Medicare Parts A, B, C, and D: what each covers and when to sign up

Medicare is the federal health insurance program for people age 65 and older, plus younger people who receive Social Security Disability Insurance (SSDI) for at least 24 months and people with end-stage renal disease. Social Security and Medicare are run by different agencies, the Centers for Medicare & Medicaid Services (CMS) administers Medicare, but you apply for Medicare through Social Security, and Medicare premiums are usually deducted from your Social Security check.

This section covers Medicare eligibility, the four parts of Medicare (A, B, C, and D), when to enroll, what happens if you miss your enrollment window, and how Medicare coordinates with Social Security retirement benefits and SSDI.

The four parts of Medicare

Medicare is not a single insurance plan. It's a set of related programs, each covering different types of health care.

• Part A, Hospital insurance. Covers inpatient hospital stays, skilled nursing facilities, hospice, and some home health care. Most people get Part A premium-free if they or their spouse paid Medicare taxes for at least 10 years.
• Part B, Medical insurance. Covers doctor visits, outpatient care, preventive services, and durable medical equipment. Part B has a monthly premium ($185.00 in 2025 for most people) that is usually deducted directly from your Social Security check.
• Part C, Medicare Advantage. An alternative to original Medicare. Private insurers offer plans that bundle Part A, Part B, and usually Part D, often with extras like dental, vision, and hearing coverage. You still pay the Part B premium, plus whatever the Advantage plan charges.
• Part D, Prescription drug coverage. A standalone prescription drug plan you add to original Medicare. Premiums vary by plan and by state. Many Medicare Advantage plans include Part D in a single package.

Medicare enrollment windows

There are several enrollment periods, and they matter a lot, miss the right one and you can owe lifetime premium penalties.

Initial Enrollment Period (IEP)

Your initial enrollment period is a 7-month window around your 65th birthday: the 3 months before, the month you turn 65, and the 3 months after. This is the simplest time to enroll and it avoids late penalties. If you're already receiving Social Security retirement benefits when you turn 65, you're enrolled in Parts A and B automatically.

Special Enrollment Period (SEP)

If you're still working at 65 and have qualifying health coverage through your employer, you can delay Part B without penalty. Once that coverage ends, you have an 8-month special enrollment period to sign up for Part B. COBRA and retiree coverage do not count as qualifying coverage for this rule, you still need the SEP before they end.

General Enrollment Period (GEP)

If you miss both your initial and special enrollment windows, the general enrollment period runs January 1 through March 31 each year. Coverage starts the month after you enroll. This is the window where late-enrollment penalties can start applying.

Annual Open Enrollment (October 15 – December 7)

Once you're enrolled, the fall open enrollment window lets you switch between original Medicare and Medicare Advantage, change Part D plans, or adjust your coverage. Changes take effect the following January 1.

How Medicare and Social Security fit together

Medicare eligibility is independent of Social Security. You can delay claiming Social Security retirement benefits until age 70 while still enrolling in Medicare at 65, and for most people, you should. Delaying Medicare past 65 without qualifying employer coverage almost always leads to a late-enrollment penalty.

If you're already drawing Social Security when you turn 65, Medicare Part A and Part B enrollment happens automatically. If you're not yet drawing Social Security, you need to sign up for Medicare yourself, online at ssa.gov/medicare, by phone, or at a Social Security office.

If you're receiving SSDI, Medicare eligibility begins automatically after 24 months of SSDI payments (there's no waiting period for ALS or end-stage renal disease). Both Parts A and B start the 25th month.

Income-related adjustments (IRMAA)

Higher-income Medicare beneficiaries pay additional Part B and Part D premiums based on their income from two years ago. The Income-Related Monthly Adjustment Amount (IRMAA) kicks in at $106,000 in modified adjusted gross income for individuals in 2025 ($212,000 for couples filing jointly). There are five IRMAA brackets; the top bracket adds roughly $443 a month to your Part B premium.

If your income has dropped since the year SSA is using, retirement, death of a spouse, loss of income, you can request a reconsideration using Form SSA-44, and SSA may adjust your IRMAA based on your current income.

Before you enroll in Medicare, a short checklist

• Mark your initial enrollment period (3 months before your 65th birthday).
• Confirm whether you'll be enrolled automatically (you will if you're already on Social Security).
• Decide between original Medicare + Part D and Medicare Advantage, based on your doctors and prescriptions.
• If you're still working at 65, confirm with your employer that the coverage qualifies as "creditable" for Medicare purposes.
• Plan for Part B and Part D premiums being deducted from your Social Security check.