Difficulty: Intermediate | Prerequisites: Module 1 (Scarcity, Allocation, and Markets)
Modules 1 and 2 looked at markets from the outside: how supply and demand interact, and what happens when governments intervene. This module moves inside the firm. It covers how firms are structured (sole proprietorships, partnerships, corporations), how they produce output using labour and capital, and how their costs behave as output changes. The cost concepts here, particularly marginal cost and the various average cost measures, are the building blocks for every market-structure analysis that follows. If you are not comfortable with opportunity cost from Module 1, revisit that first, because it is central to the distinction between economic and accounting profit.
Firms combine labour and capital to produce output, and they aim to maximise profit (total revenue minus total cost). In the short run, at least one input is fixed, which gives rise to fixed costs and variable costs. The key cost measures, including ATC, AVC, AFC, and MC, describe how per-unit costs change as a firm scales up. Economic profit differs from accounting profit because it includes opportunity costs.
Economic profit (pi)
Total revenue minus total cost, where total cost includes both explicit costs and opportunity costs. This is the measure economists care about.
Think of it as "what is left after paying everyone, including the value of the owner's own time and capital."
Accounting profit (pi_ACCY)
Total revenue minus total explicit costs only. This is the profit figure you see on a company's income statement.
In simple terms, accounting profit ignores what the owner could have earned elsewhere.
Total revenue (TR)
The total income a firm receives from selling its output. TR = price x quantity sold.
Total cost (TC)
The sum of all costs of production, including opportunity costs. TC = FC + VC.
Fixed costs (FC)
Costs that do not change with the level of output. They must be paid even if the firm produces nothing.
Think of it as the rent on the factory: you pay it whether you make one unit or a thousand.
Variable costs (VC)
Costs that change with the level of output. More output means higher variable costs.
Think of raw materials: the more you produce, the more you need to buy.
Average total cost (ATC)
Total cost divided by output (TC / q). The per-unit cost of production.
Average fixed cost (AFC)
Fixed cost divided by output (FC / q). Falls continuously as output rises, because the same fixed cost is spread over more units.
Average variable cost (AVC)
Variable cost divided by output (VC / q).
Marginal cost (MC)
The change in total cost from producing one additional unit of output. MC = dTC / dq.
Think of it as "how much does one more unit cost us?"
Sole proprietorship
A firm with a single owner who has full (unlimited) liability for the business's debts.
Partnership
A firm with two or more joint owners who share profits and losses and have full (unlimited) liability.
Corporation
A firm with shareholders that operates as its own legal entity. Owners have limited liability, meaning their personal assets are protected from the firm's debts.
Short run (SR)
A period of time in which at least one input is fixed. Typically, capital is fixed and labour is variable.
Long run (LR)
A period of time in which all inputs are variable. The firm can adjust everything, including its plant size.
Law of diminishing marginal product
When increments of a variable factor (typically labour) are added to a fixed amount of another factor (typically capital), the marginal product of the variable factor must eventually decline.
In simple terms, hiring one extra worker helps a lot when you have few workers, but the tenth extra worker in the same small kitchen adds less and less output.
Sole proprietorship: one owner, full (unlimited) liability. Simplest to set up, but the owner's personal assets are at risk.
Partnership: two or more owners share profits and losses, still with full liability.
Corporation: a separate legal entity owned by shareholders. Limited liability protects owners' personal assets. Most large firms are corporations.
The short run is defined by having at least one fixed input (usually capital). Labour is variable in both the short and long run.
The long run is when all inputs, including capital, can be adjusted.
These are not calendar periods. "Short run" and "long run" refer to the flexibility of inputs, not to months or years.
A firm's output depends on its inputs: q = f(L, K).
In the short run, capital is fixed, so the production function becomes q = f(L; K), where the semicolon indicates K is held constant.
The law of diminishing marginal product says that as you keep adding labour to a fixed amount of capital, each additional worker eventually adds less output than the one before.
Total cost = fixed costs + variable costs (TC = FC + VC).
Fixed costs do not change with output. Variable costs rise as output rises.
Average total cost (ATC) = TC / q = AFC + AVC. This is the per-unit cost.
Average fixed cost (AFC) = FC / q. Falls as output increases because the fixed cost is spread across more units.
Average variable cost (AVC) = VC / q.
Marginal cost (MC) = dTC / dq = dVC / dq. Because fixed costs do not change with output, the slope of TC equals the slope of VC, and both equal MC.
Economic profit: pi = TR - TC (where TC includes opportunity costs).
Accounting profit: pi_ACCY = TR - total explicit costs.
The difference between the two is opportunity cost. A firm can earn positive accounting profit but zero economic profit if its revenue just covers both explicit and opportunity costs.
Profit:
\pi = TR - TCEconomic profit. TC includes opportunity costs. Firms behave to maximise this.
\pi_{ACCY} = TR - \text{Total Explicit Costs}Accounting profit. The figure on an income statement.
\pi = TR - \text{Total Explicit Costs} - \text{Opportunity Costs}This shows how economic profit breaks TC into its two components.
Production function:
q = f(L, K)Output is a function of labour and capital.
q = f(L; \bar{K})Short-run production function. Capital is fixed (indicated by the semicolon or bar).
Cost formulas:
TC = FC + VCATC = \frac{TC}{q} = \frac{FC + VC}{q} = AFC + AVCAFC = \frac{FC}{q}AVC = \frac{VC}{q}MC = \frac{dTC}{dq} = \frac{\Delta TC}{\Delta q} = \frac{dVC}{dq}The last equality holds because dFC/dq = 0 (fixed costs do not change with output). MC equals the slope of both the TC curve and the VC curve.
Variables to know:
pi = economic profit
pi_ACCY = accounting profit
TR = total revenue
TC = total cost
FC = fixed costs
VC = variable costs
q = output of a single firm
L = labour (always variable)
K = capital (fixed in the short run, variable in the long run)
ATC = average total cost
AFC = average fixed cost
AVC = average variable cost
MC = marginal cost
The distinction between economic and accounting profit is why a small business owner might show a profit on paper but still feel they would be better off working for someone else. If the owner could earn 80,000 dollars a year as an employee, that forgone salary is an opportunity cost. Their business needs to clear that amount on top of explicit costs before it generates positive economic profit.
Diminishing marginal product is visible in any workplace. A restaurant kitchen with three chefs runs well; adding a fourth helps; adding a tenth means they are bumping into each other and the kitchen equipment has not changed. Output per additional worker falls.
The MC curve is central to firm decision-making. In competitive markets, firms produce up to the point where MC equals the market price. This is the profit-maximising rule you will use repeatedly in later modules.
Students often treat economic profit and accounting profit as the same thing. They are not. Economic profit subtracts opportunity costs; accounting profit does not. A firm earning zero economic profit is still covering all its costs, including the owner's opportunity cost, and has no reason to exit the industry.
"Short run" and "long run" are not about calendar time. A short run is any period in which at least one input cannot be adjusted. For a software firm, this might be weeks; for a power plant, it might be years.
Students sometimes think MC is the cost of the last unit produced. More precisely, MC is the change in total cost when output increases by one unit. The distinction matters when costs are not smooth.
AFC never reaches zero. It falls continuously as output increases, but it is always positive as long as there are fixed costs.
Expect questions that give you a total cost schedule and ask you to calculate ATC, AVC, AFC, and MC at various levels of output. Be able to derive each from the others.
The relationship MC = dTC/dq = dVC/dq is a common exam point. Know why: FC does not change, so the derivative of FC with respect to q is zero.
Questions distinguishing economic from accounting profit often present a scenario with explicit costs and an owner's forgone salary, and ask you to compute both types of profit.
The law of diminishing marginal product is the reason the MC curve eventually slopes upward. This connection is frequently tested.
True or False: Accounting profit is always less than or equal to economic profit.
Fill in the blank: In the short run, ______ is typically the fixed input.
True or False: MC equals the slope of both the TC curve and the VC curve.
Fill in the blank: ATC = AFC + ______.
True or False: A corporation's owners have unlimited liability.
Answers: 1. False (accounting profit is always greater than or equal to economic profit, because it does not subtract opportunity costs). 2. Capital (K). 3. True. 4. AVC. 5. False (corporations have limited liability).
Q: A firm has total revenue of 200,000 dollars, explicit costs of 150,000 dollars, and the owner could earn 40,000 dollars in their next best alternative. Calculate accounting profit and economic profit.
A: Accounting profit = 200,000 - 150,000 = 50,000 dollars. Economic profit = 200,000 - 150,000 - 40,000 = 10,000 dollars.
Q: A firm's total cost at 10 units of output is 500 dollars. At 11 units, total cost is 530 dollars. What is the marginal cost of the 11th unit?
A: MC = change in TC / change in q = (530 - 500) / (11 - 10) = 30 dollars.
Q: Explain why AFC falls as output rises but never reaches zero.
A: AFC = FC / q. As q increases, you are dividing the same fixed cost by a larger number, so AFC falls. But FC is always positive, so FC / q is always positive, no matter how large q gets.
Q: A bakery adds a fifth baker to a kitchen that already has four. Output rises, but by less than it did when the fourth baker was added. Which economic law does this illustrate?
A: The law of diminishing marginal product. With the kitchen (capital) fixed, each additional unit of labour eventually adds less to output.
Q: Why does MC = dVC/dq?
A: Because TC = FC + VC, and dFC/dq = 0 (fixed costs do not change with output). So dTC/dq = 0 + dVC/dq = dVC/dq.
The cost curves developed here are used directly in the analysis of perfect competition, monopoly, and oligopoly in later modules. In perfect competition, the firm's supply curve is its MC curve above AVC.
The distinction between economic and accounting profit connects back to the opportunity cost concept from Module 1. It also underpins the long-run entry and exit decisions of firms: when economic profit is zero, there is no incentive for firms to enter or leave the industry.
The law of diminishing marginal product is the microeconomic reason behind the upward-sloping MC curve, which in turn drives the U-shape of the ATC curve.
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