Each trade creates surplus when benefit exceeds cost

Price divides the gain between the two parties.

If a buyer would pay $70 and a seller’s opportunity cost is $46, the trade creates $24 of total surplus. At a $58 price, consumer surplus is $12 and producer surplus is $12. At a $50 price, consumer surplus is $20 and producer surplus is $4. The price changes distribution but not the $24 total from that particular trade.

Willingness to pay is the maximum amount a buyer would give for a unit. Seller cost is the value of resources or the minimum compensation needed to make that unit available. A voluntary trade can create gains whenever willingness to pay exceeds seller cost. Price must fall between them for both parties to accept in the simplest case, but the total gain is determined by benefit minus cost, not by where the price falls.

For the $70 buyer and $46 seller, a price of $65 leaves $5 of consumer surplus and $19 of producer surplus. A price of $48 leaves $22 and $2. In both cases, the amounts sum to $24. This invariance is a useful arithmetic check: for a completed unit, CS+PS=WTP-cost.

Across a market, consumer surplus is the area below demand and above price. Producer surplus is the area above supply and below price. For straight curves, these are often triangles. Producer surplus is not the same as profit because the supply curve typically reflects variable opportunity cost but not all fixed cost.

Demand can be read vertically as marginal willingness to pay. At each quantity, its height states the value of the marginal unit to buyers. Supply can be read as marginal seller cost. The vertical distance between them is the surplus created by that unit. Adding those vertical gains across all traded units produces total surplus.

For linear curves, use triangle area only after identifying boundaries. If demand intercepts the price axis at $30, equilibrium price is $18, and quantity is 80, consumer surplus is 1/2(80)($12)=$480. The $12 height is willingness-to-pay intercept minus price. Using $30 as the height counts the money buyers actually pay as if it were surplus.

Producer surplus differs from accounting profit because fixed cost may not appear under the short-run supply curve. If a firm’s revenue exceeds variable cost by $1,000 but fixed cost is $1,200, producer surplus can be positive while profit is negative $200. The concepts answer different questions about operation and overall return.

A market price divides, but does not create, the gain

A collector values a book at $90 and its owner values keeping it at $54. At any transaction price between those values, trade creates $36 of total surplus. Negotiation determines the division. A law that prevents the trade destroys the $36. A price change within the acceptable range merely reallocates it.

Surplus is a model-based measure, not cash sitting in an account. Consumer surplus records the difference between value and payment. Producer surplus records payment above opportunity cost. State whose valuation or cost is being compared before interpreting a shaded area.