Gross Domestic Product: Measuring an Economy's Output

Turn on the news after a jobs report or an election, and you will hear people argue about whether the economy is "doing well." But what does that even mean? An economy is billions of separate transactions, from a haircut in Ohio to a container ship leaving Shanghai. To say something useful about the whole thing, economists compress all of that activity into a handful of numbers. The most important of them, the headline figure that governments, investors, and journalists watch above all others, is gross domestic product.

What GDP Measures

Gross domestic product (GDP) is the market value of all final goods and services produced within a country's borders during a given period, usually a quarter or a year. Every word in that definition is doing work. Market value means we add up dollars, not tons or gallons, so that a laptop and a loaf of bread can share a common measuring stick. Final means we count only goods sold to their end user; the flour a bakery buys is an intermediate good, and its value is already baked into the price of the bread, so counting both would be double counting. Produced means GDP tracks new output, not the resale of used cars or existing houses. Within a country's borders makes GDP a geographic measure: a Japanese-owned factory in Kentucky counts in U.S. GDP. And during a given period makes GDP a flow, like the water passing a point in a river each minute, not the total volume of the lake.

The Expenditure Approach

Because every dollar produced is a dollar someone spends, we can measure GDP by adding up spending. Economists sort all spending into four buckets, giving the famous identity:

GDP = C + I + G + NX
  C  = consumption      (households: food, rent, haircuts)
  I  = investment       (firms: machines, factories, new housing)
  G  = government        (spending on goods and services)
  NX = net exports = exports - imports

A subtle point trips up many students: investment here means business capital, not buying stocks. And we subtract imports because the C, I, and G buckets already include spending on foreign-made goods, which were not produced domestically.

A Worked Example

Suppose in one year a country's households spend $14 trillion, firms invest $4 trillion, the government spends $4 trillion, exports are $3 trillion, and imports are $4 trillion. Then:

NX  = exports - imports = 3 - 4 = -1 trillion
GDP = C + I + G + NX = 14 + 4 + 4 + (-1) = 21 trillion

The country produced $21 trillion of output, even though it ran a trade deficit that subtracted a trillion from the total.

Nominal Versus Real, and What GDP Misses

If prices rise, GDP can grow even when the country produces nothing extra. Nominal GDP values output at current prices; real GDP values it at the prices of a fixed base year, stripping out inflation so we can compare living standards across time. When you hear "the economy grew 3 percent," that is real GDP. Finally, remember what GDP is not. It ignores unpaid work like childcare, says nothing about how output is distributed, and does not subtract pollution or count leisure. GDP is a superb measure of production, and a rough and incomplete measure of well-being. Keep both truths in mind as we build the rest of macroeconomics on this foundation.

This lesson is your free preview of the course. In the next two lessons we measure the other two vital signs of an economy, inflation and unemployment, and then assemble all three into a model of booms and busts.

Curriculum aligned with OpenStax's Principles of Macroeconomics 3e; all lesson text is original to Syllabus.