Long Period Equilibrium for Firms in Perfect Competition

A shop earning fat profits today has no guarantee those margins survive next year, especially in a market anyone can enter. That’s the entire story of a firm’s long period equilibrium under perfect competition: it’s the point where entry and exit stop happening because there’s simply no more profit left to chase. Understanding how firms reach this point tells you why prices in truly competitive markets keep drifting toward cost, and why “just enough to survive” becomes the permanent state of affairs for every firm in the industry.

What actually changes in the long period
From short period costs to long period costs
The three-way equality: LAC equals LMC equals price
Why marginal must equal average at the minimum point
Why that minimum point must also equal price
Why supernormal profits never survive in the long period
The mirror image: losses and exit
A concrete illustration
What long period equilibrium means for efficiency
Short period versus long period, side by side

What actually changes in the long period

In the short period, a firm is stuck with whatever plant, machinery, or shop size it already has. Some costs are fixed no matter how much or how little it produces. The long period removes that constraint entirely. A firm can expand its factory, downsize it, switch technology, or exit the industry altogether. Every single cost becomes variable because every input can now be adjusted.

This flexibility is also what allows entirely new firms to walk into the industry, and existing ones to walk out, whenever the price makes it worthwhile. That freedom of entry and exit is one of the defining features of a perfectly competitive market, alongside a large number of sellers offering a completely homogeneous product, which means no single firm can influence the price on its own.

From short period costs to long period costs

The long period average cost (LAC) and long period marginal cost (LMC) curves aren’t drawn from scratch. They’re built directly out of the family of short period average cost (SAC) curves that represent every possible plant size a firm could choose. For any output level, the firm picks whichever plant size lets it produce at the lowest possible cost. Plot the lowest cost achievable at every output level, and you get the LAC curve. Because of the way it wraps around and touches each short period curve at exactly one point, economists call it an envelope curve. The LMC curve is then derived mathematically from LAC, showing the extra cost of producing one more unit once the firm has already picked its optimal scale.

The three-way equality: LAC equals LMC equals price

A firm settles into long period equilibrium only when three things line up simultaneously: long period average cost equals long period marginal cost, and both equal the price the market is offering. This isn’t a coincidence built into the model. It’s the mechanical outcome of two separate forces meeting at one point.

Why marginal must equal average at the minimum point

Think about what marginal cost does to average cost. Whenever LMC sits below LAC, adding one more unit pulls the average down, so LAC keeps falling. Whenever LMC sits above LAC, that extra unit pulls the average up, so LAC keeps rising. The only place these two curves can actually cross is where LAC has stopped falling and hasn’t yet started rising, which is precisely its minimum point. This holds true regardless of market structure, which is why LAC and LMC always intersect at the lowest point on the LAC curve.

Why that minimum point must also equal price

Perfect competition adds a second condition on top of this. Since a single firm is too small to influence the market, it has to accept the price as given. Its demand curve is a flat horizontal line at the market price, meaning average revenue and marginal revenue are both equal to price at every unit sold. Profit maximisation requires marginal cost to equal marginal revenue, and in the long period, that marginal cost is LMC. Put both conditions together and you get the full equilibrium: price equals marginal revenue equals LMC equals LAC at its minimum point. A similar breakdown appears in teaching material prepared for commerce students, which lays out the same equality between short period marginal cost, long period marginal cost, price, average revenue, and both average cost curves at the point of long-run equilibrium for the firm.

Why supernormal profits never survive in the long period

Here’s the part that makes this concept genuinely useful rather than just a graph to memorise. If price happens to sit above LAC, every unit sold earns the firm more than its full cost of production, including a normal return on the owner’s own investment and effort. That gap is called supernormal profit, and it’s visible to everyone else in the market too.

Because entry is free, outsiders notice those profits and start setting up shop in the same industry. As more firms begin producing, total market supply rises. With demand unchanged, that extra supply pushes the price down. This isn’t a one-time adjustment; it continues as long as a profit incentive remains. The process only stops once price has fallen all the way down to the level where it just equals LAC again, at which point the profit motive for new entry disappears completely.

The mirror image: losses and exit

The same logic runs in reverse when the price falls below LAC. Firms making losses can’t sustain that indefinitely, so the weakest producers start shutting down and leaving the industry. As firms exit, total supply contracts, and with demand steady, the price starts climbing back up. Exit continues until the remaining firms are once again just covering their costs. Together, these two adjustment mechanisms mean the industry is self-correcting: any deviation from normal profit triggers a response that eventually restores it, whether the starting point was a boom or a slump.

A concrete illustration

Picture a cluster of small printing and photocopy shops near a college campus. When a new academic year begins and demand for course material printing spikes, existing shops might briefly charge prices well above their average cost, pocketing healthy profits. Those profits don’t stay secret. Within a few months, new shops open nearby, competing for the same footfall. As more shops enter, each one gets a smaller share of the total demand, and price competition intensifies until margins shrink back to just covering rent, staff wages, and equipment costs, nothing more.

Stage	Price vs LAC	What happens next
Short period	Price above LAC	Existing shops earn supernormal profit
Transition	New shops enter	Total supply rises, price starts falling
Long period equilibrium	Price equals LAC equals LMC	Only normal profit remains; entry stops

This is exactly why businesses in genuinely competitive, low-barrier markets rarely enjoy extraordinary profits for long. The very success that attracts customers also attracts competitors.

What long period equilibrium means for efficiency

This outcome isn’t just about firms breaking even; it also explains why perfectly competitive markets are held up as a benchmark for efficient resource use. Because every firm produces at the minimum point of its LAC curve, output is generated at the lowest possible cost per unit, a condition often called productive efficiency. And because price ends up equal to marginal cost, consumers are paying exactly what it costs society to produce one more unit, a condition known as allocative efficiency. Notes prepared for market structure teaching describe this same dual result, where firms in perfectly competitive industries such as agricultural commodities end up producing at minimum cost with no persistent excess return. It’s also why economists treat perfect competition as a useful reference point when judging how other, less competitive markets behave, since real-world industries with entry barriers, patents, or brand loyalty rarely reach this same zero-profit outcome. As one open economics resource summarises, a long-run equilibrium is reached once no new firm wants to enter and no existing firm wants to leave, because economic profits have been driven down to zero.

Short period versus long period, side by side

Feature	Short period	Long period
Fixed costs	Present	None; all costs are variable
Plant size	Fixed	Can be changed freely
Entry and exit	Not possible	Fully possible
Possible profit outcome	Supernormal profit, normal profit, or loss	Only normal profit
Equilibrium condition	Price equals SMC	Price equals LAC equals LMC (minimum LAC)

What do you think? If a market has strong entry barriers, such as heavy licensing requirements or huge upfront capital costs, does the long period equilibrium condition of “only normal profit” still hold? And can you think of an industry around you where firms seem to have been stuck earning above-normal profits for years, despite no legal restriction on new entrants?

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References

PDF 📄

Table of Contents

What actually changes in the long period 🔗

From short period costs to long period costs 🔗

The three-way equality: LAC equals LMC equals price 🔗

Why marginal must equal average at the minimum point 🔗

Why that minimum point must also equal price 🔗

Why supernormal profits never survive in the long period 🔗

The mirror image: losses and exit 🔗

A concrete illustration 🔗

What long period equilibrium means for efficiency 🔗

Short period versus long period, side by side 🔗