The circular flow model shows how money, goods and services, and the factors of production move between households and firms. It is the picture behind GDP: every dollar one group spends is a dollar of income for another group, so total spending, total income, and the value of total output describe the same flow. AP Macroeconomics uses it in Unit 2 to explain how GDP is measured. This page draws the basic model, adds banks, government, and the rest of the world, and gives you a leakages and injections checker. Educational only, not investment advice. All numbers are classroom examples, not data.
The basic model: households and firms
The simplest version has two groups and two markets.
- Households own the factors of production: labour, land, capital, and entrepreneurship.
- Firms use those factors to make goods and services.
- In the factor market (also called the resource market), households sell factors to firms and receive income in return: wages for labour, rent for land, interest for capital, and profit for entrepreneurship.
- In the product market, households spend that income on the goods and services firms produce, and that spending becomes firms' revenue.
Two loops run in opposite directions. The real flow is the physical one: factors go from households to firms, and goods and services go from firms to households. The money flow runs the other way: income goes from firms to households, and spending goes from households to firms.
The diagram
On the exam, label both markets, both groups, and at least one money flow and one real flow, with arrows pointing the right way. A free response question will often name a flow, such as wages, and ask which market it passes through and in which direction.
Adding banks, government, and trade
Real economies have three more players. Each one takes some money out of the loop between households and firms, called a leakage or withdrawal, and puts some back in, called an injection.
| Sector | Leakage (money leaves the loop) | Injection (money enters the loop) |
|---|---|---|
| Financial sector (banks and financial markets) | Saving (S) by households | Investment spending (I) by firms on capital goods |
| Government | Taxes (T), net of transfer payments | Government purchases of goods and services (G) |
| Rest of the world | Imports (M) | Exports (X) |
Transfer payments such as unemployment benefits or Social Security go from government to households without buying any output, so they are not part of G. In the simple model they are netted against taxes. If you want to see how a change in G or T ripples through income, the fiscal multipliers explainer and the fiscal multiplier calculator pick up from here.
Try it: leakages and injections checker
Enter classroom values for each flow. The checker adds up leakages (S + T + M) and injections (I + G + X) and tells you which way the model says national income would move. In the model, income settles where total leakages equal total injections.
Leakages and injections do not each have to match in pairs. A government deficit (G above T) can be offset by saving above investment, or by imports above exports, and the loop can still be in balance overall.
Why the model explains GDP
Because spending by one group is income for another, there are three ways to add up the same economy, and in principle all three give the same total.
- The expenditure approach adds spending on final goods and services: consumption plus investment plus government purchases plus net exports, written GDP = C + I + G + (X minus M).
- The income approach adds the incomes earned in production: wages, rent, interest, and profit, with adjustments the statistical agencies make for taxes on production and depreciation.
- The output or value added approach adds what each firm adds to the value of what it buys from other firms.
In the United States, the Bureau of Economic Analysis publishes GDP from the expenditure side and gross domestic income from the income side, and the gap between them is called the statistical discrepancy. In the United Kingdom, the Office for National Statistics combines the output, income, and expenditure measures. The BEA versus ONS GDP explainer compares the two, and the GDP topic page covers growth and real versus nominal GDP.
Common AP exam mistakes
- Swapping the markets. Households buy in the product market and sell in the factor market; firms do the reverse.
- Drawing money and real flows in the same direction. They always run opposite ways.
- Counting transfer payments as government purchases. Transfers are not spending on output, so they are not in G or in GDP.
- Calling saving an injection. Saving is a leakage; the investment it can fund through banks is the injection.
- Treating imports as part of GDP. Imports are subtracted because they are spending on output made in another country.
Short definitions are in the glossary. Next in the unit list, see the production possibilities curve for Unit 1, or the full AP Macro review hub.
Sources
- Bureau of Economic Analysis, Measuring the Economy: A Primer on GDP and the National Income and Product Accounts
- Bureau of Economic Analysis, Gross Domestic Product
- Office for National Statistics, GDP
- College Board, AP Macroeconomics course page
Common questions
What are the two markets in the circular flow model?
The product market, where households buy goods and services from firms, and the factor market (also called the resource market), where households sell labour, land, capital, and entrepreneurship to firms in return for wages, rent, interest, and profit.
What is the difference between real flows and money flows?
Real flows are goods, services, and factors of production moving between households and firms. Money flows are the spending and income that pay for them. The two loops always run in opposite directions.
What are leakages and injections in the circular flow?
Leakages take money out of the loop between households and firms: saving, net taxes, and imports. Injections put money in: investment, government purchases, and exports. In the model, national income settles where total leakages equal total injections.
Are transfer payments part of the circular flow?
Yes, but not as government purchases. Transfer payments move money from government to households without buying any output, so the simple model nets them against taxes, and they are not counted in GDP.