Price Determination under Perfect Competition

Because no individual firm under perfect competition can influence price, the price itself must be explained at the level of the whole industry (market), not at the level of one firm — this is the key idea that separates price determination under perfect competition from price determination under monopoly, where the single firm's own output decision is the price decision. The market price is determined at the point where the industry's total demand curve intersects the industry's total supply curve — the quantity buyers are willing to buy at a given price exactly equals the quantity sellers are willing to sell at that same price. This intersection point is called market equilibrium, and the price and quantity at which it occurs are the equilibrium price and equilibrium quantity.

Suppose, for a commodity sold in a perfectly competitive market, the total (industry) demand and supply functions are given as:

where P is price and Q is quantity. Equilibrium requires Qd = Qs:

which simplifies to

so

Substituting back, Qd = 200 − 4(24) = 200 − 96 = 104, and as a check, Qs = −40 + 6(24) = −40 + 144 = 104 — the two independently computed quantities agree, confirming P = 24 and Q = 104 as the equilibrium price and quantity. Once every firm in the perfectly competitive industry takes this price as given, each decides its own output individually by producing up to the point where its own marginal cost equals this price (since MC = MR = P is the firm's profit-maximising condition), and the sum of every firm's chosen output across the industry is exactly the 104 units the market as a whole demands and supplies at that price. …