Medicare Parts A, B, C, and D: A Plain-Language Breakdown

Confused by Medicare's alphabet? This guide explains each part, what it covers, and the gaps you may still need to plan around.

How Medicare is organized

Medicare is the federal health insurance program primarily for adults aged 65 and older, as well as certain younger people with qualifying disabilities or conditions such as end-stage renal disease. Congress structured the program in distinct parts, each covering a different category of care. Knowing what each part does prevents costly coverage gaps in retirement.

Eligibility age	Generally 65, or younger with qualifying disability or condition (Medicare.gov)
Part A premium (most enrollees)	$0 if you or a spouse paid Medicare taxes for 10+ years (CMS, 2024)
Part B standard monthly premium	$174.70 (base amount, subject to income adjustments) (CMS, 2024)
Part D late enrollment penalty	1% of national base premium per month without creditable coverage (CMS, 2024)
Original Medicare coverage of long-term care	Not covered (custodial care excluded) (Medicare.gov)
Part C plans available	Offered through private insurers approved by CMS (CMS, 2024)

Medicare pairs with broader retirement planning. If you have not yet sorted out your income sources, the Social Security benefits guide for pre-retirees covers how those payments interact with your healthcare costs.

Part A: hospital insurance

Part A covers inpatient hospital stays, care in a skilled nursing facility (following a qualifying hospital stay of at least three days), some home health services, and hospice care. Most people pay no monthly premium for Part A because they or a spouse paid Medicare taxes for at least 40 calendar quarters during their working years.

Part A does not mean unlimited free hospital care. Each benefit period carries a deductible, and for stays beyond 60 days, daily coinsurance charges apply. Custodial or long-term care (help with daily living activities) is explicitly excluded. That gap is one reason common beliefs about long-term care insurance deserve scrutiny.

Part B: outpatient medical insurance

Part B covers physician visits, outpatient procedures, preventive services, durable medical equipment, and most medically necessary services not provided in a hospital. Unlike Part A, nearly all enrollees pay a monthly premium for Part B. Higher-income beneficiaries pay more through an income-related adjustment known as IRMAA (Income-Related Monthly Adjustment Amount).

After the annual deductible, Medicare typically pays 80 percent of approved costs; you are responsible for the remaining 20 percent with no out-of-pocket maximum under Original Medicare alone. That uncapped 20 percent is a real financial exposure for a serious illness, which is why many retirees add supplemental coverage.

Premium

The monthly amount you pay to maintain insurance coverage, regardless of whether you use any medical services that month.

Deductible

The amount you pay out of pocket for covered services before your insurance begins to share costs.

Coinsurance

Your share of costs for a covered service after you have met your deductible, expressed as a percentage of the total charge.

Medicare Advantage (Part C)

An alternative way to receive Medicare benefits through a private insurer approved by the federal government, often bundling Parts A, B, and sometimes D into one plan.

Formulary

The list of prescription drugs that a Part D plan covers, organized into pricing tiers that affect your cost-sharing.

Coverage gap

A phase in Medicare Part D spending, sometimes called the donut hole, where your cost-sharing for prescription drugs changes after you and your plan reach a combined spending threshold.

Medigap

A supplemental insurance policy sold by private insurers that helps pay costs not covered by Original Medicare, such as copayments, coinsurance, and deductibles.

Part C and Part D: alternatives and prescriptions

Part C, marketed as Medicare Advantage, is not a separate pool of benefits. It is an alternative delivery method: private insurers approved by the Centers for Medicare and Medicaid Services (CMS) agree to provide at least all the benefits of Parts A and B, and most plans bundle Part D drug coverage as well. Plans typically use provider networks (HMO or PPO structures), and your costs and covered services vary by plan and geography.

Part D covers outpatient prescription drugs through standalone plans or through Medicare Advantage plans that include drug coverage. Each Part D plan uses a formulary. If you delay enrolling in Part D when you are first eligible and lack other creditable drug coverage (such as coverage from an employer or union), a late enrollment penalty applies and stays with you permanently.

Choosing between Original Medicare with a Medigap supplement and a Medicare Advantage plan involves real trade-offs in network flexibility, cost predictability, and additional benefits. The Medigap vs. Medicare Advantage comparison examines how each approach works before you make that call.

What Medicare does not cover

Several categories fall outside Medicare's scope regardless of which parts you carry: routine dental care, most vision and hearing services, custodial long-term care, and care received outside the United States. These exclusions can generate substantial expenses in retirement, and awareness of them is the first step toward planning around them.

Building a full picture of retirement costs requires understanding both your healthcare coverage and your income sources. The retirement planning key terms glossary defines concepts such as Required Minimum Distributions and defined benefit plans that interact with how you budget for healthcare premiums and out-of-pocket costs.

This article provides general educational information about Medicare and is not personalized insurance, financial, legal, or tax advice. Coverage details, premiums, and eligibility rules change annually and vary by individual circumstances. Read actual plan documents carefully and consult a licensed insurance agent, certified financial planner, or the State Health Insurance Assistance Program (SHIP) in your state before making coverage decisions.