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Medicare Part A, B, C and D explained

Last reviewed October 23, 2026 · 11 min read

Quick answer: Having Medicare Part A, B, C and D explained helps clarify your healthcare options. Part A covers inpatient hospital stays, while Part B covers outpatient medical care. Together with Part D prescription drug coverage, they form Original Medicare. Medicare Part C (Medicare Advantage) is an alternative bundled plan sold by private insurers that combines Parts A, B, and usually D.

Navigating federal healthcare benefits can feel overwhelming when you approach retirement or manage a qualifying disability. Knowing how Medicare Part A, B, C and D work together allows you to select the right coverage, budget for medical costs, and avoid permanent late enrollment penalties. This guide breaks down each part of Medicare, what each covers, how much you can expect to pay, and when to enroll.

What is Medicare Part A and how do you qualify?

Medicare Part A provides hospital insurance to cover inpatient hospital stays. Most individuals receive Part A without paying a monthly premium, though some people must pay a premium depending on their work history. A premium is the monthly fee you pay to keep health coverage active.

Premium-Free Part A Eligibility

To qualify for premium-free Part A based on age, you must be 65 or older and eligible for monthly Social Security or Railroad Retirement Board (RRB) cash benefits. Eligibility depends on earning a specific number of quarters of coverage (QCs) through Federal Insurance Contributions Act (FICA) payroll taxes. Most workers pay the full FICA tax, which qualifies them for both retirement cash benefits and premium-free Part A. Certain government employees pay only the Part A portion of the FICA tax, which earns QCs that count strictly toward premium-free Part A rather than monthly Social Security cash benefits.

If you have received Social Security or RRB benefits for at least four months before turning 65, you will get Part A automatically. If you are not receiving those cash benefits, you must contact the Social Security Administration to file an application. Part A coverage begins the month you turn 65, provided you file within six months of your 65th birthday month. If you file more than six months after turning 65, your Part A coverage will be retroactive for six months. If your 65th birthday falls on the first day of the month, your Part A coverage starts on the first day of the preceding month.

Paying a Premium for Part A

People who do not have enough work credits must pay a monthly premium for Part A. They do not get enrolled automatically at 65. To obtain premium Part A, you must apply through the Social Security Administration during an approved enrollment period, pay your monthly Part A premiums on time, and also enroll in and pay for Medicare Part B. Premium Part A coverage begins the month following the month you enroll.

Part A Coverage for Disabilities and Conditions

Medicare Part A is also available to individuals under 65 under specific medical circumstances:

Disability: Beneficiaries entitled to Social Security or RRB disability cash benefits are automatically enrolled in Part A after receiving cash benefits for 24 months. Disabled government employees become entitled after 29 months.
Amyotrophic Lateral Sclerosis (ALS): Individuals with ALS receive Part A the first month they are entitled to disability cash benefits, with no waiting period.
Child Disability Benefits: Part A entitlement based on child disability benefits cannot begin before age 20, or age 18 if the individual has ALS.
End-Stage Renal Disease (ESRD): People with ESRD who require regular dialysis or a kidney transplant qualify if they or a family member have sufficient work history. Part A coverage starts the third month after dialysis begins, the first month of dialysis if the person undergoes self-dialysis training, or the month of a kidney transplant (or two months earlier if hospitalized for transplant preparation).

What is Medicare Part B and what are its rules?

Medicare Part B provides medical insurance covering doctor visits, outpatient services, and necessary medical equipment. Part B is a voluntary program, and every enrolled individual must pay a monthly premium to maintain coverage.

If you receive Social Security or RRB benefits at least four months before becoming eligible, you are automatically enrolled in Part B alongside premium-free Part A. You have the choice to keep or refuse Part B. Residents of Puerto Rico who qualify automatically are enrolled only in Part A and must actively apply to get Part B. If you are not receiving retirement benefits, you must actively apply for Part B through the Social Security Administration.

If you must pay a premium for Part A, specific rules govern your Part B eligibility. You must be age 65 or older, reside in the United States, and be either a U.S. citizen or a lawfully admitted permanent resident who has lived in the country continuously for five years prior to applying. If you fail to enroll in Part B when you are first eligible, you will generally face a late enrollment penalty that is added to your premium for as long as you have Part B coverage.

Additionally, Medicare offers a Part B Immunosuppressive Drug benefit. This benefit is designed for individuals whose Part A coverage terminated 36 months after receiving a kidney transplant.

How do Medicare Part C and Original Medicare compare?

Original Medicare consists of Part A, Part B, and Part D, managed directly through the federal government. Medicare Part C, commonly known as Medicare Advantage, is a private insurance alternative that bundles these components into a single plan.

Medicare Advantage plans are offered by private companies and must provide the same coverage as Parts A and B, and they usually include Part D drug coverage as well. Beneficiaries who remain in Original Medicare often choose to purchase a Medigap plan, also known as Medicare Supplement insurance. Medigap policies are sold by private insurers to help pay for out-of-pocket costs that Parts A and B leave behind, including copays, coinsurance, and deductibles.

To understand how these options share costs, it helps to know standard insurance terms. A copay is a set dollar amount you pay for a medical visit or service. Coinsurance is your percentage share of the cost for a covered service. A deductible is the amount you must pay out of pocket before your insurance starts paying benefits. Medigap plans help cover these costs, but they do not include prescription drug coverage.

Coverage Feature	Original Medicare	Medicare Advantage (Part C)	Medigap (Supplement)
Plan Structure	Federal program (Parts A, B, and D)	Private bundled alternative plan	Private supplemental policy
Benefits Included	Hospital (A) and medical (B) care	Covers Part A, Part B, and usually Part D	Pays Part A and B copays, coinsurance, deductibles
Prescription Drugs	Requires adding separate Part D plan	Usually bundled into the plan	Does not include drug coverage
Quarterly Statement	Medicare Summary Notice (MSN)	Explanation of Benefits (EOB)	Not applicable

Every three months, Medicare beneficiaries receive a document detailing the care billed in their name. Original Medicare members receive a Medicare Summary Notice (MSN), while Medicare Advantage members receive an Explanation of Benefits (EOB). These statements are not bills or scams. You should review them carefully to confirm that all listed services were actually received. If you suspect an error or unauthorized charge, you can review your notice with a Statewide Health Insurance Benefits Advisors (SHIBA) volunteer for free, unbiased help.

How does Medicare Part D prescription drug coverage work?

Medicare Part D provides prescription drug coverage to help pay for both generic and brand-name medications. Part D is optional and is offered to anyone with Medicare through private insurance companies approved by Medicare.

For most people, Part D requires a monthly premium that usually costs about $33, though you may also be responsible for paying some medication costs out of pocket. You can compare plan formularies on the Medicare Plan Finder tool to see which plans cover your specific medicines. If you have limited income, the Medicare Savings Program can help pay for all or part of your Medicare premiums.

Even if you do not currently take prescription drugs, state insurance officials advise enrolling in a Part D plan when you are first eligible to prevent a late enrollment penalty later. However, you must exercise caution if you have other coverage:

Medicare Advantage Conflict: If you enroll in a Medicare Advantage plan that includes prescription drug coverage, do not sign up for a stand-alone Part D plan, or you might lose your Medicare Advantage plan entirely.
Employer or Retiree Coverage: If you are covered by an employer or retiree health plan, signing up for Part D could cause you to permanently lose your existing coverage. Always check with your benefits administrator before making changes.

What is IRMAA and how can you lower it?

An Income-Related Monthly Adjustment Amount (IRMAA) is an extra charge added to your Medicare Part B and Part D premiums if your household income exceeds certain thresholds. If your income has decreased because of a qualifying life-changing event, you can request that the Social Security Administration lower this additional amount.

Qualifying life-changing events include:

Marriage or divorce
The death of a spouse
Loss of income
Receipt of an employer settlement payment

To request a reduction, complete Form SSA-44 (Medicare Income-Related Monthly Adjustment Amount – Life-Changing Event). You can submit the form and your supporting evidence by uploading it to your online Social Security account, mailing or faxing it to your local Social Security office, or scheduling an appointment. If your income changed because of an amended tax return, call the Social Security Administration at 1-800-772-1213 (TTY 1-800-325-0778) to request an adjustment.

When can you sign up for Medicare Part A and Part B?

You can sign up for Medicare during specific enrollment periods defined by federal law. Signing up at the correct time ensures your coverage begins promptly and protects you from late enrollment penalties.

Initial Enrollment Period (IEP)

Your Initial Enrollment Period is a 7-month window based around your 65th birthday. It begins three months before the month you turn 65, includes your birthday month, and ends three months after you turn 65. If you qualify through disability, your IEP begins three months before your 25th month of disability cash benefits, includes the 25th month, and ends three months later. Coverage begins the month after you enroll during your IEP.

General Enrollment Period (GEP)

If you miss your initial window, you can sign up during the General Enrollment Period, which runs from January 1 through March 31 each year. Your coverage will begin the month after you enroll. However, you may have to pay a ongoing late enrollment penalty for Part B and premium Part A.

Special Enrollment Periods (SEPs)

Certain life circumstances allow you to sign up outside standard windows without paying a late penalty:

Working Aged and Working Disabled SEP: If you delayed enrollment because you were covered under a group health plan through your own, your spouse’s, or a family member’s current employment, you can enroll at any time while covered, or during the 8-month period that begins the month employment or group coverage ends, whichever comes first. Individuals with ESRD cannot use this SEP.
International Volunteers SEP: Individuals who volunteered outside the U.S. for at least 12 months on behalf of a tax-exempt organization and maintained health coverage have a 6-month SEP to enroll once volunteer service or coverage ends.
TRICARE Beneficiaries SEP: Individuals under 65 who gained Part A through disability or ESRD but did not take Part B because they had TRICARE Standard or Prime coverage can use this SEP to enroll in Part B.

Key takeaways

Part A covers hospital stays and is premium-free for individuals with sufficient FICA work quarters, while Part B covers outpatient medical care for a monthly premium.
Original Medicare consists of Parts A, B, and D, whereas Medicare Advantage (Part C) bundles hospital, medical, and usually drug coverage into a private plan.
Medigap policies help pay Part A and B deductibles, copays, and coinsurance, but they do not cover prescription drugs.
Medicare Part D covers generic and brand-name drugs for about $33 per month on average, and late enrollment can lead to permanent surcharges.
You can request to lower an IRMAA surcharge using Form SSA-44 if you experience a life-changing event such as job loss, divorce, or the death of a spouse.

Common mistakes to avoid

Delaying Part B without qualifying employer coverage: Missing your Initial Enrollment Period without active employment group coverage triggers a lifetime Part B penalty and delays coverage until the next General Enrollment Period.
Purchasing a Part D plan while on Medicare Advantage: Enrolling in a stand-alone drug plan can automatically cancel your Medicare Advantage Part C plan if it already includes prescription coverage.
Dropping retiree or employer benefits prematurely: Signing up for Medicare Part D while holding employer or retiree coverage can cause you to permanently lose your existing workplace health benefits.
Ignoring quarterly summary notices: Failing to check your Medicare Summary Notice or Explanation of Benefits leaves billing errors and unauthorized charges unnoticed.
Failing to dispute an IRMAA surcharge after income loss: Continuing to pay higher Part B and Part D premiums after a divorce, spousal death, or retirement without submitting Form SSA-44 wastes money.

Frequently asked questions

What is the difference between a Medicare Summary Notice and an Explanation of Benefits?

Both documents arrive every three months to list the medical care you received. You receive a Medicare Summary Notice (MSN) if you have Original Medicare, whereas you receive an Explanation of Benefits (EOB) if you are enrolled in a Medicare Advantage plan.

Can you enroll in both Medicare Advantage and a stand-alone Part D plan?

No, you should not enroll in a separate Part D prescription plan if your Medicare Advantage plan already includes drug coverage. Doing so can cause you to lose your Medicare Advantage plan.

How does Amyotrophic Lateral Sclerosis (ALS) affect Medicare Part A enrollment?

Individuals diagnosed with Amyotrophic Lateral Sclerosis (ALS) qualify for Medicare Part A the very first month they become entitled to Social Security or Railroad Retirement Board disability cash benefits, without any waiting period.

When does coverage start if your 65th birthday is on the first day of the month?

If your 65th birthday falls on the first day of a month, your Medicare Part A coverage begins on the first day of the calendar month immediately preceding your birth month.