Econ 101, Part 1: What Economics Actually Is
Stocks and Flows
A simple but powerful distinction between amounts at a point in time and amounts over a period, and why mixing them up causes confusion about wealth, debt and income.
Economists constantly distinguish between stocks and flows. It sounds technical, but it is one of the most useful ideas for understanding money and the economy.
Definitions
- A stock is an amount measured at a point in time. Example: the water in a bathtub right now.
- A flow is an amount measured over a period of time. Example: water pouring from the tap per minute, or draining out.
The stock changes because of flows: inflows add to it, outflows reduce it.
Economic examples
- Wealth, such as savings and property, is a stock; income and saving per month are flows.
- Government debt is a stock; the budget deficit per year is a flow.
- The number of unemployed people is a stock; people losing and finding jobs each month are flows.
- Capital, such as machines and buildings, is a stock; investment and depreciation per year are flows.
- Population is a stock; births, deaths and migration per year are flows.
Why it matters
- Income versus wealth: a person with a high salary (flow) may have little wealth (stock) if they spend it all. A retired person may have low income but high wealth.
- Deficits versus debt: a government can reduce its deficit (flow) while its debt (stock) still rises, as long as there is any deficit at all.
- GDP is a flow: output per year. National wealth is a stock.
Common confusions
- Comparing a company’s market value (stock) with a country’s GDP (flow) is misleading, because they measure different things.
- Saying someone is “worth” their salary mixes up flows and stocks.
Using the idea
When you see an economic number, ask: is this measured at a point in time or per period? This quickly clarifies many debates.
A student has 20,000 rupees saved (a stock). Each month, she earns 8,000 rupees from tutoring and spends 6,000 (flows). Her savings grow by 2,000 each month. If her spending rises to 9,000, her savings stock starts shrinking.
Debt keeps rising as long as there is any deficit. Only a surplus reduces the stock of debt.
- A stock is measured at a point in time; a flow is measured over a period.
- Flows change stocks: income and spending change wealth.
- Deficits are flows; debt is a stock.
- Asking "stock or flow?" clarifies many economic numbers.
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