Medicare Explained: Parts A, B, C, and D for Retirement

Medicare is the federal health insurance program for Americans aged 65 and over, built from Part A for hospital care, Part B for medical care, Part D for prescriptions, and Part C, which bundles them through a private plan. Signing up is one of the milestones of retirement, and the enrollment rules carry real penalties if you miss them. Health care is also one of the largest costs in later life, so understanding Medicare is central to a plan, not a footnote.

I enrolled in Medicare myself, and I will say plainly that the alphabet of parts and windows was the most intimidating piece of my whole retirement. Once you see the structure, it is more manageable than it looks. Here it is in plain English, reviewed by a CERTIFIED FINANCIAL PLANNER. This is general information, not advice; for your own situation, Medicare.gov and the Social Security Administration are the official sources.

Eligibility: Medicare begins at 65

You become eligible for Medicare at age 65, regardless of whether you have stopped working. A smaller group qualifies earlier through certain disabilities, but 65 is the standard age, and it lines up with several other retirement timing decisions.

Eligibility is not the same as enrollment. If you are already collecting Social Security, you are usually enrolled in Parts A and B automatically and your Part B premium is deducted from your benefit. If you are not yet collecting Social Security, you have to sign up yourself, which is where many people slip up. Note that 65 is the Medicare age even though Social Security’s full retirement age is now 67, so the two no longer arrive together.

The four parts of Medicare

Medicare is divided into four parts that cover different kinds of care, and most people combine several of them. Here is what each does:

• Part A (hospital): inpatient hospital stays, skilled nursing, hospice. Most people pay no premium because they paid Medicare taxes during their working years.
• Part B (medical): doctor visits, outpatient care, preventive services. The standard premium is about $185 a month in 2025, adjusted each year.
• Part D (drugs): prescription drug coverage, offered through private plans, with its own premium.
• Part C (Medicare Advantage): a private plan that bundles Parts A and B, and usually D, into one package, often with extras.

Parts A and B together are called Original Medicare. Because Original Medicare leaves you with deductibles and coinsurance, most people add either a Medigap policy or choose a Medicare Advantage plan to manage those out-of-pocket costs, which is the next decision.

Medicare Advantage vs Medigap

Medicare Advantage and Medigap are two competing ways to cover the gaps in Original Medicare, and you generally pick one path, not both. This is the choice that trips up the most people, so it helps to see them side by side.

• Medicare Advantage (Part C): a private plan that replaces Original Medicare. It usually rolls in drug coverage and may add dental, vision, or hearing benefits. The trade-off is provider networks and rules about which doctors you can see.
• Medigap (Medicare Supplement): a separate private policy that sits on top of Original Medicare and pays many of your deductibles and coinsurance. You keep the freedom to see any doctor who accepts Medicare, usually for a higher premium.

There is no universally right answer. People who value low predictable costs and freedom of choice often lean Medigap; people who want one bundled plan with extras often lean Advantage. The important thing is to compare specific plans for your own doctors and medications at Medicare.gov, because the details vary enormously by area.

Enrollment windows and late penalties

You usually enroll during a seven-month window around your 65th birthday, and missing it can trigger penalties that last for life. This is the part I would underline twice, because the penalties are permanent surcharges, not one-time fees.

Your Initial Enrollment Period runs for seven months: the three months before your birthday month, the birthday month itself, and the three months after. If you miss it without qualifying coverage:

• Part B late penalty: your premium can rise by 10% for each full year you could have had Part B but did not, for as long as you have it.
• Part D late penalty: a smaller ongoing surcharge based on how long you went without drug coverage.

There is an important exception. If you are still working at 65 and covered by an employer group health plan, you may be able to delay Part B and Part D without penalty and sign up later through a Special Enrollment Period. That interacts with Health Savings Accounts, since enrolling in Medicare stops new HSA contributions, so it is worth planning that timing deliberately.

IRMAA: when higher income raises your premiums

Higher earners pay an income-related surcharge, called IRMAA, on top of their standard Part B and Part D premiums. Most people never pay it, but if your income is high it can add a meaningful amount, and the timing catches people off guard.

IRMAA is based on your income from two years prior. That means a single high-income year, including large retirement account withdrawals, a Roth conversion, or a big capital gain, can push up your Medicare premiums two years later. This is one of the clearest links between your tax planning and your health costs, and it is a big reason taxes in retirement and required minimum distributions deserve attention well before you turn 65.

What Medicare does not cover

Medicare does not cover most long-term care, and it generally leaves out routine dental, vision, and hearing care. Knowing the gaps is as important as knowing the coverage, because these are exactly the costs people assume are handled and find out too late are not.

The notable gaps:

• Long-term care: Medicare does not cover most extended help with daily living, in a nursing home or at home. This is the single largest gap, and it is why long-term care costs are planned separately.
• Routine dental, vision, and hearing: generally not covered by Original Medicare, though some Medicare Advantage plans add limited benefits.
• Care outside the US: generally not covered.

So Medicare is a strong foundation, not a complete shield. Pair it with a plan for long-term care and out-of-pocket costs, lean on an HSA where you have one, and confirm every figure here at Medicare.gov, since premiums and rules change every year. For how all of this fits with your income and savings, return to retirement planning.