When all inputs can vary, how does long-run average cost behave, and what are economies and diseconomies of scale?
Topic 3.3 Long-Run Production Costs: explain the long-run average total cost curve as an envelope of short-run curves, and identify economies of scale, diseconomies of scale, and constant returns to scale.
A focused answer to AP Microeconomics Topic 3.3, covering the long run when all inputs are variable, the long-run average total cost curve as an envelope of short-run curves, economies and diseconomies of scale, constant returns to scale, and minimum efficient scale, with worked exam-style questions.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this topic is asking
Topic 3.3 moves from the short run to the long run, where the firm can vary every input, including the size of its plant. The College Board wants you to explain the long-run average total cost (LRATC) curve as an envelope of short-run curves, and to identify the three regions: economies of scale, diseconomies of scale, and constant returns to scale, along with the idea of minimum efficient scale.
The long run and the envelope curve
Each short-run average total cost curve corresponds to one fixed plant size. As the firm plans for the long run, it can pick whichever plant gives the lowest average cost for the output it wants. The LRATC curve is therefore the envelope that wraps under all the short-run curves, touching each one at the output where that plant is the cheapest choice. This is why the long run is sometimes called the "planning horizon."
Economies, diseconomies, and constant returns to scale
The U-shape of LRATC therefore tells a story of scale: small firms benefit from growing (economies), there is often a flat range of efficient sizes (constant returns), and beyond some point getting bigger raises average cost (diseconomies). Note that economies and diseconomies of scale are a long-run phenomenon about changing all inputs, which is different from the short-run diminishing marginal returns of Topic 3.1, where one input varies against a fixed input. Keeping these two apart is a common exam discriminator.
Minimum efficient scale
When minimum efficient scale is very large compared with the size of the market, average cost keeps falling over the whole relevant range, a condition that produces a natural monopoly (Unit 4), because one large firm can serve the market more cheaply than several smaller ones.
Try this
Q1. State why there are no fixed costs in the long run. [1 point]
- Cue. All inputs, including plant and capital, are variable in the long run, so nothing is fixed and there are no fixed costs.
Q2. A firm's LRATC rises as it expands output. State which scale condition this is and give one cause. [2 points]
- Cue. Diseconomies of scale; a cause is coordination or management difficulties in a very large organization.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2019 (style)1 marksMultiple choice. When a firm's long-run average total cost falls as output increases, the firm is experiencing (A) diseconomies of scale. (B) economies of scale. (C) constant returns to scale. (D) diminishing marginal returns. (E) a fixed cost increase.Show worked answer →
The answer is (B). Economies of scale occur when long-run average total cost falls as output (and plant size) increases, often due to specialization and bulk efficiencies.
(A) is the opposite (LRATC rising). (C) is flat LRATC. (D) is a short-run idea about a variable input with a fixed input, not the long run. (E) is unrelated; in the long run there are no fixed costs.
AP 2021 (style)3 marksFree response (short). (a) Explain why there are no fixed costs in the long run. (b) Define minimum efficient scale. (c) Explain one cause of diseconomies of scale.Show worked answer →
A three-point short FRQ.
(a) (1 point): in the long run all inputs (including plant and capital) are variable, so the firm can change every input; with nothing fixed, there are no fixed costs.
(b) (1 point): minimum efficient scale is the lowest output at which long-run average total cost is minimized (where economies of scale are exhausted).
(c) (1 point): a cause of diseconomies of scale, for example, coordination and communication problems in a very large firm, which raise long-run average total cost as output grows.
Related dot points
- Topic 3.2 Short-Run Production Costs: define fixed, variable, total, marginal, and average costs, calculate each from data, and explain the shapes of the short-run cost curves and how marginal cost relates to the averages.
A focused answer to AP Microeconomics Topic 3.2, covering fixed, variable, and total cost, average fixed, average variable, average total, and marginal cost, how to calculate each, and the shapes and relationships of the short-run cost curves, with worked exam-style questions.
- Topic 3.1 The Production Function: define total, marginal, and average product, explain the law of diminishing marginal returns, and relate the product curves to one another.
A focused answer to AP Microeconomics Topic 3.1, covering the production function, total product, marginal product and average product, the law of diminishing marginal returns, and how the product curves relate to one another, with worked exam-style questions.
- Topic 3.7 Perfect Competition: describe the characteristics of perfect competition, draw the short-run profit, loss, and break-even cases, explain the long-run zero-profit equilibrium, and show why perfect competition is efficient.
A focused answer to AP Microeconomics Topic 3.7, covering the characteristics of perfect competition, the price-taking firm's demand curve, short-run profit, loss, and break-even, the long-run zero-economic-profit equilibrium, and the allocative and productive efficiency of perfect competition, with worked exam-style questions.
- Topic 3.5 Profit Maximization: explain the marginal revenue equals marginal cost rule, apply it to find the profit-maximizing output, and use the average total cost curve to measure profit or loss.
A focused answer to AP Microeconomics Topic 3.5, covering the profit-maximizing rule that marginal revenue equals marginal cost, how to find the optimal output, and how to measure total profit or loss using price and average total cost, with worked exam-style questions.
- Topic 2.4 Price Elasticity of Supply: calculate price elasticity of supply using the midpoint formula, classify supply as elastic, inelastic, or unit elastic, and explain why time is the key determinant.
A focused answer to AP Microeconomics Topic 2.4, covering the price elasticity of supply, the midpoint formula, elastic versus inelastic versus unit elastic supply, the perfectly elastic and inelastic extremes, and why time is the chief determinant, with worked exam-style questions.
Sources & how we know this
- AP Microeconomics Course and Exam Description — College Board (2023)