In a perfectly competitive market, the equilibrium price is where the market supply curve (sum of all firm supply curves) intersects the market demand curve. At this price, quantity supplied equals quantity demanded. Individual firms are price-takers, accepting the equilibrium price as given. Competitive markets are efficient: equilibrium allocates resources to their highest-value uses (consumer and producer surplus are maximized).
Aggregate individual firm supply curves to derive the market supply curve. Find the intersection with market demand to determine equilibrium price and quantity. Analyze welfare by calculating consumer and producer surplus.
You already know how an individual competitive firm determines its supply: it produces where price equals marginal cost, tracing out an upward-sloping supply curve as price varies. To get from individual firm behavior to market outcomes, you aggregate. The market supply curve is the horizontal sum of every firm's supply curve — at any given price, add up all the quantities that individual firms are willing to sell. This sounds mechanical, but the insight is significant: changes in the number of firms shift the entire market supply curve. More entrants push market supply rightward; exits shift it leftward.
Market equilibrium occurs where this aggregated market supply meets the market demand curve. At the equilibrium price, the quantity all firms together want to sell exactly equals the quantity all consumers want to buy. No unsold inventory piles up; no willing buyer is turned away. This is the self-correcting mechanism of competitive markets: if price is above equilibrium, supply exceeds demand and prices are bid down; if price is below equilibrium, demand exceeds supply and prices are bid up. The equilibrium is stable because deviations generate forces that restore it.
An important subtlety: a competitive equilibrium need not be profitable for individual firms. The market price is determined by the intersection of supply and demand, regardless of firms' average costs. At equilibrium, firms might earn economic profits, break even at normal returns, or sustain losses. This matters because profitability drives long-run entry and exit — the mechanism behind the zero-profit equilibrium you'll study next. What short-run competitive equilibrium guarantees is not profit, but allocative efficiency: output is produced up to the quantity where the last unit's value to consumers (the demand curve) equals the cost of producing it (the supply curve), maximizing the total of consumer surplus and producer surplus.
The efficiency result is one of economics' deepest insights: millions of self-interested individuals, coordinating only through prices, produce an allocation that a benevolent central planner would struggle to improve upon. Each price-taker firm accepts the market price as given and responds by adjusting quantity — and the aggregate of those individual responses happens to generate the socially optimal output level. This holds, however, only under the assumptions of perfect competition — no market power, no externalities, complete information. Real markets deviate from these assumptions in important ways, which is precisely what motivates the remainder of microeconomic theory.