What does P = MC mean in economics?

P = MC has two readings, and AP Microeconomics uses both. The first is allocative efficiency. Price measures how much buyers value the last unit (the height of the demand curve), and marginal cost measures what society gives up to make it. If a buyer will pay 4 dollars for one more loaf of bread that costs a bakery 2.50 dollars to make, producing that loaf creates 1.50 dollars of value nobody is collecting, so output should rise. If the next loaf would cost 5 dollars to make but buyers value it at 4 dollars, it should not be made. Only where P = MC is every unit worth making produced and no unit worth less than its cost produced, so total surplus is as large as it can be and there is no deadweight loss. That is the definition of allocative efficiency.

The second reading is the profit rule for a perfectly competitive firm. Every firm maximizes profit where marginal revenue equals marginal cost (the general rule, covered in the MR = MC guide). A competitive firm is a price taker, so each extra unit adds exactly the market price to revenue and MR = P. The rule becomes P = MC. Say the market price is 10 dollars and the marginal cost of units 1 through 6 is 4, 5, 7, 10, 13, and 17 dollars. The firm makes 4 units: the 4th unit costs 10 and sells for 10, while the 5th would cost 13 and sell for only 10, losing 3 dollars. Because the competitive firm always produces where its MC curve meets the price, its MC curve above average variable cost is its supply curve.

P > MC means the firm has market power. A monopoly, an oligopoly, or a monopolistically competitive firm faces a downward-sloping demand curve, so marginal revenue sits below price. It still sets MR = MC, but then charges the higher price read off the demand curve. Take demand P = 60 minus Q with a constant marginal cost of 12. A competitive market produces where 60 minus Q = 12, so Q = 48 and P = 12. A monopoly sets MR = 60 minus 2Q equal to 12, so Q = 24, and charges P = 60 minus 24 = 36. Price is 24 dollars above marginal cost, units 25 through 48 are never made even though buyers value them above their cost, and the deadweight loss is one half times 24 times 24, or 288 dollars. The gap between P and MC, as a share of price, is the Lerner index: (36 minus 12) divided by 36, or about 0.67.

P = MC also shows up in regulation and in market failure. A regulator that wants a natural monopoly to produce the efficient quantity sets the socially optimal price where P = MC, but because average total cost is still falling there, that price sits below ATC and the firm needs a subsidy to stay open. When a good creates an externality, P = MC for the firm is no longer enough: the efficient quantity is where marginal social benefit equals marginal social cost, so a polluting industry where price equals private marginal cost still produces too much.

Keep three conditions separate on the exam. MR = MC is how any firm picks its profit-maximizing quantity. P = MC is allocative efficiency, and it equals the profit rule only when P = MR, under perfect competition. P = minimum ATC is productive efficiency, producing at the lowest possible cost per unit. A perfectly competitive firm in long-run equilibrium meets all three at once, P = MR = MC = minimum ATC, which is why it is the efficiency benchmark; see the perfect competition module and the allocative vs productive efficiency comparison.