Budget Deficit vs National Debt
Budget Deficit and National Debt are two Fiscal Policy concepts in AP Economics that students often mix up. A budget deficit occurs when government spending exceeds its tax revenue in a given year. The national debt is the total accumulated amount the government owes from past deficits not offset by surpluses. Here is how they compare side by side.
Governments finance deficits by borrowing, which adds to the national debt. Deficits can stimulate a weak economy but may raise interest rates and crowd out private investment. They typically grow during recessions.
It is a stock that grows whenever the government runs a deficit, unlike the deficit, which is an annual flow. Large debt can raise interest costs and crowd out private investment. It is often measured as a percentage of GDP.
Budget Deficit vs National Debt: Flow Versus Stock
| Budget deficit | National debt | |
|---|---|---|
| Stock or flow | A flow, measured across one budget year | A stock, measured at a point in time |
| Units | Dollars per year | Dollars outstanding |
| How the two connect | Each year's deficit is added to the debt | The running total of past deficits minus surpluses |
| What makes it fall | Higher tax revenue or lower spending that year | Only a budget surplus, barring default |
| Main economic concern | Crowding out via a higher real interest rate now | Interest payments squeezing future budgets |
| Usual benchmark | Annual deficit over annual GDP, flow to flow | Debt stock over one year of GDP, stock to flow |
A flow running into a stock
The cleanest way to hold these apart is to picture a bathtub. The budget deficit is the rate at which water runs in over a single year, so it is a flow measured in dollars per year. The national debt is the level of water sitting in the tub right now, so it is a stock measured in dollars at a moment in time. Every year the government spends more than it collects, that year's shortfall is added to the level; every year it collects more than it spends, that amount is drained off. Suppose a government starts the year owing 2,000 billion dollars and runs a deficit of 150 billion dollars. It ends the year owing 2,150 billion dollars, and it had to borrow that 150 billion in the loanable funds market to cover the gap. The fiscal policy tools behind that gap are covered at /macro/fiscal-policy.
Why a shrinking deficit still grows the debt
This is the single most common confusion on the topic, and news coverage invites it. Continue the example above: the next year the government cuts its deficit from 150 billion dollars to 100 billion. The deficit has fallen by a third, yet the debt still rises, from 2,150 billion to 2,250 billion, because the government is still spending more than it takes in. Cutting the deficit slows the rate at which the debt grows, and that is all it does. Barring default, the only thing that reduces the dollar amount of the national debt is a budget surplus, when tax revenue exceeds spending and the government can retire some of what it owes. Writing that the debt fell because the deficit fell is a guaranteed lost point, and the same logic explains why the debt can keep climbing through a run of years in which every single deficit is smaller than the one before.
Debt as a share of GDP, and the different worry attached to each
Both figures are usually quoted as percentages of GDP, because a dollar amount means little without the size of the economy behind it. Note that these two ratios are not the same kind of object: the deficit ratio compares one year's borrowing to one year's output, while the debt ratio compares an accumulated stock to one year's output. The debt ratio can hold steady or even fall while the government keeps running deficits, provided nominal GDP grows at least as fast as the debt. Take a country with 2,000 billion dollars of debt and 5,000 billion dollars of nominal GDP, a ratio of 40 percent. It runs a 100 billion dollar deficit, lifting the debt to 2,100 billion, while nominal GDP grows 5 percent to 5,250 billion. The new ratio is 2,100 divided by 5,250, still exactly 40 percent, because debt also grew 5 percent, even though the debt in dollars is higher than before. The economic worry attached to each measure differs as well. This year's deficit adds to the demand for loanable funds and can push the real interest rate up, crowding out private investment, as shown at /blog/crowding-out-explained. The accumulated debt shows up in every future budget as interest that has to be paid before anything else, which is why a large stock of debt narrows the room for future fiscal policy.
Frequently asked questions
What is the difference between a budget deficit and the national debt?
A budget deficit is the amount by which government spending exceeds tax revenue in a single year, while the national debt is the total accumulated amount the government owes from all past deficits net of past surpluses. The deficit is a yearly flow measured in dollars per year, and the debt is the stock, measured in dollars outstanding, that the flow adds to.
Does the national debt go down when the deficit goes down?
No, cutting the deficit only slows how fast the national debt grows, because any deficit above zero still adds to the debt that year. The dollar amount of the debt falls only when the government runs a budget surplus, although the debt-to-GDP ratio can fall while deficits continue if nominal GDP grows faster than the debt.
Is a budget deficit the same as a trade deficit?
No, a budget deficit compares a government's spending with its tax revenue, while a trade deficit compares a country's imports with its exports. One is an account of the government's finances and the other is an account of the whole country's transactions with the rest of the world.
What happens to interest rates when the government runs a larger deficit?
In the loanable funds model a larger deficit means more government borrowing, which shifts the demand for loanable funds to the right and raises the real interest rate, holding the supply of loanable funds constant. The higher real rate reduces private investment spending, and that reduction is the crowding out effect.
Want the long version? National Debt vs Deficit: The Difference Explained walks through the same comparison as a full guide, with worked examples and the exam traps. This page is the quick side-by-side.
Live Fiscal Policy graph. Drag the curves, or open the full version.
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