A constant-cost industry is one where input prices stay flat as the industry expands or contracts, so every firm faces the same production costs in the long run and the market price returns to the same level after any shift in demand. Its long-run supply curve is a horizontal line at the minimum average total cost. The model sits inside perfect competition and explains why some markets can absorb big swings in demand without any lasting price change.
Why Input Prices Stay Flat
The whole mechanism rests on resource abundance relative to the size of the industry. Firms draw on inputs that the broader economy supplies in such large quantities that the industry’s own hiring and purchasing barely registers. Scale up, and wages and material prices don’t climb. Scale down, and freed-up resources get absorbed elsewhere without prices falling.
Three conditions have to hold:
- The resources used are general-purpose rather than specialized. An industry that depends on a rare mineral or a narrow pool of highly trained workers will bid up those scarce inputs as it grows.
- The industry represents a small share of total demand for the resources it uses. A local house-cleaning market, for instance, draws from a vast pool of general labor and widely available supplies.
- Firms can enter and exit freely. No patents, exclusive licenses, or massive startup capital stand in the way.
Free entry and exit is what makes the market self-correcting. Flat input prices are what make the correction land at the original price every time.
What Happens When Demand Rises
When consumer demand climbs, the market price initially rises above the point where firms were breaking even. Existing firms start earning short-run economic profits because price now exceeds average total cost. Those profits act as a signal, and with low barriers to entry, new firms begin producing.
As new firms add output, total supply grows and price drifts back down. Entry continues until price falls all the way to the minimum average total cost, the break-even point where economic profit is zero. At that point, no further incentive exists to enter.
The constant-cost detail is what doesn’t happen during this expansion: input prices don’t move. The flood of new firms doesn’t bid up wages or material costs, because the industry is too small relative to its resource markets to shift them. Every firm, old and new, faces the same cost curves as before. The industry has absorbed the demand increase entirely through more firms producing at the original price.
What Happens When Demand Falls
The reverse plays out when demand drops. Price falls below average total cost, firms start losing money, and some exit. As they leave, supply shrinks and price climbs back toward the break-even level. Once price equals minimum average total cost again, the surviving firms earn zero economic profit and the exits stop.
The industry is now smaller, but each remaining firm operates at the same cost and charges the same price it did before demand fell. The long-run adjustment happens entirely through the number of firms, not through changes in price or per-unit costs.
Zero Economic Profit, Explained Briefly
Zero economic profit doesn’t mean firms aren’t making money. Economic profit subtracts both explicit costs (wages, rent, materials) and implicit costs, which represent the opportunity cost of using resources here instead of somewhere else. A firm earning zero economic profit is covering all its bills and fully compensating its owners for what they could earn elsewhere. Accounting profit, which only subtracts explicit costs, will typically show a positive number in the same situation. Firms in long-run equilibrium stay in business indefinitely; they just aren’t earning anything above what their resources could earn in the next-best use.
The Horizontal Long-Run Supply Curve
The signature visual is a flat line at the price equal to minimum average total cost. The industry can deliver any quantity at that single price, whether demand is low or high. The logic is straightforward: if input prices never change, the minimum cost of producing a unit never changes, so the break-even price never changes either.
How It Compares to Increasing-Cost and Decreasing-Cost Industries
The three industry types share the same adjustment mechanism. Firms enter when profits appear, exit when losses persist, and the market self-corrects toward zero economic profit. What differs is what happens to input prices along the way.
- In an increasing-cost industry, new firms compete for inputs that are limited or specialized, pushing wages and material prices higher. Every firm’s cost curves shift up, and the new long-run equilibrium settles at a higher price. The long-run supply curve slopes upward. Most real industries fit this category at sufficient scale, once resource scarcity kicks in.
- In a decreasing-cost industry, expansion actually lowers input costs, often because suppliers achieve economies of scale when the industry grows. New entry leads to a lower long-run equilibrium price, and the supply curve slopes downward. This is the rarest type.
- In a constant-cost industry, input prices don’t budge in either direction. Entry restores the original price, exit restores the original price, and the supply curve stays flat.
Examples That Approximate the Model
Perfectly constant costs are a theoretical ideal, but some industries come close enough to illustrate it. Pencil manufacturing is commonly cited: wood, graphite, and rubber are globally abundant and the industry’s demand for them is tiny relative to total supply. Domain name registration is another, with standardized server capacity and administrative processing available at scale regardless of how many registrars enter. Small-scale agricultural products like rutabagas, where the land and labor are general-purpose and the crop is a negligible share of total farming, also fit reasonably well.
The common thread is that no single input is scarce enough for the industry’s growth to create a bidding war. Once an industry starts relying on something hard to find or slow to produce, it drifts toward increasing-cost territory. That’s why the constant-cost model works best as a starting point for understanding competitive adjustment, before the complications of resource scarcity get added back in.