A federal trust fund is an accounting record kept by the U.S. Treasury to track money that Congress has set aside by law for a specific program, such as Social Security retirement benefits or highway construction.1Social Security Administration. Frequently Asked Questions About the Social Security Trust Funds It is a ledger, not a vault. More than 200 of these funds exist across the federal government, and each one shows what the government has collected for its assigned purpose, what it has paid out, and what remains committed on paper.
Why It Is Not a Trust in the Ordinary Sense
The name is misleading. A private trust involves a trustee holding assets and owing a legal duty to a beneficiary. Federal trust funds work differently. They are statutory accounting tools created by Congress under its power to tax and spend, and they do not hold segregated pools of money the way a family trust might hold stocks or real estate. When Congress passed the Social Security Act Amendments of 1939, it created the Old-Age and Survivors Insurance Trust Fund as a formal record on the Treasury’s books.2Office of the Law Revision Counsel. 42 USC 401 – Trust Funds
The consequence is that Congress can change the rules at any time. It can raise or lower the taxes that feed a fund, expand or cut the benefits it pays, or redirect the money entirely through new legislation. No one holds a legally enforceable property right to future benefits the way a private trust beneficiary might. The balance on the ledger tells you what the government has committed to a program; it does not prevent future Congresses from rewriting those commitments. Title 31 of the United States Code classifies these funds and governs how the Treasury handles their balances.3Office of the Law Revision Counsel. 31 USC 1321 – Trust Funds
Where the Money Comes From
Every federal trust fund has its own dedicated revenue stream written into its enabling statute. The three main sources are payroll taxes, excise taxes and user fees, and transfers from the general fund.
Payroll Taxes
The Social Security and Medicare trust funds draw most of their revenue from payroll taxes collected under the Federal Insurance Contributions Act and the Self-Employment Contributions Act. Employers and employees each pay a percentage of wages, and self-employed workers pay both shares.4Social Security Administration. Taxation Transfers These taxes are automatically appropriated, meaning the Treasury credits the accounts as the revenue comes in without Congress passing a separate spending bill each year.2Office of the Law Revision Counsel. 42 USC 401 – Trust Funds
Excise Taxes and User Fees
Other funds rely on consumption-based taxes tied to the services they support. The Highway Trust Fund receives roughly 18.3 cents per gallon from the federal gasoline tax and 24.3 cents per gallon from the diesel tax.5Congressional Research Service. The Highway Trust Fund’s Highway Account The Airport and Airway Trust Fund collects taxes on airline tickets and aviation fuel. The Inland Waterways Trust Fund collects a tax on fuel used by commercial barges. In each case, the users of the infrastructure pay into the fund that maintains it.
General Fund Transfers
Some funds receive money from the government’s general fund when dedicated revenue falls short of promised payments. Medicare’s Supplementary Medical Insurance Trust Fund, which covers physician visits and prescription drugs, gets about three-quarters of its funding this way rather than through payroll taxes. Congress can also authorize one-time general fund transfers to shore up a fund facing near-term depletion, though those transfers add to the overall federal deficit because the general fund has no surplus of its own to draw on.
What Happens to Surplus Money
When a fund collects more in a year than it pays out, the Treasury invests the surplus in special-issue Treasury securities. These are government debt instruments available only to federal trust funds. They cannot be bought or sold on the open market, and no private investor can own them.6Congressional Research Service. Social Security Trust Fund Investment Practices The securities earn interest, and that interest is credited back to the fund, adding to the balance available for future spending. The rate is set by a statutory formula pegged to the average market yield on outstanding Treasury obligations with four or more years to maturity.7Social Security Administration. Interest Rates on Social Security Investments The securities carry the full faith and credit of the United States.
This process creates what is called intragovernmental debt. The general fund borrows the trust fund’s surplus to pay for current government operations and issues these securities as its IOU. When the trust fund later needs to redeem them to pay benefits, the Treasury must produce actual cash, either by collecting enough tax revenue, borrowing from the public, or cutting other spending. The securities are real obligations, but redeeming them still requires the government to find real dollars, which is why a large paper balance does not eliminate the need for future fiscal choices.
What a Fund Can Pay For, and What Happens If It Runs Dry
A trust fund can only spend on the purposes spelled out in its enabling statute. Highway Trust Fund dollars cannot be redirected to healthcare. Social Security revenue cannot be spent on defense. Program administrators receive budget authority from Congress to draw on the fund, and Treasury officials redeem the special-issue securities as needed to convert the balance into cash for actual payments.
There is a hard floor on spending. If a fund’s balance reaches zero, the program can only pay out what it currently collects in revenue. The Antideficiency Act reinforces this by prohibiting federal officers from incurring obligations that exceed available funds. An agency cannot keep writing checks against an empty fund and expect Congress to backfill later. When a fund approaches depletion, the managing trustee must warn Congress, and the program faces automatic reductions unless lawmakers act.
Who Watches the Big Funds
The largest social insurance funds operate under a formal board of trustees composed of senior cabinet officials. For Social Security and Medicare, the board includes the Secretary of the Treasury, who serves as managing trustee, along with the Secretary of Labor, the Secretary of Health and Human Services, and the Commissioner of Social Security. Two public members, nominated by the President and confirmed by the Senate, serve four-year terms and cannot both belong to the same political party.8Office of the Law Revision Counsel. 42 USC 1395t – Federal Supplementary Medical Insurance Trust Fund
The board meets at least once per year and issues an annual report to Congress projecting each fund’s financial outlook over the next 75 years. Those reports are the source of the depletion-date projections that drive public debate about Social Security and Medicare solvency.
What “Running Out” Actually Means
The 2025 Trustees Report projects that the combined Social Security trust funds will be depleted by 2034. At that point, incoming payroll taxes would cover only about 81 percent of scheduled benefits. The Old-Age and Survivors Insurance fund alone, which pays retirement benefits, faces depletion in 2033, after which it could cover roughly 77 percent of scheduled payments.9Social Security Administration. The 2025 Annual Report of the Board of Trustees of the Federal Old-Age and Survivors Insurance and Federal Disability Insurance Trust Funds
Depletion does not mean the program vanishes. Payroll taxes keep flowing in every pay period, so the program always has incoming revenue. What depletion means is that the fund can no longer supplement current collections with accumulated reserves. The practical result would be an immediate across-the-board reduction in monthly benefit checks unless Congress raises taxes, adjusts benefits, changes the retirement age, or combines those options.
The Highway Trust Fund shows a different version of the same problem. Because the federal gasoline tax has been frozen at the same rate since 1993 while construction costs and fuel efficiency have risen, the fund routinely spends more than it collects, and Congress has kept it afloat through periodic general fund transfers rather than raising the gas tax.5Congressional Research Service. The Highway Trust Fund’s Highway Account A dedicated revenue stream only works as long as it keeps pace with the program’s costs.
How Trust Funds Fit Into the Federal Budget
Since fiscal year 1969, the federal government has reported all its financial activity, including trust fund transactions, in a single consolidated statement called the unified budget.10U.S. GAO. The Budget Treatment of Trust Funds The idea is to show the full cash flow between the government and the economy, rather than letting trust fund surpluses hide the cost of general operations.11Social Security Administration. The Social Security Trust Funds and the Federal Budget
Two programs carry a special designation within this framework. The Social Security trust funds and the Postal Service Fund are classified as “off-budget,” meaning their surpluses and deficits are reported separately and cannot be used to offset spending or tax cuts elsewhere. In practice, policymakers and the press focus on unified totals, which include everything. The off-budget label draws an accounting line on paper, but the underlying cash flows through the same Treasury. When Social Security ran surpluses for decades, those surpluses offset part of the general fund’s deficit in unified totals. Now that Social Security is drawing down reserves, its shortfall adds to the visible deficit instead of masking it.