Consumer surplus is the difference between the maximum price consumers are willing to pay and the actual market price they pay, represented as the area above the price level and below the demand curve. Producer surplus is the difference between the price sellers receive and the minimum price they are willing to accept, represented as the area below the price level and above the supply curve. Together they measure the total gains from trade (total surplus or social welfare) in a market. Competitive equilibrium maximizes total surplus.
Calculate surplus as triangle areas for linear supply and demand, then extend to policy analysis (taxes, price floors, price ceilings). Relating surplus to individual willingness-to-pay via step-demand curves first makes the concept tangible.
Consumer surplus builds directly on market equilibrium — the price where supply meets demand. But the equilibrium price hides something important: not every buyer values the good equally. Imagine concert tickets selling for $50. One buyer would have paid $120, another $80, another $55. The market charges them all $50. Each keeps the difference between what they *would* have paid and what they *actually* paid. That gap, summed across all buyers, is consumer surplus. Graphically, it is the triangle between the demand curve and the horizontal price line — the area above the price but below the demand curve, computed using the triangle area formula from your prerequisites.
Producer surplus is the mirror image. Not every seller has the same cost of production. A seller who can produce at $10 but receives $50 captures $40 of surplus. Summed across sellers, producer surplus is the triangle between the supply curve and the price line — below price but above supply. Total surplus (consumer + producer) represents the net gains from trade: how much better off buyers and sellers are compared to a world where this market didn't exist.
The power of this framework emerges when you apply comparative statics. A fall in price increases consumer surplus — more buyers enter the market, and existing buyers pay less — but compresses producer surplus. The elasticity of demand matters here: a more elastic demand curve (flatter) produces a different surplus triangle shape than an inelastic one, even at the same equilibrium. Understanding how these triangles change under different conditions is the core tool of welfare economics.
Competitive equilibrium is special: it maximizes total surplus. Any price different from equilibrium — a price floor above it, a price ceiling below it — creates a situation where some mutually beneficial trades don't happen. The uncaptured surplus from those missing trades is deadweight loss. This is the foundation for everything you'll study next: price controls, taxes, externalities, and market failures are all analyzed by asking how they change the size and distribution of the surplus triangles.