Long-run equilibrium combines three equalities

The constant-cost benchmark is price = marginal cost = minimum average total cost.

Each equality answers a different behavioral or efficiency question in the model.

P=MC gives allocative efficiency: buyers’ marginal willingness to pay equals marginal production cost. Production at minimum ATC gives productive efficiency. P=ATC gives zero economic profit. Normal profit remains included in cost, so owners receive a competitive return.

The equalities arise from different mechanisms. The firm chooses P=MC because it maximizes profit as a price taker. Entry and exit force P=ATC in long-run equilibrium. For the representative firm to satisfy both on standard U-shaped cost curves, MC meets ATC at its minimum. No single equality should be used as a substitute for the full reasoning.

A permanent quantity change with a temporary price change

Demand rises in a constant-cost industry. Short-run price and profit rise. Entry expands supply until price returns to the original minimum-ATC level. Long-run market quantity remains higher because more firms each produce the efficient firm output.

Suppose minimum ATC is $12 at 40 units per firm. Long-run equilibrium price is $12 and each firm produces 40. If market demand later supports 4,800 units at $12, the industry needs 120 identical firms. The market quantity and firm count adjust while the representative firm’s efficient scale remains 40.

Zero economic profit is compatible with positive revenue, accounting profit, and producer surplus. Revenue covers variable cost, fixed opportunity cost, owner labor, and the normal return on capital. “Zero” refers only to the excess after all economic costs.

The standard result assumes identical firms, free entry, and no externalities or market power. If firms have different costs, some may earn rents even in a competitive market. If entry raises input prices, the final price need not equal the original minimum ATC. That is the next topic.

On a graph, the horizontal price line is tangent to ATC at its minimum and intersects MC there. If price touches ATC elsewhere on a U-shaped curve, MC generally differs and the firm would choose another output. The three equalities identify the consistent long-run point.

Productive efficiency is the firm-level statement that average cost is minimized. Allocative efficiency is the market-level statement that buyer value equals marginal cost. Free entry connects them in the standard competitive model. If an externality is present, private P=MC can still miss social efficiency because the relevant curve is MSC.

If technology lowers every firm’s minimum ATC, the long-run zero-profit price also falls. Firms may earn temporary profit at the old price while entry and output adjust. Zero profit is an equilibrium condition, not a rule freezing price when demand or cost fundamentals change.

A permanent input-price increase similarly raises minimum ATC and the long-run break-even price, inducing exit during adjustment. The three equalities hold at the new cost curves, not necessarily at the old numerical price and output.

Equilibrium conditions travel with the curves when underlying costs change.