Fixed Costs vs Average Fixed Cost
Fixed Costs and Average Fixed Cost are two Production & Costs concepts in AP Economics that students often mix up. Fixed Costs are costs that do not change with the level of output in the short run. Average Fixed Cost is the fixed cost per unit of output produced. Here is how they compare side by side.
These include expenses like rent, insurance, or salaries for permanent staff that must be paid even if production is zero. Fixed costs are unavoidable in the short run regardless of output levels.
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.
Fixed Cost vs Average Fixed Cost: The Total Stays Put, the Per-Unit Figure Falls
| Fixed Cost (FC) | Average Fixed Cost (AFC) | |
|---|---|---|
| What it measures | The whole short run bill for inputs that do not vary with output | That same bill divided by the number of units produced |
| Formula | FC = TC minus VC | AFC = FC divided by Q |
| As output rises | Stays at exactly the same dollar figure | Falls at every quantity and never turns back up |
| Shape on a cost diagram | A horizontal line at the level of the fixed bill | A downward sloping curve that flattens toward the horizontal axis |
| At zero output | Still the full fixed bill, owed anyway | Undefined, because nothing can be divided by zero units |
| Link to the other curves | Sets the vertical gap between the TC and VC curves | Equals the vertical gap between ATC and AVC at each quantity |
The numerator never moves, so the denominator writes the whole curve
Picture a small print shop with a short run fixed bill of 60 dollars a day for rent and a leased press. That 60 dollars is owed whether the shop prints nothing or prints six jobs, so fixed cost plots as a flat line. Average fixed cost is the same 60 dollars shared out across whatever the shop actually produces. At one job a day AFC is 60 dollars. At two jobs it is 30, at three it is 20, at four it is 15, at five it is 12, and at six it is 10. Nothing about the rent changed across that whole column; only the number of units sharing it did. Because output can keep climbing while the numerator sits still, AFC keeps shrinking and edges toward zero without ever arriving. Managers call this spreading the overhead. Notice what the numbers never do: they never turn back up. Every other average cost curve in the short run is U shaped, so a sketch showing AFC with a rising right hand side has drawn something arithmetic will not allow. You can run the same division for any fixed bill at /calculate/average-fixed-cost. All figures here are illustrative rather than typical of any real firm.
A falling AFC is why average total cost drops steeply at low output
Average total cost is the sum of average fixed cost and average variable cost at each quantity, so the vertical distance between the ATC curve and the AVC curve is exactly AFC. That gap is wide on the left of the diagram and narrows as you move right, which is why ATC and AVC squeeze together at high output without ever meeting. The same fact explains the steep early fall in ATC. At low quantities the drop in AFC is large enough to swamp any rise in AVC, so ATC comes down quickly. Further right the fixed bill is already thinly spread, each extra unit shaves only a few cents off AFC, and rising AVC finally wins, which is where ATC turns up. One warning for free response questions: a falling AFC is not economies of scale. Economies of scale describe a long run comparison between different plant sizes, while AFC falls inside one unchanged plant over a single short run period. See /glossary/average-total-cost for how the two averages stack, and /micro/production-costs for the full family of short run curves and how they are drawn together.
Frequently asked questions
What is the difference between fixed cost and average fixed cost?
Fixed cost is the total dollar amount a firm owes for inputs that do not change with output, while average fixed cost is that total divided by the quantity produced. The total holds steady as output rises and the average falls, so the two are equal only at an output of exactly one unit.
Why does average fixed cost keep falling?
Because a constant amount is being divided by a larger and larger quantity. Each additional unit takes a share of the same unchanged bill, so the amount charged against any single unit shrinks, and the curve approaches the horizontal axis without touching it.
How do you find average fixed cost when a table only gives ATC and AVC?
Subtract average variable cost from average total cost at that quantity, since ATC equals AFC plus AVC. If ATC is 49 dollars and AVC is 34 dollars at four units, then AFC is 15 dollars, which also tells you the fixed bill is 60 dollars.
Live Production Costs graph. Drag the curves, or open the full version.
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