How this calculator works
Consumer surplus is the gap between what a buyer was prepared to pay and what the buyer actually paid. For a single purchase it is simply willingness to pay minus the price. Across a market, the surplus is the area under the demand curve and above the price. With a straight-line demand curve falling from the highest willingness to pay to the market price, that area is a triangle, so it is half the price gap times the quantity traded. The measure assumes that the demand curve reflects true willingness to pay and that the good has no important second-hand effects.
Formula
One buyer: consumer surplus = willingness to pay - price paid. Market: consumer surplus = 0.5 × (willingness to pay - price paid) × quantity, where willingness to pay is the price at which quantity demanded would fall to zero and the demand curve is linear.
Worked example
A buyer willing to pay $100 pays $50, so the surplus is $50. In a market where the demand curve reaches zero at $100, the price is $50 and 100 units trade, the surplus is 0.5 × $50 × 100 = $2,500.
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Frequently asked questions
Why is the market surplus a triangle?
Each buyer's surplus is the vertical gap between the demand curve and the price. Summing those gaps across all units sold gives the area of the region between the demand curve and the price line, which is a triangle when demand is a straight line.
Does consumer surplus measure happiness?
It measures the money value of the benefit buyers receive, given their willingness to pay. It treats a dollar as equally valuable to everyone, which is a simplification when incomes differ sharply.
What happens to consumer surplus when price rises?
It falls for two reasons: each remaining buyer keeps a smaller gap, and some buyers drop out of the market entirely. The lost area is a trapezoid made of the transferred surplus and the deadweight loss.