Minimum Efficient Scale and Market Structure

Every firm's long run average cost curve falls as output rises, flattens out, then eventually rises again. The output where it stops falling and bottoms out is the minimum efficient scale. Below that output, a firm is too small to capture all the cost savings from spreading fixed costs and specializing labor and equipment, so its average cost per unit is higher than it has to be. Once a firm reaches minimum efficient scale, producing more adds little or no further savings.

What matters for market structure is minimum efficient scale relative to total market demand. If one firm only needs a small slice of total demand to reach its lowest average cost, the market has room for many firms operating efficiently side by side, each carving out a share. Haircuts work this way: a single barber or small shop hits efficient scale at a tiny fraction of a city's demand for haircuts, so thousands of independent shops can all produce at low average cost. Competition among them keeps prices near that low cost and keeps any one firm from dominating.

Push minimum efficient scale up toward the size of the entire market and the picture flips. If reaching lowest average cost requires an output close to or beyond what the whole market demands, only one firm can operate at that low cost. Smaller rivals stuck below efficient scale would have higher average costs and get undercut on price. This is exactly the cost structure behind a natural monopoly, and it is a real barrier to entry: a new entrant cannot profitably stay small while climbing toward efficient scale against an incumbent already producing there, which is why water and electric utilities tend to end up as one firm rather than many.