Government Deficits
& the National Debt

Understand how budget deficits accumulate into national debt, the economic effects of government borrowing, and the tools economists use to evaluate fiscal health.

AP Macroeconomics

What Is a Budget Deficit?

A budget deficit occurs in any single fiscal year when the government's spending exceeds its revenue. In equation form:

Conversely, when $ T > G $, the government runs a budget surplus. When $ T = G $, the budget is balanced.

Visualizing Deficits and Surpluses

What Is the National Debt?

The national debt (also called public debt) is the total accumulation of all past deficits minus all past surpluses. Think of it as a running total:

Deficit

A flow variable: measured over a single year

Debt

A stock variable: measured at a point in time

Flow vs Stock: The Bathtub Analogy

Government Revenue and Spending

Revenue Sources

Income taxes (individual + corporate), payroll taxes (Social Security, Medicare), excise taxes, tariffs, fees

Spending Categories

Mandatory (entitlements: Social Security, Medicare), discretionary (defense, education), and interest on the debt

The Core Fiscal Identity

1

$ D_t $ = national debt at end of year $ t $

2

$ G_t $ = government spending (excluding interest) in year $ t $

3

$ i \cdot D_{t-1} $ = interest payments on existing debt

4

$ T_t $ = total tax revenue collected in year $ t $

Even with a primary balance ($ G = T $), debt can still grow if interest payments exceed zero.

Measuring Fiscal Health: Debt-to-GDP

Economists prefer this ratio over the raw dollar amount of debt because it shows how large the debt is relative to the economy's ability to support it.

Low Ratio

Economy can more easily service its debt; more fiscal space for emergencies

Moderate Ratio

Manageable, but rising ratios may signal future challenges

High Ratio

Risk of higher interest rates, reduced investor confidence, potential crisis

Deficit vs Debt: Side by Side

	Budget Deficit	National Debt
Definition	$ G - T $ in one year	Cumulative sum of all past deficits
Variable Type	Flow (per period)	Stock (point in time)
Analogy	Water flowing into the tub	Total water in the tub
Can Be Negative?	Yes (surplus: $ T > G $)	Not practically (always owed)
Key Metric	Deficit as % of GDP	Debt-to-GDP ratio

The Crowding-Out Effect

How Deficits Are Financed

1

Borrowing from the public: The Treasury sells bonds (T-bills, notes, bonds) to domestic and foreign investors, banks, and mutual funds.

2

Borrowing from government accounts: Surplus funds from programs like Social Security purchase special Treasury securities (intragovernmental debt).

3

Monetizing the debt: The central bank buys government bonds, increasing the money supply: this risks inflation.

How to Analyze a Fiscal Scenario

1

Identify $ G $ and $ T $: Determine government spending and tax revenue for the period.

2

Calculate the deficit or surplus: Compute $ G - T $. Positive = deficit; negative = surplus.

3

Update the debt: Add the deficit to the previous debt level: $ D_t = D_{t-1} + (G_t - T_t) $.

4

Compute debt-to-GDP: Divide total debt by GDP and multiply by 100%.

5

Assess economic effects: Consider crowding out, interest rates, and long-run growth.

Example 1: Simple Deficit Calculation

A country has government spending of \$4.5 trillion and tax revenue of \$3.8 trillion. The existing national debt is \$20 trillion.

1

Deficit: $ G - T = 4.5 - 3.8 = $ \$0.7 trillion deficit

2

New debt: $ D_t = 20 + 0.7 = $ \$20.7 trillion

3

If GDP = \$22 trillion: Debt-to-GDP $ = \dfrac{20.7}{22} \times 100\% \approx $ 94.1%

Example 2: Multi-Year Debt Accumulation

A country starts with zero debt. Over 4 years, it runs the following annual budgets:

Year	G (billions)	T (billions)	Deficit(+)/Surplus(-)	Cumulative Debt
1	\$500	\$450	$\color{#f87171}{+\$50}$	\$50
2	\$520	\$480	$\color{#f87171}{+\$40}$	\$90
3	\$490	\$510	$\color{#4ade80}{-\$20}$	\$70
4	\$550	\$470	$\color{#f87171}{+\$80}$	\$150

Example 3: Crowding Out in Action

Suppose the government increases spending by \$200 billion without raising taxes, financing it entirely through bond sales.

1

Deficit increases by \$200B: more Treasury bonds flood the loanable funds market.

2

Demand for loanable funds shifts right: the government competes with private borrowers.

3

Real interest rates rise from 3% to 4.5%.

4

Private investment falls: businesses and consumers face higher borrowing costs, reducing capital formation.

Common Mistakes to Avoid

✕ Confusing Deficit and Debt

The deficit is one year's shortfall; the debt is the cumulative total. A smaller deficit does not mean the debt is shrinking.

✕ Ignoring Debt-to-GDP

A \$1 trillion debt means very different things for a \$2T economy vs a \$25T economy. Always use the ratio.

✕ Assuming All Debt Is Bad

Borrowing during recessions (automatic stabilizers, stimulus) can boost output. The issue is sustainability, not existence.

Summary: Key Concepts at a Glance

Concept	Definition	Key Formula / Metric
Budget Deficit	$ G > T $ in one fiscal year	$ G - T $
Budget Surplus	$ T > G $ in one fiscal year	$ T - G $
National Debt	Sum of all past deficits minus surpluses	$ D_t = D_{t-1} + (G_t - T_t) $
Debt-to-GDP	Debt relative to economic output	$ \frac{D}{GDP} \times 100\% $
Crowding Out	Gov't borrowing raises interest rates	Loanable funds model: $ D_{LF} $ shifts right

Question 1 of 5

If a government collects \$3.2 trillion in tax revenue and spends \$3.9 trillion, what is the budget outcome?

Since $ G = 3.9 $ and $ T = 3.2 $, the deficit is $ G - T = 3.9 - 3.2 = 0.7 $ trillion. Spending exceeds revenue, so it is a deficit, not a surplus.

Question 2 of 5

A nation has a national debt of \$10 trillion and a GDP of \$20 trillion. What is its debt-to-GDP ratio?

Debt-to-GDP $ = \frac{10}{20} \times 100\% = 50\% $. This means the national debt is half the size of annual economic output.

Question 3 of 5

Which of the following best describes the crowding-out effect of government borrowing?

Crowding out occurs when increased government demand for loanable funds drives up real interest rates, making borrowing more expensive for businesses and consumers, thus reducing private investment.

Question 4 of 5

A country runs a deficit of \$100B in Year 1 and a deficit of \$50B in Year 2. Starting debt was \$500B. What is the debt after Year 2?

$ D_2 = 500 + 100 + 50 = 650 $ billion. Each year's deficit adds to the cumulative debt. The deficit shrank in Year 2, but the debt still grew.

Question 5 of 5

If a government runs a smaller deficit this year compared to last year, what happens to the national debt?

A smaller deficit is still a deficit: $ G > T $. The debt continues to grow, just at a slower pace. Only a surplus ($ T > G $) would actually reduce the debt.

	Budget Deficit	National Debt
Definition	\( G - T \) in one year	Cumulative sum of all past deficits
Variable Type	Flow (per period)	Stock (point in time)
Analogy	Water flowing into the tub	Total water in the tub
Can Be Negative?	Yes (surplus: \( T > G \))	Not practically (always owed)
Key Metric	Deficit as % of GDP	Debt-to-GDP ratio

Year	G (billions)	T (billions)	Deficit(+)/Surplus(-)	Cumulative Debt
1	\$500	\$450	\(\color{#f87171}{+\$50}\)	\$50
2	\$520	\$480	\(\color{#f87171}{+\$40}\)	\$90
3	\$490	\$510	\(\color{#4ade80}{-\$20}\)	\$70
4	\$550	\$470	\(\color{#f87171}{+\$80}\)	\$150

Concept	Definition	Key Formula / Metric
Budget Deficit	\( G > T \) in one fiscal year	\( G - T \)
Budget Surplus	\( T > G \) in one fiscal year	\( T - G \)
National Debt	Sum of all past deficits minus surpluses	\( D_t = D_{t-1} + (G_t - T_t) \)
Debt-to-GDP	Debt relative to economic output	\( \frac{D}{GDP} \times 100\% \)
Crowding Out	Gov't borrowing raises interest rates	Loanable funds model: \( D_{LF} \) shifts right

Government Deficits& the National Debt

What Is a Budget Deficit?

Visualizing Deficits and Surpluses

What Is the National Debt?

Flow vs Stock: The Bathtub Analogy

Government Revenue and Spending

The Core Fiscal Identity

Measuring Fiscal Health: Debt-to-GDP

Deficit vs Debt: Side by Side

The Crowding-Out Effect

How Deficits Are Financed

How to Analyze a Fiscal Scenario

Example 1: Simple Deficit Calculation

Example 2: Multi-Year Debt Accumulation

Example 3: Crowding Out in Action

Common Mistakes to Avoid

Summary: Key Concepts at a Glance

Question 1 of 5

Question 2 of 5

Question 3 of 5

Question 4 of 5

Question 5 of 5

Government Deficits
& the National Debt