Short Run vs Long Run in Economics: Costs, Shutdown and Exit

In microeconomics, the short run is any period in which a firm has at least one fixed input, usually its building or equipment, and the long run is a period long enough that every input, including plant size, can be changed. The dividing line is not a number of months: it is whether anything is still fixed, so a food truck might reach its long run in weeks while a steel mill needs years.

That one definition drives almost everything in the costs and perfect competition units. It decides which costs are fixed, which curve a firm uses to decide whether to keep producing, and whether firms can enter or leave a market. This guide builds the idea from zero, then works through the shutdown rule and the exit rule with one set of numbers. The glossary entry for short run vs long run gives the bare definition.

Fixed inputs decide the time frame

Picture a bakery that has signed a one-year lease on its oven and shop. Tomorrow it can call in more bakers, buy more flour and run longer hours. It cannot double the size of the oven or walk away from the lease. That is the short run: labor and materials are variable inputs, while the oven and the shop are fixed inputs.

Once the lease is up, everything is open. The bakery can move to a bigger shop, buy a second oven, shrink to a stall at a market, or close for good. That is the long run. Nothing about the calendar defines it. A firm with month-to-month equipment rentals reaches its long run quickly, and a firm with a decade of debt on a factory reaches it slowly.

Fixed costs vs variable costs

Inputs map directly onto costs.

The third row is the one that matters for decisions. In the short run, fixed costs are owed whether the firm makes 500 loaves or none, so they cannot change the answer to "should we bake this week?" Variable costs disappear the moment production stops, so they can. In the long run, the lease can lapse and the equipment can be sold, so every cost becomes avoidable. That shift from "some costs are stuck" to "no costs are stuck" is the whole reason the short-run and long-run rules differ.

The full set of per-unit cost measures (AFC, AVC, ATC and MC) and why they are U-shaped is in the production costs guide. Here you only need average variable cost (AVC = TVC / Q) and average total cost (ATC = TC / Q).

Short run vs long run at a glance

How a firm adjusts in each period

In the short run, the firm is stuck with one plant, so it adjusts by changing how much labor and material it pushes through that plant. Adding workers to a fixed oven runs into diminishing marginal returns, which is why short-run marginal cost eventually rises.

In the long run, the firm also chooses the plant. For each possible plant size there is a short-run average total cost curve, and the firm picks whichever one produces its target output most cheaply. Tracing the lowest of those curves at every output gives the long-run average cost curve, also called the envelope curve. Whether that curve falls or rises depends on what happens to output when every input grows together, which is the subject of returns to scale.

How a market adjusts: entry and exit

The long run also changes the number of firms. In the short run the firms in a market are the ones already there. In the long run new firms can enter and existing ones can leave.

When firms earn economic profit, outsiders enter, market supply shifts right, and the price falls. When firms take economic losses, some leave, market supply shifts left, and the price rises. In a perfectly competitive market this continues until price equals minimum ATC and each firm earns zero economic profit. The perfect competition guide draws that two-graph sequence step by step, and you can shift the market yourself in the perfect competition sandbox.

See it move

Make it move

A perfectly competitive market and one price-taking firm earning a short-run profit. Tap it to drag the curves, open the full version, or put it on your own site free, or turn it into a five-minute class activity.

Zero economic profit is not zero income. ATC includes the owner's opportunity cost, so a firm earning zero economic profit still earns a normal return. The economic profit vs accounting profit guide explains the gap.

The short-run shutdown rule vs the long-run exit rule

These two rules ask different questions because different costs are avoidable.

Shutdown rule (short run). Keep producing if total revenue covers total variable cost, TR ≥ TVC, which is the same as P ≥ AVC. Shut down temporarily if P < AVC. Fixed costs are left out because the firm pays them either way.

Exit rule (long run). Stay in the industry if total revenue covers total cost, TR ≥ TC, which is the same as P ≥ ATC. Leave if price stays below ATC at every output. Fixed costs now count because leaving makes them go away.

Worked example: one bakery, three prices

A bakery in a competitive bread market pays $1,000 per week for its oven lease and shop, its total fixed cost. At each market price it produces where price equals marginal cost. The table shows its costs at that output.

Price $7. Total revenue is 7 x 500 = $3,500. Total cost is 2,050 + 1,000 = $3,050. Profit is 3,500 minus 3,050 = $450. Check per unit: P minus ATC is 7.00 minus 6.10 = $0.90, and 0.90 x 500 = $450. The bakery produces and stays, and in the long run that profit draws in new bakeries, pushing the price down.

Price $5. Total revenue is 5 x 400 = $2,000. Total cost is 1,450 + 1,000 = $2,450, so producing loses 2,450 minus 2,000 = $450. Shutting down would lose the full $1,000 of fixed cost. Producing is better by $550. Check it another way: price covers AVC by 5.00 minus 3.625 = $1.375 per loaf, and 1.375 x 400 = $550 goes toward the fixed cost, leaving 1,000 minus 550 = $450 uncovered. Since P = $5 is above AVC = $3.625, the shutdown rule says produce. But P = $5 is below ATC = $6.125, so the exit rule says leave once the lease ends, because staying means a $450 loss every week while exiting means a loss of zero. The only reasons to stay would be a price recovery or a different plant size that brings ATC below $5.

Price $3. Total revenue is 3 x 300 = $900, which does not even cover the $1,050 of variable cost. Producing loses 2,050 minus 900 = $1,150, while shutting down loses $1,000. Check per unit: AVC minus P is 3.50 minus 3.00 = $0.50, a shortfall on every loaf, and 0.50 x 300 = $150 of extra loss on top of the $1,000 fixed cost, giving $1,150. Since P < AVC, the bakery shuts down now, and if the price stays there it exits in the long run.

The middle column is the one exams love: a firm that is losing money but should keep producing in the short run, and should still leave in the long run if nothing changes. You can rerun the shutdown test with your own numbers in the shutdown point calculator, and the break-even point vs shutdown point comparison sets the two thresholds side by side.

Short run and long run in macroeconomics

Macroeconomics uses the same two words with a different meaning. In AP Macroeconomics the short run is the period when some wages and prices are sticky and have not yet adjusted, and the long run is when all of them have. That is why short-run aggregate supply slopes upward while long-run aggregate supply is vertical at full-employment output. There are no fixed plants in that story; the dividing line is price flexibility. The SRAS vs LRAS guide covers the macro version. When a question mentions firms, costs or entry, use the micro definition; when it mentions the price level, wages or real GDP, use the macro one.

Common mistakes

Defining the short run as "less than a year." The test is whether any input is fixed, not how many months have passed.

Using ATC for the shutdown decision. Short-run shutdown compares price with AVC. ATC is for profit, break even, and the long-run exit decision.

Saying a firm with a loss should always shut down. If price is above AVC, producing loses less than shutting down, as the $5 column shows.

Forgetting fixed costs vanish in the long run. There are no fixed costs in the long run, so the loss from leaving is zero, not total fixed cost.

Mixing up shutdown and exit. Shutdown is temporary and the firm stays in the industry; exit means leaving the market, which shifts market supply left.

Carrying the micro definition into macro. In macro the short run is about sticky wages and prices, not fixed plants.