Foundationsmicroeconomics
Short Run vs Long Run in Production
Overview
The short run and long run are not calendar periods but descriptions of input flexibility: in the short run at least one input is fixed, while in the long run all inputs are variable. This lesson explains the distinction, introduces quasi-fixed costs and the planning horizon, and shows how the long-run average cost curve envelopes the short-run curves.
Learning objectives
- Define the short run and long run in terms of input flexibility.
- Explain why the distinction varies across firms and industries.
- Describe quasi-fixed inputs and the planning horizon.
- Interpret the long-run average cost curve as an envelope of short-run curves.
Flexibility, Not the Calendar
The short run is a period in which at least one input is fixed; the long run is a period in which all inputs are variable. A lemonade stand's short run might be an afternoon (the cart is fixed), while a carmaker's short run spans years (the factory is fixed). The distinction is about which decisions are still reversible, not about days versus years.
Fixed Inputs Anchor the Short Run
Typical fixed inputs are plant size, heavy machinery, and long-term leases - the commitments a firm cannot quickly undo. Variable inputs like labor hours, raw materials, and energy adjust freely. Because the fixed input constrains production, the short run is where diminishing marginal returns operates: variable inputs pile onto fixed capacity.
A fixed input is fixed only within the decision horizon being analyzed. The factory that is fixed this quarter becomes variable when the firm plans next year's capacity - fixity is relative to the question asked.
Quasi-Fixed Inputs and the Planning Horizon
Some inputs are quasi-fixed: available in lumps that can be adjusted only at a cost, like hiring a supervisor or opening a second shift. The planning horizon is the long-run perspective in which the firm chooses its fixed inputs before the short run begins. Good decisions are made long-run (choose the right plant) and executed short-run (operate it efficiently).
Short-Run Costs vs Long-Run Costs
In the short run, total cost splits into fixed and variable components, giving the U-shaped AVC and ATC curves. In the long run there are no fixed costs - every cost is variable because every input can change. Long-run cost analysis therefore asks a different question: for each output level, what is the cheapest possible plant size and input mix?
The Envelope Relationship
Draw short-run average cost curves for several plant sizes: each is U-shaped, and each is tangent to the long-run average cost (LRAC) curve at the output where that plant is optimal. The LRAC is the lower envelope of all possible short-run curves - the firm can always do at least as well by choosing the right plant, and usually better. This is why LRAC is flatter and U-shaped over a wider range.
At any output, long-run cost cannot exceed short-run cost: the long-run planner can always mimic the short-run input mix, and will improve on it whenever a different plant fits better. Flexibility has nonnegative value.
Using the Distinction in Analysis
Apply the right horizon to the question: a sudden demand spike is a short-run problem (overtime, extra shifts); a permanent demand shift is a long-run problem (build or close plants). Shutdown decisions are short-run; exit decisions are long-run. Keeping the horizons straight prevents the most common cost-curve mistakes.
Key points
- Short run: at least one input fixed. Long run: all inputs variable.
- The distinction is about flexibility, not calendar time, and varies by firm.
- Quasi-fixed inputs adjust only in lumps or at a cost.
- Long-run costs have no fixed component; every input is a choice variable.
- LRAC is the lower envelope of short-run average cost curves.
- Flexibility means long-run cost is never higher than short-run cost at any output.
Common misconceptions
✓ Truth: No calendar cutoff exists. The short run lasts as long as some input remains fixed - an afternoon for a food cart, several years for a semiconductor fab. It is defined by fixity, not by the clock.
✓ Truth: All inputs are variable in the long run, so there are no fixed costs - but variable costs remain. 'No fixed costs' is not 'no costs'; the firm still pays for every input it uses.
✓ Truth: LRAC is an envelope of many short-run curves, each tangent at the output where its plant size is optimal. It embodies plant-size choice, which no single short-run curve contains.
Practice questions
Q1. A bakery cannot change its oven capacity this month but can vary bakers and flour. This month is best described as the:
- long run.
- short run.
- planning horizon.
- shutdown period.
At least one input (ovens) is fixed, which defines the short run regardless of calendar length. Next year, when ovens can change, the bakery faces the long run.
Q2. Which statement about long-run cost is correct?
- Long-run total cost includes fixed costs.
- All inputs are variable, so there are no fixed costs in the long run.
- Long-run average cost is always above short-run average cost.
- The long run has no marginal cost.
With every input variable, the fixed/variable split disappears - all costs are variable in the long run. LRAC lies nowhere above the short-run curves; it is their lower envelope.
Q3. The long-run average cost curve is best described as:
- a single U-shaped short-run curve drawn larger.
- the lower envelope of all possible short-run average cost curves.
- always upward sloping.
- identical to the marginal cost curve.
Each plant size generates one short-run ATC curve; LRAC traces the minimum cost across all plant choices, touching each short-run curve where that plant is optimal - the envelope relationship.
Q4. A sudden, temporary surge in demand is best analyzed as a:
- long-run plant-size decision.
- short-run problem of varying variable inputs against fixed capacity.
- reason to exit the industry.
- change in technology.
Temporary spikes must be met with the current plant: overtime, extra shifts, more materials. Permanent demand changes instead trigger long-run capacity decisions.
Click a question to reveal the answer and explanation.
Frequently asked questions
Can the long run arrive suddenly, like after a lease expires?
Yes - fixity is contractual and technological, not physical. When the lease ends, the building becomes a variable input and the firm enters a new long-run planning moment. Horizons shift as commitments expire.
Why do economists bother with two horizons instead of one?
Because constraints bind differently: diminishing returns and fixed costs shape short-run supply, while plant choice and entry/exit shape long-run supply. Mixing them up produces wrong predictions, e.g., about how firms respond to price changes.
Is labor always the variable input?
Usually but not always. Union contracts, hiring frictions, or specialized skills can make labor quasi-fixed in the short run, while rented equipment adjusts quickly. Which input is fixed depends on the firm's actual commitments.
What does the envelope result imply for empirical cost studies?
Observed costs at a given plant trace a short-run curve; the long-run curve must be inferred from firms with different plant sizes. Comparing costs across plants of different sizes approximates the LRAC envelope.