Average Fixed Cost vs Average Total Cost
Average Fixed Cost and Average Total Cost are two Production & Costs concepts in AP Economics that students often mix up. Average Fixed Cost is the fixed cost per unit of output produced. Average Total Cost is the total cost per unit of output produced. Here is how they compare side by side.
It is found by dividing total fixed cost by quantity of output. Since fixed costs do not change with output, average fixed cost continuously declines as output increases.
It is found by dividing total cost by the quantity of output. Average total cost includes both average fixed and average variable costs and typically forms a U-shaped curve due to spreading fixed costs and diminishing returns.
Average Fixed Cost vs Average Total Cost: What Each Curve Does as Output Grows
| Average Fixed Cost | Average Total Cost | |
|---|---|---|
| What sits in the numerator | Total fixed cost only | Fixed and variable cost together |
| Shape of the curve | Falls at every output, flattening toward the axis | U-shaped, falling and then rising |
| Minimum point | None at any positive quantity | At the break-even output, where marginal cost crosses it |
| Multiply it by quantity and you get | The same total fixed cost at every output | Total cost at that output |
| Effect of diminishing returns | None, it keeps falling regardless | Pulls it upward once average variable cost climbs fast enough |
| Decision it feeds | None, it explains a shape rather than a choice | Profit per unit, break-even price, long-run entry and exit |
Average fixed cost has no minimum, so nothing meaningful crosses it
Every other average curve on the short-run diagram has a bottom. Average total cost and average variable cost each fall, reach a minimum and rise, and marginal cost passes through both of those minimum points. Average fixed cost has no bottom at all. Divide an unchanging number by a larger and larger quantity and the answer keeps shrinking without ever turning around, so the curve slides toward the horizontal axis and never reaches it. Marginal cost may well cut the average fixed cost curve somewhere on a hand-drawn diagram, and that crossing carries no meaning. Two drawing errors follow from missing this. The first is sketching average fixed cost as a shallow U, which no cost schedule can produce. The second is letting average total cost and average variable cost meet at high output. The vertical distance between those two curves is exactly average fixed cost, so in the schedule above the gap is 30 at 2 units, 15 at 4 units, 10 at 6 units and 7.5 at 8 units. The gap narrows steadily, which is why the curves look like they are converging, but it stays positive at every finite quantity and the two curves never touch.
Only average total cost answers a question the exam actually asks
Average total cost carries decisions. Profit per unit is price minus average total cost, total profit is that gap multiplied by quantity, and the break-even price is the minimum of average total cost, which is also the long-run equilibrium price under perfect competition. Average fixed cost appears in no decision rule at all. The short-run shutdown test compares price with average variable cost, the break-even test compares price with average total cost, and neither asks about fixed cost, because fixed cost is owed whether the firm produces or not. Put a firm facing a price of 29 into the schedule above. At 6 units average total cost is 28, so it earns 1 per unit and 6 in total. Its average fixed cost of 10 says nothing about whether to produce, and a student who lines the price of 29 up against an average fixed cost of 10 and concludes the firm is thriving has answered a question nobody asked. The one job average fixed cost does have is explanatory: when a question asks why average total cost falls so steeply at low output, the answer is that a fixed cost is being spread over very few units. Drill both at /calculate/average-fixed-cost and /calculate/average-total-cost.
Frequently asked questions
What is the difference between average fixed cost and average total cost?
Average fixed cost divides only the firm's fixed cost by output, while average total cost divides every cost, fixed and variable, by output. The two are linked because average total cost equals average fixed cost plus average variable cost. Average fixed cost falls at every output without exception, and average total cost falls, reaches a minimum and then rises once diminishing returns push the variable side up.
Why does average total cost rise when average fixed cost is still falling?
Average total cost rises once average variable cost climbs faster than average fixed cost falls. With a fixed cost of 60, moving from 6 units to 8 units cuts average fixed cost by 2.5 while average variable cost rises by 5, so average total cost increases by 2.5. Diminishing marginal returns drive the variable side up, and past a point that increase outweighs the shrinking fixed-cost share.
Does average fixed cost have a minimum point?
Average fixed cost has no minimum at any positive output. Dividing an unchanged total fixed cost by a larger quantity always gives a smaller answer, so the curve falls continuously and approaches the horizontal axis without ever touching it. Marginal cost therefore never cuts average fixed cost at a meaningful point, unlike average total cost and average variable cost, which marginal cost crosses at their minimums.
Live Production Costs graph. Drag the curves, or open the full version.
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