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

Medicare is the federal health program most Americans rely on starting at age 65, yet its structure confuses many people at exactly the moment they need it. The program is divided into four parts, each covering different services and carrying different rules. Knowing what Parts A, B, C, and D do makes your enrollment decisions far clearer.

1. Who Qualifies and When

Most people become eligible for Medicare at age 65. If you are already receiving Social Security, enrollment in Parts A and B is often automatic. If you are not, you generally need to sign up yourself during a set window, so do not assume it happens for you.

Some people qualify before 65 due to certain disabilities or specific medical conditions. Eligibility and the mechanics of enrollment can vary, so confirm your situation and current rules at Medicare.gov. Getting the timing right matters, because late enrollment can trigger lasting penalties that we cover later in this article.

2. Part A: Hospital Insurance

Part A is the hospital side of Medicare. It helps cover inpatient hospital stays, care in a skilled nursing facility, some home health care, and hospice. Most people do not pay a monthly premium for Part A because they or a spouse paid Medicare taxes while working.

Premium free does not mean cost free. Part A still has deductibles and, for longer stays, daily coinsurance amounts. Those figures change each year, so check current figures at Medicare.gov. Part A is the foundation of your coverage, but on its own it leaves a lot of everyday medical care untouched, which is where Part B comes in.

3. Part B: Medical Insurance

Part B covers the outpatient side of medicine: doctor visits, preventive services, lab tests, durable medical equipment, and many other services you use outside a hospital. Unlike Part A, Part B charges a monthly premium that most enrollees pay.

The standard Part B premium is set each year, and higher-income enrollees pay more through a surcharge known as IRMAA, the income-related monthly adjustment amount. Because IRMAA is driven by your income, readers still earning may want to review our guide on working in retirement and Social Security. Part B also has an annual deductible and typically covers 80 percent of approved services, leaving you responsible for the rest.

4. Part C: Medicare Advantage

Part C, better known as Medicare Advantage, is an alternative way to receive your benefits. Private insurers approved by Medicare bundle Part A and Part B, and usually Part D, into a single plan. Many plans add extras such as dental, vision, or hearing coverage.

Medicare Advantage plans often use provider networks and require referrals or prior authorizations, trading some flexibility for potential cost savings and added benefits. This is a meaningfully different path from Original Medicare. If you are weighing the two approaches, our detailed comparison of Medicare versus Medicare Advantage walks through the tradeoffs in depth.

5. Part D: Prescription Drug Coverage

Part D helps pay for prescription medications. It is offered through private plans, either as a standalone policy added to Original Medicare or built into a Medicare Advantage plan. Each plan has its own list of covered drugs, called a formulary, and its own pricing tiers.

Because formularies differ, the best Part D plan depends on the specific medications you take. A plan that is cheap for one person can be expensive for another. Review the drug list carefully and compare total expected costs, not just the premium. Confirm current details and plan options at Medicare.gov during your enrollment window.

6. Enrollment Periods and Penalties

Medicare timing is strict, and missing a window can cost you for years. The main windows to know are summarized below.

• Initial Enrollment Period: the seven-month window around your 65th birthday
• General Enrollment Period: for those who missed their initial window
• Annual Open Enrollment: a yearly chance to change plans
• Special Enrollment Periods: triggered by certain life events
• Late enrollment penalties for Part B and Part D can be permanent

The Initial Enrollment Period is the one most people use. Signing up late for Part B or Part D without qualifying coverage can add a lasting surcharge to your premium. If you still have employer coverage at 65, special rules may apply, so verify your situation before delaying.

7. Filling the Gaps

Original Medicare, meaning Parts A and B, does not cover everything and has no annual out-of-pocket maximum by itself. To limit exposure, many people add a Medigap policy, also called Medicare Supplement Insurance, which helps pay deductibles and coinsurance.

You generally cannot pair a Medigap policy with a Medicare Advantage plan, so this is an either-or decision at a high level. This article is educational and not official advice. Your health needs, budget, and preferred doctors should guide the choice, and you should confirm specifics at Medicare.gov before enrolling.

The key takeaway: Part A covers hospitals, Part B covers outpatient care, Part C bundles everything through a private plan, and Part D covers drugs, so match the parts to your needs and enroll on time to avoid penalties.

Frequently Asked Questions

Do I have to pay for Part A?

Most people get premium-free Part A because they or a spouse paid Medicare taxes for enough years. Even so, Part A has deductibles and coinsurance for longer stays. Check the current year’s amounts at Medicare.gov, since they change annually.

What is IRMAA and who pays it?

IRMAA is an income-related surcharge added to Part B and Part D premiums for higher-income enrollees. It is based on your tax return from a couple of years earlier. If your income has dropped due to a life event, you may be able to request a review.

Can I have both Medicare Advantage and Medigap?

No. Medigap works only with Original Medicare, not with a Medicare Advantage plan. You choose one path or the other. To understand which fits you, read our comparison of Medicare versus Medicare Advantage.