Short-Run Cost Structure: Fixed and Variable Costs

Core Idea

In the short run, some inputs (e.g., factory buildings) are fixed, while others (e.g., labor and raw materials) are variable. Total cost (TC) equals fixed costs (FC) plus variable costs (VC). Average total cost (ATC = TC/Q), average fixed cost (AFC = FC/Q), and average variable cost (AVC = VC/Q) all vary with output. Marginal cost (MC = ΔTC/ΔQ) is the cost of producing one more unit. The firm cannot avoid fixed costs even if it produces nothing in the short run.

How It's Best Learned

Work with numerical examples calculating FC, VC, TC, AFC, AVC, ATC, and MC for different output levels. Graph the cost curves and observe their shapes and relationships.

Common Misconceptions

• Confusing average cost with marginal cost—they are equal only at the minimum of average cost.
• Thinking fixed costs are irrelevant to production decisions—they're sunk in the short run but influence entry/exit decisions and the shutdown point.

Explainer

Your prerequisite work on factor demand established that firms hire inputs up to the point where their marginal product justifies their cost. Now we look at how those input costs aggregate into the cost curves that govern output decisions. The key structural distinction is between inputs the firm can and cannot adjust in the short run.

Fixed costs (FC) are costs that don't change with output — you pay them whether you produce zero units or a million. Think of the rent on a factory, a piece of capital equipment under a long-term lease, or a salaried manager. These are locked in because the input cannot be adjusted quickly. Variable costs (VC) respond to output — more production requires more labor hours, more raw materials, more energy. Total cost (TC = FC + VC) rises with output because variable inputs rise, but the fixed component means TC is strictly positive even at zero output.

Dividing each cost component by quantity gives three average curves that reveal the firm's per-unit economics. Average fixed cost (AFC = FC/Q) falls continuously as output rises — the fixed overhead is spread over more units, a phenomenon called "spreading the overhead." Average variable cost (AVC = VC/Q) typically falls at first (due to increasing returns to labor — more workers can specialize) and then rises (as the factory becomes congested and workers get in each other's way — diminishing marginal returns). Average total cost (ATC = TC/Q = AFC + AVC) is the vertical sum: it inherits both the declining AFC and the U-shape of AVC, producing its own U-shape, but its minimum occurs at higher output than AVC's minimum because AFC is still declining.

Marginal cost (MC = ΔTC/ΔQ) is the most important cost concept for decisions. Because fixed costs don't change with output, ΔFC = 0, so MC = ΔTCC/ΔQ = ΔVC/ΔQ — marginal cost is purely determined by how variable costs change. The MC curve passes through the minimum points of both AVC and ATC — this is a mathematical necessity, not a coincidence. When MC is below average cost, producing more pulls the average down; when MC is above average cost, producing more pulls the average up; they must be equal at the average's minimum. This relationship between marginal and average is the cornerstone of all the cost-curve geometry you'll use when studying profit maximization and market structure.