Economic and Business Environment · Basics of Demand and Supply and Forms of Market Competition

Perfect Competition and Monopoly: Features and Price-Output

Updated 11 October 2026 · Fact-checked

Perfect competition has many buyers and sellers, a homogeneous product, free entry and exit, and firms that are price takers. Monopoly has one seller, no close substitutes and blocked entry, so the firm is a price maker. Both firms maximise profit where marginal revenue equals marginal cost (MR = MC).

Understand Perfect Competition and Monopoly

A market structure describes how many sellers there are, what they sell and how easily new firms can enter. Perfect competition and monopoly sit at the two extremes.

In perfect competition, there are so many small sellers and buyers that no one can change the price. The product is homogeneous (identical). Firms can enter or leave freely. Buyers and sellers know prices fully. The market sets the price through demand and supply. The firm accepts it and is a price taker. Its demand curve is a horizontal line at the market price, so Price = AR = MR.

In a monopoly, there is a single seller of a product with no close substitute. Entry is blocked by things like a patent, a government licence, control of a key raw material or very high costs. The monopolist is a price maker. It faces the whole market demand curve, which slopes downward. To sell more, it must cut the price. So MR is less than AR (price) and the MR curve lies below the demand curve.

Both types of firm maximise profit at the output where MR = MC, with MC cutting MR from below. The difference lies in what happens next. A perfectly competitive firm in the long run earns only normal profit, because new firms enter when profits are abnormal and exit when there are losses. Price equals minimum average cost in the long run. A monopolist can keep abnormal profit in the long run because entry is blocked, and it charges a price above MC.

A monopolist may also practise price discrimination: selling the same product at different prices to different buyers, where the difference is not due to cost. It needs market power, the ability to separate markets and no resale between them. Example: a cinema charging different rates, or an electricity supplier charging different rates to homes and factories.

Key rules to remember

Profit-maximising condition: MR = MC (MC cuts MR from below); Applies to every market structure, including perfect competition and monopoly.
Perfect competition: firm's revenue: Price (P) = AR = MR; The demand curve is horizontal. Each extra unit sells at the same price.
Perfect competition: equilibrium: P = MR = MC; In the long run, P = minimum AC, so firms earn normal profit.
Monopoly: revenue: AR = P, and MR < AR; Demand slopes downward, so MR falls faster than price.
Monopoly: equilibrium: MR = MC, and P > MC; Price is read from the demand (AR) curve at the equilibrium output.
Total and average revenue: TR = P × Q; AR = TR ÷ Q; MR = change in TR ÷ change in Q; Use these to find MR from a table.
Profit: Profit = TR − TC = (AR − AC) × Q; If AR > AC the firm earns abnormal profit; if AR = AC, normal profit.

How to solve Perfect Competition and Monopoly questions

Use this method for any question on features, comparison, price-output or discrimination.

1Identify the market from the clues: many sellers and identical product mean perfect competition; one seller and no substitutes mean monopoly.
2For a theory question, list the features in a fixed order: number of sellers, product type, entry and exit, price control, information, and the shape of the demand curve.
3For a numerical question, find TR (P × Q) and then MR as the change in TR for each extra unit.
4Pick the output where MR = MC. Check that MC is rising at that point.
5Find price. In perfect competition it is the market price. In monopoly read it from the demand (AR) curve or table at that output.
6Compare AR with AC to state whether the firm earns abnormal profit, normal profit or a loss.
7For a comparison question, write a short point-by-point contrast and end with the effect on consumers.
8For price discrimination, state the meaning, the three conditions and one example.

Quickest way: Clue-and-condition shortcut

When to use it: For MCQ-style or short-answer questions when time is short.

Look for the keywords: 'price taker', 'homogeneous', 'free entry' point to perfect competition. 'Price maker', 'single seller', 'no close substitute' point to monopoly.
Remember: perfect competition has P = MR; monopoly has P > MR.
Remember: perfect competition ends at P = MC; monopoly ends at P > MC.
In a table, compute MR first, then find the row where MR is equal to or just above MC.
For discrimination, check three things: market power, separable markets, no resale.

Common mistakes in Perfect Competition and Monopoly

Saying a monopolist can fix both price and quantity at will.
The word 'price maker' is read as unlimited power.
Fix: Write that the monopolist can fix either price or output, but the demand curve decides the other. Higher price means lower quantity sold.
Writing MR = P for a monopoly.
The rule from perfect competition is carried over.
Fix: In monopoly, MR is below price because a price cut applies to all units sold. Only in perfect competition is P = MR.
Reading monopoly price from the MR curve.
The output is found at MR = MC, and the student stops there.
Fix: Go up from that output to the demand (AR) curve and read the price there.
Saying a perfectly competitive firm never makes profit.
Long-run normal profit is confused with no profit.
Fix: In the short run it can earn abnormal profit or a loss. In the long run only normal profit remains, because of entry and exit.
Calling any price difference price discrimination.
Price differences caused by cost are ignored.
Fix: It is discrimination only when the same product is sold at different prices for reasons not linked to cost differences.
Treating a monopoly as always earning abnormal profit.
Blocked entry is taken to guarantee profit.
Fix: A monopolist can earn abnormal profit in the long run, but it can also make a loss if demand is weak relative to cost.

Worked examples

Example 1

A monopolist faces this demand: at price ₹10 sells 1 unit, ₹9 sells 2 units, ₹8 sells 3 units, ₹7 sells 4 units, ₹6 sells 5 units. Marginal cost is constant at ₹4. Find the profit-maximising output and price.

Show the solution

Total revenue: 1 × 10 = ₹10; 2 × 9 = ₹18; 3 × 8 = ₹24; 4 × 7 = ₹28; 5 × 6 = ₹30.
Marginal revenue: ₹10, then 18 − 10 = ₹8, then 24 − 18 = ₹6, then 28 − 24 = ₹4, then 30 − 28 = ₹2.
MC is ₹4. MR = MC at the 4th unit (MR = ₹4).
Price at 4 units is read from demand: ₹7.
Check: the 5th unit adds MR of ₹2, less than MC of ₹4, so it would reduce profit.

Answer: Output is 4 units and price is ₹7. Price (₹7) is above MC (₹4), as expected for a monopoly.

Example 2

Distinguish between perfect competition and monopoly, and explain price discrimination.

Show the solution

Number of sellers: perfect competition has very many; monopoly has one.
Product: homogeneous in perfect competition; unique with no close substitute in monopoly.
Entry: free in perfect competition; blocked in monopoly.
Price control: the firm is a price taker in perfect competition and a price maker in monopoly.
Revenue: P = AR = MR in perfect competition; AR > MR in monopoly.
Equilibrium: P = MC in perfect competition; P > MC in monopoly.
Long-run profit: normal in perfect competition; abnormal profit possible in monopoly.
Price discrimination means a monopolist sells the same product at different prices to different buyers without a matching cost difference. It needs market power, separable markets and no resale between them.

Answer: Perfect competition has many sellers, a homogeneous product, free entry and P = MC. Monopoly has one seller, blocked entry and P > MC. A monopolist may use price discrimination, for example by charging different rates to different groups of buyers.

Exam tips

For a 'features' question, give a numbered list of 6 to 8 points with a one-line explanation each. Do not just name them.
For 'differences', use a clear point-by-point format and keep both sides parallel.
Always say which curve you read price from in a monopoly. Examiners reward this.
Learn one real-world example for each structure and for price discrimination, but note that pure forms are rare.
In Paper 3 Part A, finish with the consumer impact: monopoly usually means higher price and lower output than perfect competition.

Practice questions from Basics of Demand and Supply and Forms of Market Competition

Perfect Competition and Monopoly 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 and Monopoly: frequently asked questions

What are the main features of perfect competition?

There are many buyers and sellers, the product is homogeneous, entry and exit are free, and buyers and sellers have full information. No single firm can influence price, so each is a price taker.

Why is MR less than price in a monopoly?

The demand curve slopes downward. To sell one more unit, the monopolist must lower the price on all units, not just the extra one. So the gain from the extra unit is less than its price.

What is price discrimination in monopoly?

It is selling the same product at different prices to different buyers, where the difference is not due to cost. It works only if the seller has market power, can keep markets separate and resale is not possible.

Does a monopolist always earn abnormal profit?

No. Blocked entry lets it keep abnormal profit in the long run if it exists, but if demand is low compared with costs, a monopolist can also make a loss.