Producer Surplus (Linear Supply)

The mirror image of the consumer's triangle, and the same geometry. A supply curve read horizontally is a queue of sellers sorted by what it costs them to produce. The first would have supplied at the shutdown price; the last barely covers the market price. Everyone receives the same market price, so everyone except the last collects more than their cost. Sum the differences and you get the area between the price and the supply curve.

What producer surplus is NOT is profit. The supply curve is the marginal cost curve, so the area above it and below the price is revenue minus total VARIABLE cost — the contribution margin, in the language of the break-even page. Fixed costs have not been taken out. A firm with a large producer surplus and larger fixed costs is losing money, and this happens routinely: it is exactly the condition under which a business keeps operating in a bad year rather than shutting the doors, because the contribution still covers something toward the rent.

The shutdown price deserves a word, because it is where a real supply curve stops being a line. In the short run a firm supplies whenever the price covers its AVERAGE VARIABLE cost, since anything above that contributes to fixed costs it must pay regardless. In the long run it supplies only when the price covers average TOTAL cost, because the fixed costs come up for renewal. So the same industry has two different shutdown prices depending on the horizon, and the surplus triangle drawn on one is not the triangle drawn on the other.

The other name for part of this area is economic rent — the payment to a factor above what was needed to bring it into use. For an inelastic factor almost the entire payment is rent: the surplus accruing to a plot of land, a mineral deposit, a broadcast licence, or a performer nobody can substitute for. That distinction matters for taxation, because taxing pure rent does not change behaviour — the land is there whatever the tax — while taxing the return on something elastic drives it out of the market. It is the same insight as the deadweight-loss page approached from the other side.

The linearity assumption is a real limitation and it usually cuts the other way from demand. Actual supply curves are typically flattish over a range of spare capacity and then turn sharply upward as capacity binds, which no straight line represents well. The triangle drawn across that kink is wrong in a specific direction: it overstates the surplus in the flat region and understates it near the ceiling. As with consumer surplus, the calculation is most defensible when comparing two scenarios under identical assumptions and least defensible when quoted as an absolute figure.