The national debt of the United States is the total national debt owed by the federal government of the United States to Treasury security holders. The national debt at any point in time is the face value of the then-outstanding Treasury securities that have been issued by the Treasury and other federal agencies. The terms "national deficit" and "national surplus" usually refer to the federal government budget balance from year to year, not the cumulative amount of debt. In a deficit year the national debt increases as the government needs to borrow funds to finance the deficit, while in a surplus year the debt decreases as more money is received than spent, enabling the government to reduce the debt by buying back some Treasury securities. In general, government debt increases as a result of government spending and decreases from tax or other receipts, both of which fluctuate during the course of a fiscal year. There are two components of gross national debt:

"Debt held by the public" – such as Treasury securities held by investors outside the federal government, including those held by individuals, corporations, the Federal Reserve, and foreign, state and local governments.

"Debt held by government accounts" or "intragovernmental debt" – is non-marketable Treasury securities held in accounts of programs administered by the federal government, such as the Social Security Trust Fund. Debt held by government accounts represents the cumulative surpluses, including interest earnings, of various government programs that have been invested in Treasury securities.Historically, the U.S. public debt as a share of gross domestic product (GDP) increases during wars and recessions and then subsequently declines. The ratio of debt to GDP may decrease as a result of a government surplus or via growth of GDP and inflation. For example, debt held by the public as a share of GDP peaked just after World War II (113% of GDP in 1945) but then fell over the following 35 years. In recent decades, aging demographics and rising healthcare costs have led to concern about the long-term sustainability of the federal government's fiscal policies. The aggregate, gross amount that Treasury can borrow is limited by the United States debt ceiling.As of August 31, 2020, federal debt held by the public was $20.83 trillion and intragovernmental holdings were $5.88 trillion, for a total national debt of $26.70 trillion. At the end of 2020, debt held by the public was approximately 99.3% of GDP, and approximately 37% of this public debt was owned by foreigners. The United States has the largest external debt in the world; as of 2017, its debt-to-GDP ratio was ranked 43rd out of 207 countries and territories. The total number of U.S. Treasury securities held by foreign countries in June 2020 was $7.04 trillion, up from $6.63 trillion in June 2019. A 2018 report by Congressional Budget Office (CBO) forecast that publicly-held debt will rise to nearly 100% of GDP by 2028, perhaps higher if current policies are extended beyond their scheduled expiration date. As of February 2022, total US federal Government debt breached $30 trillion mark for the first time in the history.During the COVID-19 pandemic, the federal government spent trillions in virus aid and economic relief. The CBO estimated that the budget deficit for fiscal year 2020 would increase to $3.3 trillion or 16% GDP, more than triple that of 2019 and the largest as % GDP since 1945.