Consumer and Producer Surplus

Consumer surplus is the extra benefit buyers get when they pay less than the most they were willing to pay. Producer surplus is the extra benefit sellers get when they receive more than the least they were willing to accept. Both are measured against the market price set at equilibrium.

On a supply-and-demand graph, consumer surplus is the triangle below the demand curve and above the market price. If you were willing to pay $10 for a book but buy it for $6, your personal surplus is $4. Adding this up across every buyer gives the total consumer surplus.

Producer surplus is the mirror image: the triangle above the supply curve and below the market price. A seller willing to accept $3 who sells at $6 earns $3 of surplus. Summed across all sellers, this is the total producer surplus.

Because each region is a triangle, you can use area = ½ × base × height. The base is the equilibrium quantity and the height is the gap between the price and the curve's intercept. The sum of consumer and producer surplus is total surplus, a common measure of the gains from trade in a market at a price floor or price ceiling.

Anything that moves the price away from equilibrium changes both surpluses. A tax drives a wedge between what buyers pay and sellers receive, shrinking total surplus and creating deadweight loss. The same shrinkage appears with taxes and binding price controls.

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