Introduction

The United States is entering a period in which the size of federal debt is becoming increasingly difficult to separate from the broader economic outlook. With U.S. federal debt moving beyond the $40 trillion mark, attention is shifting from the headline size of the debt to the cost of carrying it. The issue is not simply that Washington owes a very large amount of money. The more immediate concern is that the government must continually refinance maturing Treasury securities while also borrowing to cover annual budget deficits. When borrowing costs remain elevated, this process can make the fiscal burden grow faster.

The Congressional Budget Office’s 2026 outlook illustrates why investors and policymakers are watching the situation closely. CBO projects a federal budget deficit of about $1.9 trillion in fiscal 2026, while debt held by the public is expected to equal roughly 101% of gross domestic product. Under its baseline assumptions, that ratio could reach 120% by 2036.

Interest expenses are an especially important part of the discussion. CBO estimates that net federal interest outlays will exceed $1 trillion in 2026, compared with about $970 billion in 2025. It projects those costs could rise to approximately $2.1 trillion by 2036.

That creates a difficult policy environment. The United States has enormous financial resources, a large economy and a Treasury market that remains central to the global financial system. However, a rising debt balance combined with persistent deficits and relatively high interest rates can gradually reduce the government’s flexibility.

The important question is therefore not whether the United States can suddenly “run out of money.” It is how expensive continued borrowing becomes, how much of the federal budget is eventually directed toward interest payments, and what consequences that could have for taxes, public spending, private investment, financial markets and future economic growth.

Why the $40 Trillion Debt Level Matters

The headline figure of $40 trillion is enormous, but the absolute number alone does not provide a complete picture of the United States’ fiscal position. Government debt needs to be considered alongside the size of the economy, federal revenues, government spending and the interest rate paid on outstanding obligations.

One useful measure is debt relative to GDP. CBO expects federal debt held by the public to stand at approximately 101% of GDP in 2026 and rise to 120% by 2036 under its baseline projections. That is significantly above the historical average and above the previous postwar peak of 106% of GDP recorded in 1946.

The distinction between total federal debt and debt held by the public is also important. Total federal debt includes obligations held by different parts of the government as well as debt held outside the federal government. Debt held by the public is more directly relevant when considering how much the government must finance in financial markets.

The problem becomes more complicated when annual deficits remain large. A deficit means the government spends more than it collects in revenues during a fiscal year. That gap generally has to be financed through additional borrowing. Consequently, debt can continue rising even when economic growth is reasonably strong.

CBO projects the federal deficit to increase from approximately $1.9 trillion in 2026 to $3.1 trillion in 2036. As a share of GDP, the deficit is projected to rise from 5.8% to 6.7%. These levels are considerably higher than the average deficit of 3.8% of GDP recorded over the previous five decades.

This matters because debt can create a feedback mechanism. Larger deficits require additional borrowing. More borrowing increases the amount of debt outstanding. If interest rates remain elevated, refinancing that debt becomes more expensive. Higher interest costs then add to the deficit, which can require still more borrowing.

This does not mean that every additional dollar of borrowing automatically produces a crisis. Government borrowing can support productive investment, emergency spending and economic stabilization. During recessions, for example, deficit spending can help protect households and businesses and prevent an economic downturn from becoming worse.

The concern arises when large deficits become persistent even without an extraordinary economic emergency. At that point, policymakers have fewer options when another recession, financial shock or national emergency arrives.

The $40 trillion milestone therefore serves as a powerful warning about the scale of the fiscal challenge. It does not, by itself, establish that a crisis is imminent. Instead, it highlights how important future borrowing costs and fiscal decisions have become.

Rising Borrowing Costs and the Interest Burden

The most important change in the debt discussion is the growing role of interest payments. A government with a large debt balance can manage that debt more easily when interest rates are low. The same debt becomes considerably more expensive when securities mature and have to be replaced with new debt carrying higher yields.

This is particularly important for the United States because Treasury securities have different maturities. The government does not refinance its entire debt portfolio at once. Instead, securities mature at different times and are replaced gradually. This means higher interest rates can take years to fully influence the government’s average borrowing cost.

CBO estimates that the average interest rate on federal debt held by the public is about 3.4% in 2026 and projects it to rise toward 3.9% in the later years of its 2026–2036 outlook. Net interest costs are projected to increase from about $1 trillion in 2026 to $2.1 trillion in 2036.

That growth is significant because interest payments do not directly provide a government service in the way that spending on infrastructure, education or public safety can. They represent the financial cost of past borrowing.

There is also a compounding element. If the government must borrow additional money to cover part of its interest burden, that borrowing increases the debt balance. Future interest is then calculated on a larger amount of outstanding obligations.

This dynamic is one reason higher interest rates can have a disproportionately large effect over time. CBO’s analysis shows that if interest rates were higher than expected, additional borrowing costs would accumulate as Treasury securities mature and are replaced. In one recent scenario, a sustained increase in rates eventually produces substantially larger debt levels over the long term.

The effect is not limited to the federal budget. Government borrowing takes place in the same broad financial environment that affects businesses, households and investors. Treasury yields serve as an important reference point for many other borrowing costs.

When investors demand higher returns to hold government bonds, yields on other forms of debt can also face upward pressure. Corporate borrowing, mortgages, consumer loans and other financial products may therefore become more expensive, depending on market conditions and credit risk.

Higher Treasury yields can also change investment decisions. Businesses may compare the return available from investing in a new project with the return available from relatively safe government securities. If financing costs increase or expected returns decline, some private investment may be postponed.

This is why the debt problem is not simply a Washington accounting issue. Over time, the interaction between government borrowing and interest rates can influence the wider economy.

At the same time, Treasury securities remain among the most important assets in global financial markets. The depth and liquidity of the U.S. Treasury market provide the government with substantial financing capacity. That strength should not be ignored. But the larger the debt becomes, the more sensitive the fiscal outlook becomes to changes in interest rates.

Economic and Financial Consequences of Persistent Borrowing

A growing federal debt burden can affect the American economy through several channels. One of the most important is the allocation of government resources.

As interest payments consume a larger share of federal spending, policymakers may have less room to increase spending in other areas without raising taxes or borrowing even more. This can make future budget decisions increasingly difficult.

CBO projects that net interest costs will rise from 3.3% of GDP in 2026 to 4.6% by 2036. By then, interest spending would be close to the size of total federal discretionary spending under the agency’s projections.

That creates an important long-term trade-off. Every additional dollar directed toward debt service is a dollar that cannot be used elsewhere unless the government increases revenues or reduces other expenditures.

The impact can become especially serious during an economic downturn. Governments often respond to recessions by increasing spending or reducing taxes. If debt and interest costs are already high before the recession begins, policymakers may have less fiscal capacity to provide that support.

Another concern is private investment. CBO has noted that higher government borrowing costs can contribute to higher borrowing costs throughout the economy, reducing private investment and slowing economic output.

The consequences can also reach financial markets. Investors continuously evaluate whether government debt offers adequate compensation for inflation, interest-rate and fiscal risks. If concerns about long-term fiscal sustainability become stronger, markets could demand higher yields.

Such an outcome could create a difficult cycle. Higher yields would increase the cost of government borrowing, which could increase deficits and require additional debt issuance. Larger debt requirements could then place further pressure on interest rates.

This is not necessarily the most likely short-term outcome, but it explains why fiscal credibility matters. CBO has warned that rising debt increases vulnerability to higher interest rates and raises the risk of a fiscal crisis in which investors lose confidence in government debt.

The dollar’s international role is another consideration. The U.S. dollar remains central to global commerce, reserves and financial markets. Persistent fiscal deterioration does not automatically eliminate that position, but prolonged concerns about inflation, debt management or political stability could eventually affect international confidence.

There is also a generational dimension. Borrowing allows current policymakers to finance today’s priorities while shifting part of the financial burden into the future. If debt grows faster than the economy for an extended period, future taxpayers may face higher taxes, reduced government services or both.

CBO’s long-term projections illustrate the scale of the potential challenge. Under its extended baseline, debt held by the public rises to about 175% of GDP by 2056. In a scenario where interest rates are one percentage point higher than in the baseline, CBO estimates that the ratio could reach 222% of GDP in 2056.

These are projections rather than predictions. Laws, economic growth, inflation, interest rates, tax policy and spending decisions can all change the outcome. Nevertheless, they demonstrate why borrowing costs are becoming as important as the debt total itself.

Conclusion

The United States’ debt crossing the $40 trillion threshold represents a major fiscal milestone, but the deeper issue is what happens next. A large debt balance is manageable when economic growth, government revenues and borrowing costs remain favorable. The challenge becomes more serious when large annual deficits continue while interest rates remain elevated.

Current CBO projections show the direction of the problem clearly. Federal debt held by the public is projected to rise from about 101% of GDP in 2026 to 120% in 2036, while annual net interest costs are projected to increase from roughly $1 trillion to $2.1 trillion over the same period.

The key concern is therefore not simply whether the United States has too much debt. It is whether the country’s economic growth and fiscal policies can keep the cost of servicing that debt under control.

The United States still possesses significant advantages. Its economy is large, its Treasury market is exceptionally deep, and the dollar occupies a unique position in international finance. These factors give Washington considerable borrowing capacity. But those advantages do not eliminate the arithmetic of compound interest.

If deficits remain large, the debt stock will continue expanding. If borrowing costs rise at the same time, interest payments can consume an increasingly important portion of federal resources. That can reduce the government’s flexibility, put pressure on private investment and make future fiscal adjustments more difficult.

The most sustainable solution would ultimately require addressing the underlying gap between federal revenues and spending rather than focusing only on the debt ceiling or individual years of borrowing. A combination of stronger economic growth, responsible spending decisions, sustainable revenue policies and careful management of borrowing costs could reduce the pressure over time.

For financial markets, businesses and households, the development deserves close attention. The $40 trillion milestone is striking, but the more important number may be the growing annual cost of carrying that debt. As interest expenses become a larger part of the federal budget, America’s fiscal choices will increasingly influence the country’s economic outlook, financial markets and policy flexibility for years to come.