How Medicare Is Financed: Trust Funds and Revenue Sources

Medicare's financing structure rests on two legally distinct trust funds that receive revenue from separate streams and pay out for different categories of services. Understanding how those funds are capitalized — and what happens when their balances shift — clarifies why payroll tax rates, premium levels, and general appropriations each carry direct consequences for the program's solvency. This page covers the definition of each trust fund, the revenue mechanisms that feed them, the scenarios in which multiple revenue streams interact, and the boundaries that separate what the trust funds can and cannot finance.

Definition and scope

Medicare operates through two statutory trust funds established under the Social Security Act: the Hospital Insurance (HI) Trust Fund and the Supplementary Medical Insurance (SMI) Trust Fund. The two funds are legally separate accounts held in the U.S. Treasury and managed by the Medicare Board of Trustees, a six-member body that includes the Secretaries of Treasury, Labor, and Health and Human Services, along with the Social Security Commissioner and two public trustees (CMS Medicare Trustees Report).

The HI Trust Fund — commonly associated with Medicare Part A (Hospital Insurance) — finances inpatient hospital care, skilled nursing facility stays, hospice, and some home health services. The SMI Trust Fund is divided into two accounts: one for Medicare Part B (Medical Insurance), covering physician services and outpatient care, and one for Medicare Part D (Prescription Drug Coverage).

The structural distinction matters because the HI Trust Fund can — in principle — be depleted, whereas the SMI Trust Fund cannot. By statute, the SMI Trust Fund is automatically re-funded each year through a combination of beneficiary premiums and general revenue appropriations, meaning Congress must provide sufficient funding for Part B and Part D regardless of prior balance levels (CMS.gov, Medicare Trust Fund Overview).

How it works

HI Trust Fund revenue sources:

1. Payroll taxes — The primary revenue stream. Employees and employers each pay 1.45% of wages, for a combined rate of 2.9%, under the Federal Insurance Contributions Act (FICA) (IRS Publication 15, Circular E). High earners pay an additional 0.9% on wages above $200,000 (individual) or $250,000 (joint), enacted under the Affordable Care Act.
2. Taxation of Social Security benefits — Beneficiaries with income above statutory thresholds pay federal income tax on up to 85% of their Social Security benefits; a portion of those revenues flows to the HI Trust Fund.
3. Interest on Treasury securities — The HI Trust Fund holds its reserves in special-issue U.S. Treasury securities and earns interest on those holdings.
4. Premiums from voluntary enrollees — Workers who did not pay Medicare taxes for 40 quarters may purchase Part A by paying a premium, contributing a smaller revenue stream.

SMI Trust Fund revenue sources:

1. Beneficiary premiums — In 2024, the standard Part B monthly premium was set at $174.70 (CMS 2024 Medicare Parts A & B Premiums and Deductibles). Premiums are income-adjusted for higher earners through the Income-Related Monthly Adjustment Amount (IRMAA); more detail on that mechanism is at /medicare-income-related-adjustment-irmaa.
2. General federal revenues — Approximately 74% of Part B spending is financed through general Treasury appropriations, making it the largest single revenue source for the SMI Trust Fund (CMS Medicare Trustees Report).
3. State transfers — For Part D, states contribute payments related to the dual-eligible population (individuals eligible for both Medicare and Medicaid) through a "clawback" formula.

The broader overview of Medicare's dimensions and scope provides context for how these financing mechanisms connect to enrollment and benefit structure.

Common scenarios

Payroll tax shortfall: When HI Trust Fund expenditures exceed payroll tax income — a condition that has occurred repeatedly since 2008 according to annual Trustees Reports — the fund draws down its reserves. The Trustees project dates of potential depletion in each annual report, triggering legislative attention to either increase revenues or reduce expenditures.

IRMAA surcharges: Higher-income beneficiaries pay more than the standard Part B premium. In 2024, the top IRMAA tier for individuals with modified adjusted gross income above $500,000 set the Part B premium at $594.00 per month (CMS 2024 Premiums Fact Sheet). These additional premium revenues flow into the SMI Trust Fund's Part B account.

Dual-eligible transitions: When a beneficiary shifts between Medicaid and Medicare as a primary payer, the SMI Trust Fund absorbs Part B costs while state Medicaid programs continue to cover some cost-sharing. This interaction is detailed further under Medicare as Secondary Payer.

General revenue trigger: Under the Medicare Modernization Act of 2003, if general revenues are projected to finance more than 45% of total Medicare spending for two consecutive years, the Trustees must issue a "Medicare funding warning." That warning obligates the President to submit legislation addressing the imbalance within 15 days of the next budget submission.

Decision boundaries

Several hard distinctions govern what each trust fund can and cannot do:

• No cross-fund borrowing: The HI Trust Fund cannot borrow from the SMI Trust Fund or vice versa. If HI reserves reach zero, the Social Security Act authorizes payment only up to the amount of incoming revenue — meaning benefits could be paid at a reduced rate, not eliminated but proportionally curtailed, without new congressional action.
• Premium adequacy requirement: Part B premium rates are set annually by the Secretary of Health and Human Services to cover 25% of projected Part B expenditures. The actuarial calculation is performed by CMS and published in the Federal Register each fall.
• General revenue limits on HI: General appropriations do not fund Part A. The HI Trust Fund is structurally dependent on payroll taxes and benefit taxation; there is no automatic backstop from general revenues the way Part B has.
• Medicare Advantage and Part D passthrough: Medicare Part C (Medicare Advantage) plans receive capitated payments drawn from both the HI and SMI Trust Funds, proportional to the benefits provided, rather than operating a separate trust fund of their own.

Beneficiaries seeking a full breakdown of cost obligations tied to these revenue mechanisms can refer to Medicare Costs: Premiums, Deductibles, and Copays. The home page provides a structured entry point to all related Medicare topics on this site.