consumer surplus
Have you ever bought something and thought, "I'd have happily paid more than that"? That little glow of getting a deal has a name in economics: consumer surplus. It's the gap between the most you were willing to pay for something and the price you actually paid. If you'd have given 30 dollars for a concert ticket but bought it for 20, you walk away with 10 dollars of consumer surplus — value you got but didn't have to hand over.
Add this up across all the buyers in a market and you get the total consumer surplus — a measure of how much benefit, in money terms, the whole crowd of buyers squeezes out of being able to trade at the going price. On a supply-and-demand diagram it has a tidy home: it's the area below the demand curve and above the price line. Why that shape? Because the demand curve traces each buyer's willingness to pay, from the keenest (high up) to the most lukewarm (down near the price), and everyone above the price gets the difference as surplus. The eager buyers get a lot; the marginal buyer, who'd pay just barely the price, gets almost none.
Consumer surplus is one half of how economists measure the gains from trade and the wellbeing a market creates (the other half is producer surplus; together they make total surplus). It's the analytic backbone for judging policies: a price ceiling, a tax, or a monopoly can all be assessed by how much consumer surplus they create or destroy. The honest caveats are worth stating: surplus is measured in money, so a dollar of surplus to a billionaire counts the same as a dollar to a pauper, which sidesteps fairness; and willingness to pay is partly a measure of ability to pay, so the metric quietly favours those with deeper pockets. It captures efficiency, not justice.
Ten people would pay, respectively, 9, 8, 7… down to 1 dollar for a sandwich now priced at 4. The six who buy each pocket the gap above 4 — 5, 4, 3, 2, 1, and 0 dollars — adding up to 15 dollars of consumer surplus shared among them.
The benefit each buyer gets above the price, summed up — the triangle under demand, above price.
Consumer surplus measures money value, so it treats a dollar to the rich and a dollar to the poor as equal. That makes it a clean efficiency yardstick but a poor fairness one — a policy can raise total surplus while leaving vulnerable people worse off.