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M06.L03 — Profit Maximisation: The MR = MC Rule

Module: Module 06 — Market Equilibrium and Perfect Competition Lesson: L03 of 05 Duration: ~30 minutes Level: Introductory Provenance: MIT OCW 14.01 Principles of Microeconomics | OpenStax Principles of Microeconomics 3e

Learning Objective

Apply the MR = MC rule to determine profit-maximizing output.

The Golden Rule of Profit Maximisation

All firms, regardless of market structure, maximize profit where:

Marginal Revenue (MR) = Marginal Cost (MC)

Why this works: - If MR > MC: Producing more adds to profit - If MR < MC: Producing less reduces losses - At MR = MC: No further profit gains possible

Australian context: This rule applies equally to: - Small farms (perfect competition) - Local cafes (monopolistic competition) - Major banks (oligopoly)

Four-Step Profit Analysis

  1. Find Q* where MR = MC
  2. Find P from demand curve at Q*
  3. Find ATC from cost curve at Q*
  4. Calculate profit: (P - ATC) × Q*

Example: Australian wheat farmer - Market price: $300/tonne (P = MR) - MC = $280 at Q* = 1000 tonnes - ATC = \(290 at Q* - Profit = (\)300 - $290) × 1000 = $10,000

Worked Example

Australian Café Economics

  1. Given:
  2. P = $5 per coffee (price taker)
  3. MC = $4 at Q* = 200 coffees/day

  4. ATC = $4.50 at Q*

  5. Profit calculation:

  6. MR = P = $5
  7. MR = MC at 200 units

  8. Profit = ($5 - $4.50) × 200 = $100/day

  9. Decision:

  10. If demand falls to $4.25, should they operate?
  11. Yes, if P > AVC (covers variable costs)

Common Misconception

"Firms should produce where MC is lowest"

Actually, firms produce where MR = MC, which may be at higher MC levels. Minimizing MC alone would lead to underproduction and lost profit opportunities.

Key Takeaways

  • Profit is maximized when MR = MC
  • The rule applies universally across market structures
  • Requires comparing revenue and cost at the margin
  • Per-unit profit equals P - ATC

Practice

  1. If a firm's MR = $10 and MC = $8, should it produce more?
  2. How does perfect competition simplify profit analysis?
  3. Why can't we use average revenue instead of marginal?

Further Resources

Provenance: MIT OCW 14.01 Principles of Microeconomics | OpenStax Principles of Microeconomics 3e