Cost Curves & Profit Maximization

You’ll be able to

Apply the MR = MC rule to find the profit-maximizing quantity
Calculate economic profit and distinguish it from accounting profit
Use the shutdown rule to decide whether a firm should operate in the short run

The profit-maximizing rule: MR = MC

Every firm — in any market structure — maximizes profit by producing the quantity where marginal revenue (MR) equals marginal cost (MC). The logic is marginal: as long as MR > MC, the next unit adds more to revenue than to cost, so produce it; once MR < MC, that unit loses money, so stop. Profit is greatest exactly where MR = MC. This single rule governs output decisions throughout the course; what differs across market structures is the shape of the demand and MR curves the firm faces.

Economic vs. accounting profit

Economic profit subtracts all opportunity costs — both explicit (money paid out) and implicit (the value of the owner’s own resources and forgone alternatives). Accounting profit subtracts only explicit costs, so it is larger. A firm earning zero economic profit is doing exactly as well as its next-best alternative — a normal profit — which is a perfectly acceptable outcome. Economic profit is measured on the graph as (Price − ATC) × Quantity: positive if price exceeds ATC, a loss if price is below ATC.

The shutdown decision

In the short run a firm must pay its fixed costs whether it operates or not, so it should keep producing as long as it covers its variable costs. The shutdown rule: operate if price ≥ AVC; shut down if price < AVC. When price is below AVC, the firm loses more by operating (it cannot even cover variable costs) than by shutting down and losing only its fixed costs. Between AVC and ATC, the firm operates at a loss in the short run but still minimizes that loss. In the long run, a firm exits if price stays below ATC.

Profit and the decision rules

Profit-max: MR = MC · Economic profit = (P − ATC) × Q · Shut down if P < AVC

Produce where MR = MC, then check profit against ATC and viability against AVC. Zero economic profit = normal profit (breaking even including opportunity cost).

Worked example

A competitive firm faces a market price of $12. At its profit-maximizing output of 50 units (where MR = MC), its ATC is $10 and its AVC is $7. Should the firm produce, and what is its economic profit?

1.The firm produces where MR = MC; for a price-taker, MR = P = $12, and it is told this occurs at 50 units.
2.Check the shutdown rule: price ($12) ≥ AVC ($7), so the firm should operate.
3.Profit per unit = P − ATC = 12 − 10 = $2.
4.Economic profit = (P − ATC) × Q = $2 × 50 = $100.

Answer: Yes, the firm should produce: price ($12) exceeds AVC ($7). It earns a positive economic profit of (12 − 10) × 50 = $100. Because price also exceeds ATC, this is a genuine profit above the normal return.

Checkpoint

A firm is producing at an output where marginal revenue is $18 and marginal cost is $12. To maximize profit, the firm should:

Watch out

The shutdown rule compares price to AVC, not ATC. A firm operating between AVC and ATC is losing money yet should keep producing in the short run, because it still covers variable costs and part of its fixed costs. Only price below AVC triggers shutdown.

Checkpoint

A firm earns zero economic profit. Which statement is correct?

On the exam

Run the two-step firm analysis: (1) find output where MR = MC, then (2) compare price to ATC for profit/loss and to AVC for the shutdown decision. Zero economic profit is breaking even (normal profit), not a loss — a distinction the exam tests directly.

Answer the 2 checkpoints as you read.