AP Microeconomics Short-Run Costs — AP Microeconomics
1. Core Definition of Short-Run Costs ★★☆☆☆ ⏱ 3 min
In microeconomics, short-run costs are all production costs a firm faces when at least one input (most often capital like factory space or equipment) is fixed and cannot be adjusted to change output. Unlike the long run where all inputs are variable, the short run's fixed input creates the split between fixed and variable costs that defines all short-run cost measures. This topic makes up roughly 20% of Unit 3, which accounts for 20-25% of the total AP exam score, appearing in both MCQs and FRQs.
2. Total and Average Short-Run Costs ★★☆☆☆ ⏱ 4 min
All short-run costs are split into two core categories: fixed costs and variable costs. Total fixed cost ($TFC$) is the cost of fixed inputs, constant at all output levels, including when output is zero. Firms incur $TFC$ even if they shut down temporarily in the short run. Total variable cost ($TVC$) is the cost of variable inputs (most often labor and raw materials), which increases with output because more output requires more variable inputs.
TC = TFC + TVC
Average costs are per-unit cost measures, used to compare cost per unit to the market price to calculate profit. The three average cost definitions are:
AFC = \frac{TFC}{Q} \quad \quad AVC = \frac{TVC}{Q} \quad \quad ATC = \frac{TC}{Q} = AFC + AVC
A key feature of average fixed cost is that it always falls as output increases, because the same total fixed cost is spread over more units of output. This causes the gap between $ATC$ and $AVC$ to shrink as output rises, since $ATC - AVC = AFC$.
Exam tip: If a question asks for total cost when output is zero, the answer is just total fixed cost—there is no variable cost when no output is produced.
3. Marginal Cost and Diminishing Marginal Returns ★★★☆☆ ⏱ 4 min
Marginal cost ($MC$) is the additional cost of producing one more unit of output. Since total fixed cost does not change with output, the change in total cost equals the change in total variable cost, so marginal cost can be calculated two equivalent ways:
MC = \frac{\Delta TC}{\Delta Q} = \frac{\Delta TVC}{\Delta Q}
Marginal cost is inversely related to the marginal product of the variable input ($MP_L$). Higher marginal product means each additional worker produces more output, so the marginal cost of that output is lower, given by the relationship $MC = w / MP_L$, where $w$ is the wage rate.
The law of diminishing marginal returns explains the U-shape of the short-run marginal cost curve. Diminishing marginal returns states that as more variable input is added to a fixed amount of capital, eventually the marginal product of the additional variable input falls. When marginal product falls, marginal cost rises. Initially, specialization of new workers increases marginal product, so MC falls, but once diminishing returns set in, MC rises, creating the U-shape.
Exam tip: When marginal cost is calculated for output increments larger than 1 unit, always divide by the change in quantity—never just report the change in total cost as MC. This is a common point deduction on FRQs.
4. Relationship Between Marginal Cost and Average Costs ★★★☆☆ ⏱ 3 min
A core relationship tested repeatedly on the AP exam is the interaction between marginal cost and average costs: if marginal cost is less than average cost, it pulls the average down; if marginal cost is greater than average cost, it pulls the average up. As a result, marginal cost intersects both AVC and ATC exactly at their minimum points.
This relationship is intuitive if you think of your GPA: your current GPA is your average grade, and your new (marginal) course grade changes your average. If your new grade is lower than your current average, your average falls; if it is higher, your average rises. Only when your new grade equals your current average does your average stay the same, which occurs at the minimum point of the average curve.
Exam tip: On graph-drawing FRQs, AP graders require you to draw MC crossing ATC and AVC exactly at their minimum points to earn full credit. Drawing the intersection anywhere else loses points.
5. AP-Style Concept Check ★★★☆☆ ⏱ 3 min
Common Pitfalls
Why: Students memorize that AFC always falls and incorrectly extend this pattern to total fixed cost
Why: Mixes up average total cost and marginal cost definitions, since both use total cost
Why: Memorizes 'MC crosses average at minimum' but mixes up which average
Why: Forgets that fixed costs are sunk and do not change with output in the short run
Why: Confuses short-run diminishing returns with long-run scale concepts