Businesses and the Costs of Production

Behind every supply curve is a firm transforming inputs into output. To understand how firms decide how much to produce and what to charge, we first need to understand their costs. This chapter examines the relationship between inputs and output and derives the family of cost curves that underlie every production decision in the chapters that follow.

Economic Costs and Economic Profit

Economists measure cost more broadly than accountants do. Explicit costs are the money payments a firm makes to outsiders for resources — wages, rent, materials. Implicit costs are the opportunity costs of resources the firm already owns, such as the salary an owner forgoes by running the business or the return on funds invested in it.

This distinction produces two notions of profit. Accounting profit is revenue minus explicit costs. Economic profit is revenue minus both explicit and implicit costs. Because it counts opportunity costs, economic profit is the more meaningful figure: a normal profit (zero economic profit) means the firm is earning exactly what its resources could earn elsewhere, while positive economic profit means it is doing better than its next-best alternative.

The Short Run and the Long Run

The firm's options depend on the time horizon. In the short run, at least one input — usually plant size or capital — is fixed, so the firm can change output only by varying its variable inputs, such as labor. In the long run, all inputs are variable: the firm can build a bigger factory, and new firms can enter or leave the industry. The short run is about using a given plant; the long run is about choosing the plant.

The Law of Diminishing Returns

Short-run production is governed by the law of diminishing returns: as successive units of a variable input (labor) are added to a fixed input (capital), the marginal product — the extra output from each additional worker — eventually declines. With a fixed amount of equipment, the first few workers may raise output rapidly through specialization, but beyond some point each new worker adds less than the one before, because they have less capital to work with. Diminishing returns is the technical fact that shapes the firm's short-run cost curves.

Short-Run Costs

From the production relationship we derive the firm's costs. Total cost splits into two parts:

• Fixed costs (FC) do not vary with output — rent, insurance, loan payments. They must be paid even if output is zero.
• Variable costs (VC) rise with output — labor, materials, energy.

Dividing by output gives the per-unit measures that drive decisions: average fixed cost (AFC), which falls continuously as fixed costs spread over more units; average variable cost (AVC); and average total cost (ATC = AFC + AVC). Most important is marginal cost (MC) — the additional cost of producing one more unit. Because of diminishing returns, marginal cost eventually rises as output expands.

A key geometric fact ties these together: the marginal cost curve intersects both the average variable cost and average total cost curves at their minimum points. When marginal cost is below average, it pulls the average down; when it is above, it pulls the average up. This MC-ATC relationship is central to the profit analysis of later chapters.

Long-Run Costs and Economies of Scale

In the long run, with all inputs variable, the firm chooses its scale of operation, traced by the long-run average total cost curve. This curve is typically U-shaped for three reasons:

• Economies of scale — Over the initial range, expanding output lowers average cost, thanks to specialization, more efficient large-scale equipment, and the spreading of certain costs.
• Constant returns to scale — Over a middle range, average cost may be roughly flat.
• Diseconomies of scale — Eventually, average cost rises as the firm grows too large to manage efficiently, with communication and coordination problems mounting.

The extent of economies of scale helps explain industry structure: where they are vast, a few large firms dominate; where they are exhausted quickly, many small firms can compete.

Why Costs Matter

Costs are the foundation of supply. A firm's decision about whether to produce, how much to produce, and whether to stay in business all flow from the cost curves developed here. With this toolkit in hand, the next chapters examine how firms behave in different market structures, beginning with the benchmark case of pure competition.

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Next chapter: we put the cost curves to work in the simplest market structure, where firms have no control over price — pure competition.

Further Listening & Reading

• 🎥 Maximizing Profit Under Competition (MRU) — defines total/fixed/variable cost and marginal cost and revenue.