost ÷ Output
- Economies of scale mean this falls as the firm grows; diseconomies mean it rises.
- Variable cost
- Total variable cost = Variable cost per unit × Output
- Variable cost per unit is assumed constant within the normal range of output.
- Scale rule
- Economies: average cost falls as scale rises. Diseconomies: average cost rises as scale rises.
- Applies to the long run, when all inputs can change.
How to solve Costs, Economies of Scale and Firm Size questions
Use this method for any question on costs, scale and firm size.
- 1Read the question and decide what it asks: a definition, a cost calculation, a type of economy or a reason for growth.
- 2If it is a cost question, classify each item as fixed or variable by asking whether it changes when output changes.
- 3Do any calculation: total cost = fixed + variable, then average cost = total cost ÷ units.
- 4If average cost falls as output grows, it points to economies of scale. If it rises, it points to diseconomies.
- 5For a scenario, find the source. Bulk discounts mean purchasing. New machinery means technical. Cheaper loans mean financial. Industry-wide benefits mean external.
- 6Check whether the benefit comes from the firm's own size (internal) or from the industry or location (external).
- 7Eliminate options that confuse short run with long run or fixed with variable, then choose the best answer.
Quickest way: Three-question shortcut
When to use it: Use this for multiple choice questions on cost types and scale in the exam.
- Ask: does the cost change when output changes? Yes means variable, no means fixed.
- Ask: is average cost falling or rising as the firm gets bigger? Falling means economies, rising means diseconomies.
- Ask: who gets the benefit? Only the growing firm means internal. Every firm in the industry or area means external.
Common mistakes in Costs, Economies of Scale and Firm Size
Saying fixed cost per unit stays fixed.
The word fixed makes students think everything about it is constant.
Fix: Total fixed cost is constant in the short run, but fixed cost per unit falls as output rises.
Treating economies of scale as a short-run effect.
Students mix them up with spreading fixed costs over more units.
Fix: Economies of scale are long-run, when the firm changes the scale of all its inputs. Spreading fixed costs within one plant is a short-run effect.
Classing external economies as internal.
Both lower costs and both come with growth.
Fix: If the saving comes from the industry or location growing, such as a local skilled workforce, it is external. If the firm's own size causes it, it is internal.
Assuming bigger always means cheaper.
Students remember only the benefits of growth.
Fix: Past a certain size, diseconomies such as poor communication and slow decisions raise average cost.
Confusing diseconomies of scale with a fall in demand or a rise in input prices.
Any cost increase looks the same at first glance.
Fix: Diseconomies are rising average cost caused by the firm's own larger size and management problems, not by outside price changes.
Worked examples
Example 1
A company has fixed costs of $120,000 a year and variable costs of $5 per unit. Calculate the average cost per unit at output of 20,000 units and 40,000 units. What does the change show?
Show the solution
- At 20,000 units: variable cost = 20,000 × $5 = $100,000.
- Total cost = $120,000 + $100,000 = $220,000.
- Average cost = $220,000 ÷ 20,000 = $11.
- At 40,000 units: variable cost = 40,000 × $5 = $200,000.
- Total cost = $120,000 + $200,000 = $320,000.
- Average cost = $320,000 ÷ 40,000 = $8.
- Average cost falls from $11 to $8 because the fixed costs are spread over more units.
Answer: $11 per unit at 20,000 units and $8 per unit at 40,000 units. Average cost falls because fixed costs are spread over more output.
Example 2
A fast-growing clothing maker negotiates bigger discounts from its fabric suppliers. Separately, its factory district gains a new specialist textile college that trains workers for all local firms. Identify the type of economy of scale in each case.
Show the solution
- Bigger discounts are due to the firm's own larger orders. This is an internal economy, specifically a purchasing (bulk buying) economy.
- The textile college benefits all firms in the district, not just this one. This is an external economy.
- The firm's own growth did not create the college, which confirms the benefit is external.
Answer: Supplier discounts are an internal purchasing economy. The new college is an external economy.
Exam tips
- Read the wording carefully: questions often hinge on whether the benefit is internal or external.
- Classify costs by behaviour first. Check if a cost changes with output before deciding.
- When a number entry question asks for average cost, divide total cost, not variable cost alone, by units.
- Remember that economies of scale are long run, and watch for answers that mix in short-run ideas.
- Link firm size to motives for growth, such as market power, risk spreading and lower unit costs.
Practice questions from Microeconomic factors
- Which of the following would cause a movement along a product's demand curve, rather than a shift of the curve?
- Which of the following is the defining feature of a monopoly market structure?
- A government imposes a per-unit tax on a product with highly inelastic demand and fairly elastic supply. Which statement best describes the …
- A bakery rents its premises for a fixed monthly amount and pays its bakers by the hour for hours actually worked. In the short run, which of…
- A government sets a maximum price for rental housing below the free-market equilibrium rent. What is the most likely immediate result?
Costs, Economies of Scale and Firm Size in other exams
The same ground in other exams, if you are preparing for more than one or want another angle on it.
Costs, Economies of Scale and Firm Size: frequently asked questions
What is the difference between internal and external economies of scale?
Internal economies come from a single firm growing larger, such as bulk buying or better machinery. External economies come from the whole industry or area growing, such as a local skilled workforce. Any firm in that industry or area can benefit from external economies.
What is the difference between fixed and variable costs?
Fixed costs do not change with output in the short run, for example rent. Variable costs change with output, for example raw materials. Total cost is the sum of both.
What are examples of diseconomies of scale?
Examples are slow communication between many layers of management, poor coordination across sites, and low motivation as workers feel distant from decisions. These push average cost up as the firm grows.
Why do firms grow in size?
Firms grow to gain economies of scale, increase profit, gain market power, spread risk and enter new markets. Managers may also want the status and rewards that come with running a larger business.