Econ Lab · Welfare
Total surplus and the gains from trade
You have measured each side of the coffee market on its own. Buyers gain the upper triangle, sellers the lower one. Now put them in the same picture, at the price where the market actually settles, and add them up.
Shift demand or supply and watch the combined gain change.
Total surplus is the two triangles added together, the buyers' winnings stacked on the sellers'. It is the whole gain a market creates, the reason trade is worth doing at all.
This is the baseline market. Buyers gain about dollars and sellers about , for a total of . No quantity other than the market's cups makes that combined area larger. Trade less and you skip deals both sides wanted, force more and you push through deals that cost more than they are worth.
Shift demand or supply with the sliders and watch both triangles resize.
Two triangles, one number
Total surplus is just the sum of the two gains you already know.
It is the entire value the coffee market creates, split between the people who drink the coffee and the people who make it.
Why the market quantity wins
Here is the quiet result. The quantity the market lands on makes that combined area as large as it can possibly be. Brew fewer cups and you skip sales where a buyer valued the coffee above what it cost to make, a gain left on the table. Force more and you brew cups that cost more than anyone wants to pay for them, destroying value. Either way the total shrinks.
Left alone, the market finds the most surplus available. That is the precise sense in which it is efficient, and it is the benchmark every later module measures against.
What you just did
You built the ruler economists use to judge a market. From here on, a tax, a subsidy, or a price control is good or bad depending on what it does to this total. When you saw the tax on coffee open a wedge, it was quietly carving a piece out of exactly this area.