In Economics, distinction is often made between the short-run and long-run.

By short-run is meant that period of time within which a firm can vary its output by varying only the amount of variable factors, such as labour and raw material.

In the short-run period, the fixed factors such as capital equipment, management personnel, the factory buildings, etc., cannot be altered.

If, therefore, a firm wants to increase production in the short-run, it can do so only by hiring more workers or buying and using more raw materials. It cannot, in the short-run, enlarge the size of the existing plant or build a new plant of a bigger capacity. Thus, in the short-run, only variable factors can be varied, while the fixed factors remain the same.

On the other hand, long-run is a period of time during which the quantities of all factors, variable as well as fixed, can be adjusted. Thus, in the long-run, output can be increased by increasing capital equipment or by increasing the size of the existing plant or by installing

Short-Run Cost Curves

We may repeat that, in the short-run, a firm will adjust output to demand by varying the variable factors. If all the factors of production can be used in varying proportions, it means that the scale of operations of the firm can be changed. Each time, the scale of operations is changed, a new short-run cost curve will have to be drawn for the firm such as SAC’, SAC” and SAC” in the next diagram.

To begin with, let us suppose that the firm has the short-run cost curve SAC “. ,In this case, the optimum output will be OM’. Now, if it is desired to increase the output to OM” in the short-run, it can be obtained at the average cost M”L” along the short-run cost curve SAC”, because in the short-run, the scale of operations is fixed. But, in the long run, a new and bigger plant can be built on which OM ” is the optimum output. That is, the firm has now a short-run average cost curve SAC “‘, and by increasing the scale of its operations, the firm can produce the OM” output at a cost of M “L” ‘ instead of M “L”

Thus, it will be seen that, at any scale of operations in the short-run, a firm will have regions of rising and falling costs. But, in the long-run, the firm can produce on a completely different cost curves to the left (i.e., SAC’) or right (i.e., SAC”’) of the original cost curve (i.e., SAC”). For each different scale represented by a different short-run cost curve, there will be an output where the average cost is the minimum. This is the optimum output.

Long-Run Cost Curve

In the diagram, SAC,, SAC,, and SAC, are the short-run cost curves corresponding to the different scales of operations. In each case, the firm in question will be producing the desired output at the lowest cost. For example, OM”’ output is produced at PM”’ in the scale of operations represented by the curve SAC OM will be produced on SAC, and so on.

It should be clearly understood that only in the long-run can the scale of operations be altered; in the short-run, it will be fixed, and the average cost of output above or below the optimum level will necessarily rise along the short-run cost curve in question, whether it be SAC, SAC ₂ and SAC₃. A long-run average cost will show what the long-run cost of producing each output will be. It will be seen, in the Fig. that the short-run average cost curve SAC, has a lower minimum point than either the curves SAC, and SAC₃. The optimum output of the firm is obtained at OM.

The long-run average cost curve LAC is a tangent to all the short-run cost curves SAC, SAC₂ and SAC. The LAC curve will, therefore, be U-shaped like the short-run cost curves, but its U-shape will be less pronounced than that of the short-run cost curves. It will be flatter. That is why the long-run cost curve is called an ‘Envelope’, because it envelops all the short-run cost curves.

The cost curves, whether short-run or long-run, are U-shaped because the cost of production first starts falling as output is increased owing to the various economies of scale. But after touching the lowest point at the optimum output level, it starts rising, and goes on rising if production is continued beyond the optimum level.

This obviously makes a U-shape. But, as we have said already, the U-shape of the long-run cost curves is less pronounced. In other words, the long-run average costs are flatter than the short-run curves. The longer the period to which the curve relates, the less pronounced will be the U-shape of the cost curves. By the long period, we mean the period during which the size and organization of the firm can be altered to meet the changed conditions.

In the short-run, if output is reduced, average cost will rise because the fixed costs will work out at a higher figure. But, in the long-run, fixed costs can be reduced if the output is continued at the low level. Hence, average fixed cost will be lower in the long than in the short run.

The variable costs will not rise as sharply in the long-run as in the short-run, because in the long-run, the size of the firm can be increased to deal more economically with an increased output. Thus, LAC curves are flatter than the short-run cost curves, because, in the long-run, the average fixed cost will be lower, and variable costs will not rise to sharply as in the short period.

Why Are Long-Run Cost Curves U-Shaped?

We have explained above that the long-run cost-curves are U-Shaped. That is, as output is increased, the cost per unit falls, then it reaches a minimum after which it starts ascending so that it takes the shape of U. How do we account for this U-shape? The reason is that the cost curve falls on account of the various economies of scale. But when the firm has expanded too much, economies are changed into diseconomies and the cost curve starts rising.

Short-run Fixed and Variable Costs

We have already drawn a distinction between prime (or variable) costs and supplementary (or fixed) costs. During the short period, only the prime costs relating to labour and raw materials can be varied, whereas the fixed costs remain the same. But, during the long period, even the fixed costs relating to plant and machinery, staff salaries, etc., can be varied. That is, in the long run, all costs are variable, and no costs are fixed.