Long Run Cost Curves: Planning and Production Efficiency

Every business owner eventually asks the same question: should I stick with my current setup, or is it time to build a bigger factory, hire more staff, or invest in new machinery? That decision sits at the heart of long run cost analysis. Unlike the short run, where at least one input like factory size or machinery is fixed, the long run treats every single input as variable. This gives firms the freedom to redesign their entire production process, and long run cost curves are the tool economists use to map out what that freedom actually costs.

Why the long run is a different planning horizon 🔗

In the short run, a firm is stuck with whatever plant size it already has. If demand rises, it can only add more labour or raw material to a fixed set of machines, which is why short run costs eventually rise sharply due to diminishing returns. The long run removes that constraint. A firm can expand its factory, buy new equipment, or shut down entirely and start fresh elsewhere.

Because every input is adjustable, the long run is essentially a planning period. A firm is not just reacting to today’s demand, it is deciding what scale of operation will serve it best going forward. This is why the long run average cost curve is sometimes called a planning curve. It does not describe what a firm is currently doing, it describes the cheapest way to produce at each possible output level if the firm were starting from scratch.

How the long run average cost curve is built 🔗

The long run average cost curve, or LRAC, is not drawn independently. It is derived directly from a series of short run average cost curves, each one representing a different possible plant size. Imagine a firm that could build a small plant, a medium plant, or a large plant. Each of these choices has its own short run average cost curve, U-shaped for the usual reasons of fixed capacity and diminishing returns.

For any output level, the firm will naturally pick whichever plant size produces that output at the lowest cost. When you trace the lowest point achievable across all these short run curves for every output level, you get the long run average cost curve. This is why the LRAC is frequently called an envelope curve, it wraps around and touches each short run curve at exactly one point, without necessarily touching any of them at their own minimum.

The envelope is not a simple sum of minimum points 🔗

A common misunderstanding is that the LRAC is formed by joining the lowest points of every short run curve. That is not quite accurate. On the downward-sloping portion of the LRAC, the tangency point with a short run curve lies to the left of that curve’s own minimum. On the upward-sloping portion, it lies to the right. Only at the very bottom of the LRAC does the tangency coincide with the minimum point of a short run curve. This detail matters because it shows that a firm choosing a plant size for a given output is not always operating that plant at its own most efficient point, it is operating it at whichever point is cheapest across all its options.

Why the LRAC curve is U-shaped 🔗

Just like its short run counterpart, the long run average cost curve is typically U-shaped, but the reasons behind the shape are completely different. In the short run, the U-shape comes from diminishing returns to a fixed factor. In the long run, since every factor can change, the shape comes from economies and diseconomies of scale.

Economies of scale 🔗

As a firm expands from a small operation, average costs usually fall. This happens because larger firms can divide tasks among specialised workers, negotiate better rates for bulk raw material purchases, spread fixed costs like machinery and administration over more units, and use larger, more efficient equipment. A textile unit that buys yarn in bulk pays a lower price per kilogram than a small tailoring shop buying in small lots, and that saving shows up directly in average cost.

Constant returns to scale 🔗

At some point, expansion stops producing extra savings. Average cost flattens out over a range of output. The firm has already captured the major efficiencies available from scaling up, and doubling inputs simply doubles output without changing the cost per unit.

Diseconomies of scale 🔗

Beyond a certain size, growing further can actually push average costs up. Large organisations often struggle with coordination, communication breaks down across departments, decision-making slows, and monitoring a sprawling workforce becomes expensive. A firm that has grown too large may find that management overheads and internal inefficiencies outweigh any remaining benefits of scale.

The point of lowest cost: minimum efficient scale 🔗

The output level at which the LRAC curve reaches its lowest point is known as the minimum efficient scale, often shortened to MES. This is the scale at which a firm has captured all available economies of scale without yet running into diseconomies. Producing below this level means the firm is leaving cost savings on the table, while producing beyond it means average costs start climbing again.

The size of the minimum efficient scale varies enormously across industries. Heavy manufacturing, such as automobile or steel production, typically needs a very large minimum efficient scale because of the massive upfront investment in plant and machinery. Service-based businesses, by contrast, often reach their most efficient scale at a much smaller size. This difference explains why some industries are dominated by a handful of large players while others have room for thousands of small firms competing side by side, as explained by the Corporate Finance Institute.

This concept has direct relevance in India, where micro, small, and medium enterprises play an outsized role in the economy. According to recent data, MSMEs contribute close to a third of India’s GDP and account for a similarly large share of manufacturing output. Many of these units operate below the minimum efficient scale of larger industries, which is exactly why access to cheaper credit, shared infrastructure, and cluster-based manufacturing zones matters so much for their competitiveness.

The long run marginal cost curve 🔗

The long run marginal cost curve, or LRMC, measures the change in total cost from producing one more unit when all inputs are variable. Like LRAC, it is typically U-shaped, though it tends to fall and rise more steeply than the average cost curve.

The relationship between LRMC and LRAC follows the same logic that connects any marginal and average curve. When LRMC is below LRAC, average cost is being pulled down, so the LRAC curve slopes downward. When LRMC is above LRAC, average cost is being pulled up, so LRAC slopes upward. The two curves meet at exactly one point, and that point is the minimum of the LRAC curve. This is a purely mathematical relationship: whenever the marginal value sits below the average, it drags the average down, and whenever it sits above, it pulls the average up. The only place the two can be equal is at the average curve’s turning point, as summarised in this standard treatment of short run and long run costs.

Why this matters for real production decisions 🔗

Understanding long run cost curves is not just an academic exercise. Firms use this framework to decide capacity before construction even begins. A bakery deciding between a single large central kitchen versus several smaller neighbourhood kitchens is effectively choosing a point on its long run cost curve. A textile exporter deciding whether to invest in automated looms is doing the same thing, comparing the long run average cost of the current scale against a larger one.

These decisions are rarely reversible in the short term, which is why the planning aspect of the long run curve is so important. Once a firm commits to a particular plant size, it is locked into the corresponding short run cost curve until it chooses to expand or contract again. Getting the scale decision right the first time avoids costly restructuring later.

What do you think? If you were advising a small manufacturing firm in India on whether to expand its plant, what signals would you look for to know whether it is still capturing economies of scale or approaching diseconomies? Can you think of an industry around you where the minimum efficient scale seems unusually high or unusually low, and what that tells you about how many firms compete in it?

References

1. https://ecampusontario.pressbooks.pub/uvicmicroeconomics/chapter/7-4-the-structure-of-costs-in-the-long-run/
2. https://www.open.edu/openlearn/society-politics-law/sociology/innovation-markets-and-industrial-change/content-section-3.3
3. https://en.wikipedia.org/wiki/Minimum_efficient_scale
4. https://corporatefinanceinstitute.com/resources/accounting/minimum-efficient-scale-mes
5. https://www.drishtiias.com/daily-updates/daily-news-editorials/scaling-indias-msme-growth
6. https://courses.lumenlearning.com/suny-microeconomics/chapter/short-run-vs-long-run-costs/