Sunk Cost vs Fixed Costs
Sunk Cost and Fixed Costs are two Production & Costs concepts in AP Economics that students often mix up. A sunk cost is a cost that has already been incurred and cannot be recovered. Fixed Costs are costs that do not change with the level of output in the short run. Here is how they compare side by side.
Sunk costs should not influence future economic decisions because they are irreversible. Rational firms ignore sunk costs when deciding whether to continue production or shut down, focusing instead on marginal costs and revenues.
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.
Sunk Cost vs Fixed Cost: Recoverability Is the Line, Not Output
| Sunk Cost | Fixed Costs | |
|---|---|---|
| The test that defines it | Can the money be recovered? If not, the cost is sunk | Does the amount change when output changes? If not, the cost is fixed |
| Time frame it refers to | The past, money already committed | The short run, the period in which at least one input cannot be varied |
| Effect on the next decision | None, a rational firm leaves it out of every forward calculation | None for the output choice, since it never enters marginal cost, but it does set profit and drive long run exit |
| Where it appears on a cost diagram | Nowhere, sunkness is not a shape on the graph | The vertical gap between ATC and AVC, which equals AFC and narrows as output rises |
| Can one be the other? | A fixed cost that cannot be resold or subleased is also sunk, and so is a non-returnable order of a variable input | A fixed cost that can be resold or subleased is fixed but not sunk |
| In the long run | Past spending stays spent, but no new commitment has to be renewed | No costs are fixed, every input including plant size becomes variable |
The two overlap heavily, and the exceptions are where the points live
Rent on a warehouse for a year is a fixed cost, since it does not vary with how much the firm produces. Whether it is also sunk depends on the lease. If the firm can sublease the space and recover the payments, the cost is fixed but not sunk, and stepping away costs nothing. If the lease forbids subleasing, the identical payment is both fixed and sunk. Run the logic the other direction and the overlap breaks again. A batch of custom printed packaging already ordered and not returnable is sunk, but packaging is a variable cost, because the firm orders more of it when it produces more. So neither term contains the other. Fixed answers a question about output, and sunk answers a question about recovery. The distinction earns its keep because only sunkness justifies ignoring a cost. A recoverable fixed cost is a live choice, and the firm should weigh keeping the asset against selling it. A sunk cost is settled, and the only rational response is to leave it out of every calculation from this moment forward.
The shutdown rule uses AVC precisely because fixed costs are sunk in the short run
A firm carries a fixed cost of 40 that it cannot recover this period. It can produce 10 units at an average variable cost of 6, and the market price is 7. Revenue is 70 and variable cost is 60, so operating leaves 10 dollars of contribution toward the fixed 40, and the loss is 30. Shutting down brings in no revenue and incurs no variable cost, but the 40 is owed regardless, so the loss is the full 40. Operating at a loss is the better of two bad outcomes, and that is why the short run test compares price to average variable cost rather than average total cost. Change the price to 5 and the arithmetic flips. Revenue is 50 against variable cost of 60, so operating loses 10 on top of the 40 for a total of 50, and shutting down is now better. Notice that the fixed cost never appears in the comparison itself. Being sunk is exactly what removes it from the decision.
A lump sum fee changes profit without changing the quantity produced
Suppose a city charges every restaurant a flat annual license fee of 90 dollars, payable whether the kitchen serves one meal or many. That fee is a fixed cost, and once paid it is sunk. It raises total cost by 90 at every output level, so average total cost rises by 90 divided by quantity. Marginal cost does not move at all, because producing one more meal costs precisely what it did before. Since the firm sets output where marginal revenue equals marginal cost, the profit maximizing quantity is unchanged in the short run, and profit simply falls by the full 90. This is a favorite exam setup, usually dressed as a lump sum tax or a franchise fee, and a full-credit answer has two halves: quantity and price stay the same in the short run, and profit falls by the amount of the fee. Contrast that with a per unit tax, which does shift marginal cost upward, raises price, and cuts quantity. In the long run the fee still bites, since a firm whose profit before the fee falls short of 90 will drop the licence rather than renew it.
Frequently asked questions
Are all fixed costs sunk costs?
Fixed costs are sunk only when the money cannot be recovered. A machine bought for 90 dollars that can be resold for 70 is a fixed cost with just 20 dollars sunk, since that 20 is the portion the firm can never get back. Rent under a lease that permits subleasing is fixed and not sunk at all. The two labels answer different questions: fixed asks whether the cost varies with output, while sunk asks whether the money is retrievable. A question that calls a cost fixed has not yet told you whether it is sunk.
Why should a firm ignore sunk costs when deciding whether to keep operating?
Sunk costs are identical under every option on the table, so they cannot change which option is better. A firm that already spent 40 on non-refundable equipment loses that 40 whether it produces or shuts down, so the real comparison narrows to revenue against variable cost. Adding the sunk amount to both sides of a comparison just inserts the same number twice and cancels out. The behavioral failure to do this carries a name, the sunk cost fallacy, and it shows up as continuing a losing project because of what has already been spent on it.
Does anything become unsunk in the long run?
A sunk cost stays sunk in the long run, since money already spent cannot be unspent, but the long run means every input can be varied, so the firm faces no fixed costs going forward and can exit without owing anything new. The correct long run test is price against average total cost. If price sits below minimum average total cost, the firm exits, because it no longer has to renew leases or replace equipment. The short run test, price against average variable cost, applies only while the existing fixed commitments are still binding.
Live Production Costs graph. Drag the curves, or open the full version.
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