Business Economics · Profit maximisation under perfect competition and monopoly

Perfect Competition: Short-Run and Long-Run Equilibrium Explained

Updated 11 October 2026 · Fact-checked

In perfect competition, firms take the market price as given, so price equals marginal revenue. A firm maximises profit where P = MC, with MC rising. In the short run it can earn supernormal profit or a loss, and it shuts down if P < minimum AVC. In the long run, entry and exit force P = minimum ATC and zero economic profit.

Understand Perfect Competition: Short-Run and Long-Run Equilibrium

Perfect competition is a market with many small buyers and sellers, a identical (homogeneous) product, free entry and exit, and perfect information. No single firm can move the market price. Each firm is a price taker.

Because the firm is a price taker, it can sell any quantity at the market price P. Its demand curve is horizontal at P. So each extra unit adds exactly P to revenue: AR = MR = P. The market demand curve still slopes downward. Only the demand facing one firm is flat.

The firm picks the output where MR = MC, with MC cutting MR from below. Here MR = P, so the rule becomes P = MC. Then compare price with average cost at that output. If P > ATC, the firm earns supernormal profit of (P − ATC) × Q. If P = ATC, it earns normal profit (zero economic profit). If P < ATC, it makes a loss.

In the short run, at least one input is fixed, so fixed costs must be paid whatever the output. A loss-making firm should still produce if price covers average variable cost (P ≥ AVC), because it then recovers some fixed cost. If P < AVC, it loses less by closing. The shutdown point is the minimum of AVC, where MC cuts AVC. So the firm's short-run supply curve is its MC curve above minimum AVC.

In the long run, all inputs are variable and firms can enter or leave. Supernormal profit attracts entrants. Market supply shifts right, price falls and profit shrinks. Losses push firms out, supply shifts left and price rises. Entry and exit stop when profit is zero. The long-run equilibrium is at P = MR = MC = minimum ATC. The firm is productively efficient (minimum ATC) and allocatively efficient (P = MC). Assume constant input prices for this standard result.

Key rules to remember

Price-taker revenue: AR = MR = P; Holds for a firm facing a horizontal demand curve. Market demand still slopes down.
Profit-maximising rule: MR = MC, so P = MC (MC rising); The second-order condition is that MC cuts MR from below.
Profit: π = (P − ATC) × Q; Positive means supernormal profit, zero means normal profit, negative means a loss.
Shutdown rule (short run): Produce if P ≥ min AVC; shut down if P < min AVC; At P = AVC the firm is indifferent, because it loses its fixed costs either way.
Short-run supply curve: Firm supply = MC curve above min AVC; Market supply is the horizontal sum of firms' supply curves.
Long-run equilibrium: P = MR = MC = min ATC; Zero economic profit, so no entry or exit.

How to solve Perfect Competition: Short-Run and Long-Run Equilibrium questions

Use this method for any question on a competitive firm, whether it is a diagram, a calculation or a discussion.

1Confirm the market is perfectly competitive and note the market price P. Set MR = P.
2Find the output where MC = P. If costs are given as a function, differentiate total cost to get MC and solve. Check that MC is rising there.
3Calculate total revenue = P × Q and total cost at that Q. Find ATC = TC ÷ Q.
4Compare P with ATC to state supernormal profit, normal profit or loss. Compute π = TR − TC.
5If there is a loss, compute AVC at that output. If P ≥ AVC, keep producing in the short run. If P < AVC, shut down.
6For the supply curve, state that supply is MC above minimum AVC. Find minimum AVC by setting AVC = MC if needed.
7For the long run, describe entry (if profit) or exit (if loss), the shift of market supply and the price change until P = min ATC.
8State the efficiency conclusion: productive and allocative efficiency at long-run equilibrium.

Quickest way: Three-line check: P = MC, then P vs ATC, then P vs AVC

When to use it: Use it for MCQs and short numerical parts where you have price and cost data.

Set P = MC and solve for Q. Ignore any solution where MC is falling.
Compute profit as (P − ATC) × Q. The sign gives the profit status and the direction of entry or exit.
If profit is negative, compare P with AVC. P below AVC means shut down. P at or above AVC means continue.
For long-run questions, jump straight to P = min ATC and zero profit.

Common mistakes in Perfect Competition: Short-Run and Long-Run Equilibrium

Drawing a downward-sloping demand curve for the individual firm.
Students mix up market demand with the demand facing one firm.
Fix: Draw two side-by-side diagrams: the market with sloping curves, and the firm with a horizontal line at the market price.
Saying a loss-making firm should always shut down immediately.
The loss feels like a reason to stop, and the role of fixed costs is forgotten.
Fix: Compare P with AVC, not ATC. If P ≥ AVC, the firm covers variable costs and part of fixed costs, so it continues in the short run.
Using minimum ATC as the shutdown point.
Both points lie on MC and look alike on a diagram.
Fix: Shutdown is at minimum AVC. Minimum ATC is the break-even point, where profit is zero. AVC is below ATC.
Saying zero economic profit means the owners earn nothing.
The word 'zero' is read literally.
Fix: Economic cost includes the opportunity cost of owners' capital and time. Zero economic profit means normal profit, which is just enough to keep the firm in the industry.
Taking the whole MC curve as the supply curve.
Students remember 'supply is MC' and stop there.
Fix: Only the part of MC at or above minimum AVC is supply. Below that, output is zero.
Choosing an output where P = MC but MC is falling.
With quadratic or cubic costs the equation has two roots.
Fix: Pick the root where MC is rising, meaning the slope of MC is positive. Check that profit is higher there.

Worked examples

Example 1

A competitive firm has total cost TC = Q³ − 6Q² + 20Q + 50 (in ₹ thousand). The market price is ₹56 thousand per unit. Find the profit-maximising output and the profit.

Show the solution

MR = P = 56.
MC = dTC/dQ = 3Q² − 12Q + 20.
Set MC = P: 3Q² − 12Q + 20 = 56, so 3Q² − 12Q − 36 = 0, so Q² − 4Q − 12 = 0.
Factorise: (Q − 6)(Q + 2) = 0. Q = 6 (reject −2).
Check slope of MC: 6Q − 12 = 24 > 0, so MC is rising. Good.
TR = 56 × 6 = 336.
TC = 216 − 216 + 120 + 50 = 170.
Profit = 336 − 170 = 166.

Answer: Output Q = 6 units, profit = ₹166 thousand (supernormal profit).

Example 2

A competitive firm has TC = 2Q² + 10Q + 200, where 200 is fixed cost, in ₹. Price is ₹30. Find the output, profit and say whether it should continue in the short run.

Show the solution

MR = P = 30. MC = 4Q + 10.
Set MC = P: 4Q + 10 = 30, so Q = 5. MC is rising.
TR = 30 × 5 = 150.
TC = 2(25) + 50 + 200 = 300.
Profit = 150 − 300 = −150, a loss of ₹150.
Variable cost = 2Q² + 10Q = 50 + 50 = 100. AVC = 100 ÷ 5 = 20.
P = 30 > AVC = 20, so revenue covers variable cost with 50 left over towards fixed cost.
If it shut down, the loss would be the fixed cost of ₹200, which is bigger than ₹150.

Answer: Q = 5, loss of ₹150. The firm should continue producing in the short run because P (30) exceeds AVC (20). Shutting down would lose ₹200.

Exam tips

Draw the firm and market diagrams side by side. Examiners reward the link: market price sets the firm's horizontal line.
Label MC, ATC and AVC clearly, and mark the shutdown point and the break-even point separately.
In calculation questions, always check the second-order condition and say why you rejected any root.
For long-run answers, name the process: entry or exit, shift in market supply, price adjustment, then P = min ATC.
When asked about efficiency, name both types: productive (min ATC) and allocative (P = MC).

Practice questions from Profit maximisation under perfect competition and monopoly

Perfect Competition: Short-Run and Long-Run Equilibrium in other exams

The same ground in other exams, if you are preparing for more than one or want another angle on it.

Perfect Competition: Short-Run and Long-Run Equilibrium: frequently asked questions

What are the main characteristics of perfect competition?

There are many buyers and sellers, a homogeneous product, free entry and exit, and perfect information. Each firm is a price taker. These conditions give a horizontal demand curve for the firm.

How do you derive the supply curve of a competitive firm?

The firm sets P = MC to maximise profit. So at each price, it supplies the output where MC equals that price. This holds only if P is at least minimum AVC, so the supply curve is the MC curve above minimum AVC.

What is the shutdown point in perfect competition?

It is the minimum point of the AVC curve. If price falls below it, the firm cannot cover variable costs and loses less by producing nothing. Above it, the firm keeps producing even if it makes a loss.

Why is long-run profit zero in perfect competition?

Free entry and exit remove any supernormal profit or loss. Profit attracts entrants, which lowers price. Losses drive firms out, which raises price. The process stops when P equals minimum ATC and economic profit is zero.