Econ Lab · Welfare
Consumer surplus
Stay in the coffee market a moment longer, but look only at the buyers. Some of them would happily have paid six dollars for their cup. They pay four like everyone else. That two-dollar gap is pure gain to them, and it has a name.
Drag the price line and watch the buyers' winnings, the shaded triangle, swell and shrink.
Consumer surplus is what buyers gain from a deal. Each buyer along the demand curve would have paid more than the price they actually pay, and that gap is their private win. Stack all those wins and you get the shaded triangle.
This is the usual market price of a cup, where the coffee market clears. The buyers who turn up pocket about dollars between them. Drag the price either way and watch what happens to that triangle.
Drag the dashed price line, or use the slider. Lower price, bigger triangle.
The demand curve is a queue
Read the demand curve as a line of buyers sorted by how much they want coffee. The keenest stands at the top left, ready to pay almost anything. The most reluctant waits at the bottom right, barely willing to pay the going rate. Everyone above the price line is getting their cup for less than it was worth to them.
Why lower prices help buyers twice
Drag the price down and the triangle grows for two reasons at once. The buyers already in the market keep more of their money, and new buyers who were priced out now step in. Both effects pile onto the same shaded area, which is why a price cut is such an obvious win for consumers.
What you just did
You measured one side of the gains from trade. Sellers have their own version of this, producer surplus, and once you put the two together you can weigh whether anything done to a market, a tax, a subsidy, a price cap, left people better or worse off. That is the next step.