Average Fixed Cost vs Law of Diminishing Marginal Returns
Average Fixed Cost and Law of Diminishing Marginal Returns 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. The law of diminishing marginal returns states that adding more of a variable input to fixed inputs eventually yields smaller increases in output. 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.
As a firm adds workers to a fixed amount of capital, marginal product may rise at first but eventually falls. This causes marginal cost to rise, shaping the upward-sloping part of the cost curves. It applies only in the short run, when at least one input is fixed.
AFC vs Diminishing Marginal Returns: The Two Forces Inside a U
| Average Fixed Cost | Law of Diminishing Marginal Returns | |
|---|---|---|
| Kind of statement | Arithmetic; a constant divided by a rising number | Technological; a claim about how output answers inputs |
| Push on average total cost | Downward, at every output | Upward, once it sets in |
| Depends on the production function | No; two firms with equal fixed costs share one AFC curve | Yes; it is a property of that function |
| Where you read it | The AFC curve, falling toward the axis | The marginal product column turning down, and MC turning up |
| Strength as output grows | Weakens; each extra unit spreads a little less | Strengthens; each extra worker adds a little less |
| In the long run | Absent, since no cost is fixed | Not defined, since it needs a fixed input |
| Effect of doubling total fixed cost | AFC doubles at every quantity | Unchanged; technology was not touched |
The U in average total cost is a tug of war you can watch in the numbers
Average total cost turns up at the output where the rise in average variable cost finally beats the fall in average fixed cost, and diminishing returns starts well before that moment. Take fixed costs of 60 and workers paid 40 each. One worker makes 10 units, two make 24, three make 33 and four make 38, so marginal product runs 10, 14, 9 and 5, and diminishing marginal returns sets in with the third worker. Average fixed cost across those outputs is 6, then 2.50, then about 1.82, then about 1.58, falling the whole way. Average variable cost is 4, then about 3.33, then about 3.64, then about 4.21, turning up after the second worker. Between 24 and 33 units, average fixed cost drops by about 0.68 while average variable cost rises by about 0.30, so average total cost still falls, from about 5.83 to about 5.45. Between 33 and 38 units the drop in average fixed cost shrinks to about 0.24 while average variable cost climbs about 0.57, and average total cost rises to about 5.79. The curve turned when the tug of war changed sides, not when returns began diminishing.
Both effects come from the same fixed input, and neither one causes the other
A single fixed machine produces both, which is why students so often fuse them into one sentence. Because the machine's cost does not move with output, dividing it across more units shrinks the cost per unit, and that is pure arithmetic: it would hold for a brilliantly run firm and a badly run one alike. Because the machine cannot be duplicated in the short run, each extra worker has less of it to work with and adds less than the worker before, and that is a claim about technology which could fail at low staffing levels. Test the independence directly. Give two firms the same fixed cost of 60 and completely different production functions: their average fixed cost curves match at every quantity while their average variable cost curves sit nowhere near each other. Writing that average fixed cost falls because of diminishing returns, or that diminishing returns pushes average fixed cost up, is marked wrong for that reason. Marginal cost is where the technological force lands, since it rises exactly when marginal product falls and contains no fixed cost at all, as worked at /calculate/marginal-cost.
Frequently asked questions
Does average fixed cost ever rise?
No, not as output increases. Dividing a constant total fixed cost by a larger quantity always returns a smaller number, so the curve falls throughout and flattens without reaching the axis. The one way to see a higher average fixed cost at the same output is for total fixed cost itself to change, such as a rent increase, which lifts the whole curve to a new position rather than bending the old one upward.
Why does average total cost keep falling after diminishing returns begins?
Because the two forces pull opposite ways and the fixed-cost force is still the stronger one. In the figures above, diminishing marginal returns starts with the third worker, yet moving from 24 to 33 units cuts average fixed cost by about 0.68 while average variable cost rises only about 0.30, so average total cost falls about 0.38. Only once the saving on average fixed cost drops below the increase in average variable cost does the curve turn up.
If fixed costs are larger, does the minimum of average total cost move?
Yes, it moves to a higher output. A bigger total fixed cost means average fixed cost keeps falling by more at each quantity, so it offsets rising average variable cost for longer. Doubling fixed cost in the example above from 60 to 120 shrinks the increase in average total cost between 33 and 38 units from about 0.34 to about 0.10, pushing the turning point to the right. Marginal cost and average variable cost do not move at all.
Live Production Costs graph. Drag the curves, or open the full version.
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