Short Run Costs

Learning Objectives

By the end of this page, you should be able to:

1. Define the short run and distinguish fixed costs from variable costs.
2. Compute and interpret the seven cost measures: TC, TFC, TVC, AFC, AVC, ATC, and MC.
3. Explain why MC and the average cost curves are U-shaped, using diminishing marginal returns.
4. State and apply the marginal–average relationship (MC cuts AVC and ATC at their minimum points).
5. Use MR = MC to find a firm's profit-maximizing output.
6. Apply the shutdown rule: produce in the short run only if price covers average variable cost.

Quick Answer

In the short run, at least one input (usually plant and machinery) is fixed, so a firm's costs split into fixed costs (rent, salaries, insurance — unchanged whatever the output) and variable costs (materials, casual labour, power — rising with output). From these two building blocks come the cost curves every economics exam draws: average fixed cost (always falling), average variable cost and average total cost (U-shaped), and marginal cost (the cost of one more unit, also U-shaped, cutting both average curves at their minimums). These curves matter because they answer the firm's two central questions: how much should I produce? (where marginal revenue = marginal cost) and should I produce at all? (only if price covers average variable cost).

Overview

Every firm — a dosa stall, a garment factory, an airline — faces the same accounting reality: some bills arrive regardless of sales, others grow with every unit made. The short-run cost framework organizes this reality into a small set of curves whose shapes are all driven by one production fact: diminishing marginal returns to the variable input. Master the logic once and you have the supply side of microeconomics — these same curves reappear in perfect competition, monopoly, and every market structure after this chapter.

Core Concepts

1. The Short Run and the Fixed/Variable Split

Definition: The short run is the period during which at least one input is fixed (typically capital: plant, machinery, premises). Fixed costs (TFC) don't vary with output; variable costs (TVC) do. Total cost: TC = TFC + TVC.

Explanation: "Short run" is defined by flexibility, not the calendar — it's however long the firm is stuck with its current plant. A dhaba can expand its kitchen in months; a steel plant needs years. Fixed costs must be paid even at zero output (rent is due whether or not you cook); variable costs start at zero and climb with production.

Example: A small garment unit: rent ₹30,000/month, sewing-machine lease ₹10,000, supervisor salary ₹20,000 → TFC = ₹60,000. Cloth, thread, and piece-rate tailors' wages vary with shirts stitched → TVC. At zero shirts, TC = ₹60,000; at 1,000 shirts with TVC ₹1,40,000, TC = ₹2,00,000.

Real-World Example: During COVID lockdowns, restaurants earned zero revenue yet still owed rent and salaries — pure fixed cost bleeding. That's why many negotiated rent waivers (attacking TFC) or pivoted to delivery (generating revenue above variable cost).

Why It Matters: The split determines short-run decisions: fixed costs are unavoidable (sunk for the period), so smart decisions look only at variable costs and revenue — the basis of the shutdown rule below.

Common Misunderstanding: Classifying costs by category name rather than behaviour. Labour is not automatically variable: permanent salaried staff are fixed; piece-rate workers are variable. Ask "does this bill change if output changes?" — that's the only test.

2. The Average Cost Family: AFC, AVC, ATC

Definition: Per-unit versions of the totals: AFC = TFC/Q, AVC = TVC/Q, ATC = TC/Q = AFC + AVC.

Explanation: Averages tell you cost per unit, which is what you compare with price to check profitability. Their shapes differ predictably:

• AFC always falls as Q rises — the same fixed sum spread over more units ("spreading the overhead"). It never rises.
• AVC is U-shaped — falls first (specialization gains), rises later (diminishing returns).
• ATC is U-shaped and sits above AVC by exactly AFC; the gap narrows as Q grows because AFC shrinks.

Example: With TFC = ₹60,000: AFC is ₹600 at 100 shirts but only ₹60 at 1,000 shirts. If AVC at 1,000 shirts is ₹140, ATC = ₹200. Price above ₹200 → profit per shirt.

Real-World Example: Airlines obsess over "cost per available seat-kilometre" — an ATC measure. A flight's fixed costs (aircraft lease, crew) are huge, so filling more seats slashes AFC per passenger — the economics behind cheap last-minute fares and high load-factor targets.

Why It Matters: ATC vs. price determines profit or loss; AVC vs. price determines whether to operate at all. Every profitability diagram you'll draw in market-structure chapters compares price with these two curves.

Common Misunderstanding: Expecting ATC's minimum to sit directly above AVC's minimum. ATC bottoms out at a larger output than AVC: after AVC starts rising, the still-falling AFC keeps dragging ATC down for a while.

3. Marginal Cost (MC)

Definition: The additional cost of producing one more unit: MC = ΔTC/ΔQ = ΔTVC/ΔQ (fixed costs don't change with output, so MC comes entirely from variable cost).

Explanation: MC is the decision-making cost. "Should I make one more unit?" depends only on what that unit adds to cost versus what it adds to revenue — averages and fixed costs are irrelevant to that question. MC is the mirror image of marginal product: MC = wage ÷ marginal product of labour, so when MP rises, MC falls, and when diminishing returns set in and MP falls, MC rises. The U-shape of MC is diminishing returns in cost clothing.

Example: Shirts 1–100 need little extra labour per shirt (workers specialize) → low MC. By shirt 900, the workshop is crowded and machines queue → each extra shirt needs much more labour time → MC climbs steeply.

Real-World Example: A cloud-kitchen quoting a bulk order at midnight: the kitchen and staff are already paid for (fixed); the marginal cost of 50 more biryanis is just ingredients and gas. Accepting a price above that MC adds profit even if it's below full average cost — correct marginal reasoning that "cover-your-full-cost" intuition would wrongly reject.

Why It Matters: MC is half of the universal profit-maximization rule (MR = MC) and — in perfect competition — the firm's supply curve is its MC curve above AVC. No curve in this chapter works harder.

Common Misunderstanding: Thinking fixed costs influence MC. They never do: ΔTFC = 0 by definition. A rise in rent shifts ATC and AFC up but leaves MC (and hence the profit-maximizing output) completely unchanged — a favourite exam trap.

4. The Marginal–Average Relationship

Definition: The MC curve intersects both the AVC and ATC curves at their minimum points, from below.

Explanation: Pure arithmetic, no economics needed: whenever the marginal unit costs less than the average, it pulls the average down; whenever it costs more, it pulls the average up. So the average can only stop falling and start rising at the exact point where marginal equals average — the minimum.

Example: Your exam average is 70. Score 60 on the next test (marginal < average) and your average falls; score 80 (marginal > average) and it rises. Score exactly 70 and the average is momentarily flat — its turning point.

Real-World Example: A delivery fleet's average cost per parcel falls as long as each additional route added costs less per parcel than the current average — dispatch managers effectively track this marginal-vs-average comparison when deciding whether expansion is diluting or worsening unit costs.

Why It Matters: It disciplines your diagrams (examiners check that MC passes through both minimums) and defines productive efficiency: minimum ATC is the least-cost output, the benchmark perfect competition achieves in long-run equilibrium.

Common Misunderstanding: Believing MC must be below AVC whenever output is "small" and above whenever "large." The rule is positional, not size-based: MC < average ⇔ average falling; MC > average ⇔ average rising; MC = average ⇔ average at minimum.

5. Optimal Output: MR = MC

Definition: A firm maximizes profit by producing every unit for which marginal revenue exceeds marginal cost, stopping where MR = MC (with MC rising).

Explanation: Each unit with MR > MC adds to profit; each with MR < MC subtracts. So keep expanding until the two meet. In perfect competition MR equals the market price, so the rule becomes P = MC. Profit per unit at that output is P − ATC; total profit is (P − ATC) × Q.

Example: A competitive shirt-maker faces price ₹250. If MC of the 800th shirt is ₹230 (make it — adds ₹20 profit) and MC of the 901st is ₹260 (don't — loses ₹10), the optimum lies where MC climbs to exactly ₹250.

Real-World Example: Sugar mills during crushing season expand daily throughput while the extra cane, labour, and fuel per quintal (MC) stay below the sugar price, and throttle back when overtime and machine-strain costs push MC above it — MR = MC in operational form.

Why It Matters: This single rule is the firm-behaviour engine of every market-structure chapter to come; only the shape of MR changes across structures.

Common Misunderstanding: Thinking firms maximize profit per unit or revenue. Producing where ATC is lowest, or where revenue peaks, generally sacrifices total profit; only MR = MC maximizes the total.

6. The Shutdown Decision

Definition: In the short run, a firm should operate as long as price ≥ minimum AVC; if price falls below minimum AVC, it should shut down temporarily. The point (min AVC) is the shutdown point.

Explanation: Fixed costs are owed whether or not the firm produces — they're sunk for the period. So the real comparison is: operating loss vs. shutdown loss (= TFC). If revenue covers all variable costs and something toward fixed costs (P > AVC), operating loses less than shutting. If revenue can't even cover variable costs (P < AVC), every unit produced deepens the loss — better to close and lose only TFC.

Example: Shirt firm: TFC ₹60,000; at best output, AVC = ₹140, price = ₹160. Price < ATC (₹200) → loss. But each shirt earns ₹20 above its variable cost; 1,000 shirts contribute ₹20,000 toward rent. Operating loss = ₹40,000 < shutdown loss ₹60,000 → keep producing at a loss.

Real-World Example: Airlines fly some off-season routes at fares below full cost but above fuel-and-crew (variable) cost — contributing to aircraft lease payments that are owed anyway. Similarly, hotels in the monsoon off-season stay open at discounted tariffs rather than closing floors entirely, as long as tariffs beat housekeeping and utility costs per room.

Why It Matters: It explains the everyday puzzle of firms "losing money but staying open," and it defines the competitive firm's supply curve: MC above the AVC minimum.

Common Misunderstanding: "A loss-making firm should close immediately." Wrong whenever P > AVC — closing makes the loss bigger. Shutdown is also temporary; exit (leaving the industry) is a long-run decision made when price persistently fails to cover ATC.

Visual Learning

How diminishing returns generates every curve's shape:

The firm's short-run decision tree:

Key Terms

Term	Definition
Fixed cost (TFC)	Cost that does not vary with output in the short run (rent, salaries, lease payments)
Variable cost (TVC)	Cost that rises with output (raw materials, hourly wages, electricity for machines)
Marginal cost (MC)	The addition to total cost from producing one more unit; MC = ΔTC/ΔQ
Average total cost (ATC)	Total cost per unit; ATC = TC/Q = AFC + AVC
Average variable cost (AVC)	Variable cost per unit; the floor price below which a firm shuts down
Average fixed cost (AFC)	Fixed cost per unit; falls continuously as output spreads TFC thinner
Shutdown point	Minimum of AVC — below this price, producing anything deepens the loss
Diminishing marginal returns	Beyond some point, each extra unit of the variable input adds less output than the one before

Common Mistakes

1. Misconception: A firm making a loss should shut down immediately. Why it's wrong: Fixed costs are owed regardless. If price covers variable cost (P > AVC), each unit sold contributes something toward those fixed costs, so operating loses less than closing. Correct understanding: Shut down in the short run only when P < minimum AVC; exit is a separate long-run decision made when P persistently stays below ATC.

2. Misconception: MC intersects the average curves anywhere on their falling sections. Why it's wrong: Arithmetic of averages: while the marginal is below the average, the average must still be falling; while above, rising. The crossing can only happen exactly at the minimum. Correct understanding: MC cuts both AVC and ATC precisely at their minimum points — a favourite exam check.

3. Misconception: Firms maximize profit by producing where ATC is lowest. Why it's wrong: Minimizing per-unit cost is not the same as maximizing total profit; at min-ATC output the firm may be leaving profitable units unproduced (or producing unprofitable ones). Correct understanding: Profit is maximized where MR = MC; the ATC at that output then determines whether the maximum profit is positive, zero, or a minimized loss.

Comparison and Connections

Concept	Short run	Long run
Inputs	At least one fixed (usually capital)	All inputs variable
Cost split	TFC + TVC meaningful	No fixed costs — all costs variable
Curve shape driver	Diminishing marginal returns	Economies/diseconomies of scale
Firm's exit option	Only temporary shutdown (lose TFC)	Full exit or entry
Key decision rule	Produce if P ≥ min AVC; output at MR = MC	Stay only if P ≥ min LRAC

Practice Questions

Recall

1. Write the three per-unit cost identities that connect ATC, AVC, and AFC. Answer guidance: ATC = TC/Q, AVC = TVC/Q, AFC = TFC/Q, with ATC = AVC + AFC.
2. At what points does MC intersect AVC and ATC, and why there? Answer guidance: At each curve's minimum — marginal below average pulls the average down, above pulls it up.

Understanding 3. Explain why AFC falls continuously while AVC eventually rises. Answer guidance: AFC = TFC/Q mechanically shrinks as Q grows; AVC eventually rises because diminishing marginal returns raise the labour needed per extra unit. 4. Why is MC's U-shape a mirror of the marginal product curve? Answer guidance: MC = wage ÷ MP of labour — when MP rises MC falls, when MP falls MC rises.

Application 5. A bakery has TFC ₹30,000/month; at its best output price is ₹25/loaf, AVC ₹20, ATC ₹32. Should it operate this month? Show the comparison. Answer guidance: Yes — P > AVC, so each loaf contributes ₹5 toward fixed costs; operating loss < ₹30,000 shutdown loss. 6. If TC rises from ₹5,000 to ₹5,180 when output goes from 100 to 102 units, compute MC and state whether the firm on a P = ₹95 market should expand. Answer guidance: MC = 180/2 = ₹90 < P = ₹95, so expand — each extra unit still adds more revenue than cost.

Analysis 7. "Because airlines sell some seats below full cost, they must be irrational." Evaluate using short-run cost concepts. Answer guidance: Rational when fare > variable cost per seat — contributes to aircraft leases owed anyway; irrational only if fares fell below AVC. 8. Analyse why the competitive firm's short-run supply curve is the MC curve only above minimum AVC, not the whole MC curve. Answer guidance: Below min AVC the firm supplies zero (shutdown); above it, P = MC picks the profit-maximizing quantity for every price.

FAQ

1. Are salaries a fixed or variable cost? Contracted monthly salaries are fixed in the short run; hourly or piece-rate wages that move with output are variable. The test is whether the cost changes when output changes.

2. Why do economists include "normal profit" in costs? Because the owner's time and capital have opportunity costs. "Break even" (P = ATC) therefore already includes a normal return — economic profit is anything above that.

3. Can fixed cost affect the profit-maximizing output level? No. TFC shifts total and average cost but not marginal cost, so the MR = MC output is unchanged — TFC only affects whether that output yields profit or loss.

4. What happens to these curves if wages rise? MC, AVC, and ATC all shift upward (variable input became dearer); AFC is untouched. The shutdown point rises too.

5. Is the shutdown decision the same as going out of business? No — shutdown is temporary (produce zero, keep paying TFC). Exit means leaving the industry entirely and is a long-run decision when losses look permanent.

Quick Revision

• Short run = at least one fixed input; costs split into TFC + TVC
• AFC = TFC/Q falls continuously; ATC–AVC gap = AFC narrows as Q grows
• Diminishing marginal returns make MC, AVC, ATC U-shaped
• MC = wage ÷ MP — marginal cost mirrors marginal product
• MC cuts AVC and ATC exactly at their minimum points
• Profit-maximizing output: MR = MC (for competitive firms, P = MC)
• Operate at a loss if AVC < P < ATC — revenue helps pay fixed costs
• Shut down when P < minimum AVC; loss then equals TFC
• Firm's short-run supply curve = MC above minimum AVC
• Fixed costs never change the optimal output, only the profit level

Related Topics

Prerequisites

• Production Theory — marginal product and diminishing returns, the engine behind every cost curve here

Related

• Economies of Scale — the long-run counterpart of cost behaviour

Next

• Long Run Costs — what happens when every input, including capital, becomes variable