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In general, for each output level there is some choice of fixed cost that minimizes
The long-run average total cost
the firm’s average total cost for that output level. So when the firm has a desired out-
curve shows the relationship between
put level that it expects to maintain over time, it should choose the optimal fixed cost output and average total cost when
for that level—that is, the level of fixed cost that minimizes its average total cost. fixed cost has been chosen to minimize
Now that we are studying a situation in which fixed cost can change, we need to take average total cost for each level of output.
time into account when discussing average total cost. All of the average total cost curves
we have considered until now are defined for a given level of fixed cost—that is, they are
defined for the short run, the period of time over which fixed cost doesn’t vary. To rein-
force that distinction, for the rest of this module we will refer to these average total cost
curves as “short-run average total cost curves.”
For most firms, it is realistic to assume that there are many possible choices of fixed
cost, not just two. The implication: for such a firm, many possible short-run average Section 10 Behind the Supply Curve: Profit, Production, and Costs
total cost curves will exist, each corresponding to a different choice of fixed cost and so
giving rise to what is called a firm’s “family” of short-run average total cost curves.
At any given time, a firm will find itself on one of its short-run cost curves, the one cor-
responding to its current level of fixed cost; a change in output will cause it to move along
that curve. If the firm expects that change in output level to be long-standing, then it is
likely that the firm’s current level of fixed cost is no longer optimal. Given sufficient time,
it will want to adjust its fixed cost to a new level that minimizes average total cost for its
new output level. For example, if Selena had been producing 2 cases of salsa per day with a
fixed cost of $108 but found herself increasing her output to 8 cases per day for the fore-
seeable future, then in the long run she should purchase more equipment and increase her
fixed cost to a level that minimizes average total cost at the 8-cases-per-day output level.
Suppose we do a thought experiment and calculate the lowest possible average total
cost that can be achieved for each output level if the firm were to choose its fixed cost
for each output level. Economists have given this thought experiment a name: the long-
run average total cost curve. Specifically, the long-run average total cost curve, or
LRATC, is the relationship between output and average total cost when fixed cost has
been chosen to minimize average total cost for each level of output. If there are many pos-
sible choices of fixed cost, the long-run average total cost curve will have the familiar,
smooth U shape, as shown by LRATC in Figure 56.2.

figure 56.2

Short-Run and Long-Run Cost of
case
Average Total Cost Curves
Short-run and long-run average total cost Economies of scale Diseconomies of scale
curves differ because a firm can choose its
fixed cost in the long run. If Selena has chosen
ATC 3 ATC 6 ATC 9 LRATC
the level of fixed cost that minimizes short-run
average total cost at an output of 6 cases, and
actually produces 6 cases, then she will be at B Y
point C on LRATC and ATC 6 . But if she produces
only 3 cases, she will move to point B. If she
expects to produce only 3 cases for a long time,
in the long run she will reduce her fixed cost
and move to point A on ATC 3 . Likewise, if she
produces 9 cases (putting her at point Y ) and
expects to continue this for a long time, she will
increase her fixed cost in the long run and A X
move to point X. C

0 3 4 5 6 7 8 9 Quantity of
salsa (cases)

module 56 Long-Run Costs and Economies of Scale 561