<P> 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 . In practice, Treasury securities are not issued or redeemed on a day - by - day basis, and may also be issued or redeemed as part of the federal government's macroeconomic management operations . </P> <P> Historically, the US public debt as a share of gross domestic product (GDP) has increased during wars and recessions, and subsequently declined . The ratio of debt to GDP may decrease as a result of a government surplus or due to 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 . </P> <P> As of April 30, 2018, debt held by the public was $15.3 trillion and intragovernmental holdings were $5.7 trillion, for a total or "National Debt" of $21 trillion . Debt held by the public was approximately 77% of GDP in 2017, ranked 43rd highest out of 207 countries . The Congressional Budget Office forecast in April 2018 that the ratio will rise to nearly 100% by 2028, perhaps higher if current policies are extended beyond their scheduled expiration date . As of December 2017, $6.3 trillion or approximately 45% of the debt held by the public was owned by foreign investors, the largest being China (about $1.18 trillion) then Japan (about $1.06 trillion). </P> <P> The United States government has continuously had a fluctuating public debt since its formation in 1789, except for about a year during 1835--1836 . To allow comparisons over the years, public debt is often expressed as a ratio to gross domestic product (GDP). </P>

Who is the biggest buyer of us debt