Medicare Part A vs. Part B: Understanding the Coverage Divide

What Each Part Was Designed to Do

Medicare was structured in 1965 with a deliberate split: one part for hospital care, one part for physician and outpatient services. That division still holds today, and it explains why the program uses the names it does — Part A is literally called Hospital Insurance, and Part B is called Medical Insurance.

Think of it this way: if you end up admitted to a hospital overnight, Part A takes the lead. If you walk into a doctor's office for a checkup, a blood test, or a chemotherapy infusion, Part B handles it. The care setting — inpatient versus outpatient — is the clearest signal for which part applies.

That said, the boundary isn't always obvious in practice. A patient can spend two nights in a hospital and never technically be "admitted" — a status called observation care — which means Part B, not Part A, picks up the bill. Understanding these distinctions before you need care can save you from a surprise invoice. See our guide on how care setting affects coverage for a broader look at this dynamic.

Both parts fall under what Medicare calls Original Medicare — the fee-for-service program administered directly by the federal government. If you've heard of Medicare Advantage (Part C), that's a private-plan alternative that typically bundles both A and B into a single plan. But to understand any form of Medicare, you need a firm grip on Parts A and B first. Our article on your first year on Medicare walks through how all four parts fit together if you're just getting started.

Coverage Breakdown: What Each Part Actually Pays For

Let's move from concept to specifics. Below is a plain-language look at what each part covers — and just as importantly, what it doesn't.

Medicare Part A: What's Included

• Inpatient hospital stays — semi-private room, meals, nursing care, and most hospital services and supplies after you're formally admitted
• Skilled nursing facility (SNF) care — but only following a qualifying three-day inpatient hospital stay, and only for skilled (not custodial) care
• Hospice care — for terminally ill beneficiaries who elect palliative rather than curative treatment
• Home health services — limited to medically necessary, part-time skilled nursing or therapy when you're homebound

What Part A does not cover: long-term custodial care (think daily assistance with bathing or dressing), private-duty nursing, private hospital rooms unless medically necessary, or most prescription drugs administered outside a hospital. For a deeper dive, see our dedicated Part A explainer.

Medicare Part B: What's Included

• Doctor visits — office visits with your primary care physician or specialist
• Outpatient hospital services — same-day procedures, emergency room visits where you are not admitted
• Preventive care — annual wellness visits, flu shots, cancer screenings, and cardiovascular risk assessments at no cost-sharing when you see a participating provider
• Lab work and diagnostic imaging — blood tests, X-rays, MRIs, and CT scans ordered by your doctor
• Durable medical equipment (DME) — wheelchairs, walkers, home oxygen equipment, and CPAP machines when medically necessary
• Outpatient mental health services — therapy and psychiatric evaluations outside a hospital
• Some drugs administered in a clinical setting — notably chemotherapy infusions, certain biologics, and drugs administered by a provider (not self-administered prescriptions, which fall to Part D)

For a detailed look at Part B's scope, including what it doesn't pay for, visit our article Medicare Part B: Medical Insurance and What It Pays For.

Costs: Premiums, Deductibles, and Cost-Sharing Side by Side

The cost structures of Parts A and B are completely separate — and quite different from each other. Many new beneficiaries assume they share a single deductible the way a typical employer plan works. They don't.

Part A Costs

Most people — those who worked and paid Medicare taxes for at least 40 quarters (10 years) — pay no monthly premium for Part A. If you worked between 30 and 39 quarters, you pay a reduced premium; fewer than 30 quarters means the full premium applies. The "free" Part A story is more nuanced than it sounds — there are still costs to understand even when the premium is zero.

Beyond the premium:

• Inpatient deductible: Applies per benefit period, not per calendar year. In 2024 that's $1,632 per benefit period.
• Coinsurance: Days 1–60 in a hospital are covered after the deductible. Days 61–90 require daily coinsurance ($408/day in 2024). Beyond day 90, lifetime reserve days kick in at higher coinsurance.
• SNF coinsurance: Days 1–20 are fully covered; days 21–100 carry coinsurance ($204/day in 2024); day 101 and beyond, you pay 100%.

Part B Costs

Part B always comes with a monthly premium. The standard amount in 2024 is $174.70/month, though higher-income beneficiaries pay more through IRMAA surcharges.

• Annual deductible: $240 in 2024, reset each January 1.
• Coinsurance: After the deductible, Medicare pays 80% of the approved amount; you pay the remaining 20% with no out-of-pocket cap in Original Medicare.
• Preventive services: Most are covered at 100% with no cost-sharing when you use a Medicare-participating provider.

That uncapped 20% coinsurance is one reason many beneficiaries add a Medigap (Medicare Supplement) plan — to limit their exposure on expensive outpatient procedures. For a full cost reference, see Medicare Costs at a Glance.

Enrollment Rules: When and How Each Part Starts

Both parts share the same initial enrollment window — a seven-month period centered on your 65th birthday month. But the consequences of missing that window differ significantly between the two parts.

Part A Enrollment

If you're already collecting Social Security before age 65, you're enrolled in Part A automatically. If not, you need to sign up actively. Since most people pay no premium for Part A, there's rarely a reason to delay — though technically you can if you prefer to time your start date. The late-enrollment penalty for Part A is relatively minor: 10% added to your premium for twice the number of years you went without coverage. Because the premium is zero for most, this penalty often has no practical bite.

Part B Enrollment

Part B is where enrollment decisions get consequential. If you don't enroll during your Initial Enrollment Period and don't have qualifying creditable coverage (such as active employer group health insurance through current employment), you face a permanent late-enrollment penalty of 10% added to your premium for every 12-month period you went without Part B. That penalty never goes away.

There's an important distinction between having employer coverage through current work versus retirement coverage or COBRA — only the former qualifies as creditable coverage that lets you delay Part B penalty-free. Retiree coverage and COBRA do not protect you from the penalty.

Once enrolled, your Special Enrollment Period (SEP) activates when your employer coverage ends, giving you eight months to enroll in Part B without penalty.

If you're weighing Original Medicare against a Medicare Advantage plan, understanding enrollment is equally important — read Original Medicare vs. Medicare Advantage before making that call.

Coverage Gaps: What Neither Part Handles

Even together, Parts A and B leave meaningful gaps. Understanding those gaps upfront prevents the most common Medicare surprises.

Prescription Drugs

Neither Part A nor Part B covers self-administered medications — the kind you pick up at a pharmacy. That's the domain of Part D, a standalone drug plan you add on top of Original Medicare. Part B does cover a limited set of drugs administered by a provider in a clinical setting (like infused cancer therapies), but that's the exception, not the rule. Our article Does Part B Cover Prescriptions? separates the common assumptions from the actual rules.

Dental, Vision, and Hearing

Routine dental care, eyeglasses, and hearing aids are not covered by either part. Medicare Advantage plans sometimes include these as supplemental benefits, but Original Medicare does not. This is one of the most consistently surprising gaps for new beneficiaries.

Long-Term Custodial Care

Part A covers skilled nursing care after a hospital stay — occupational therapy, wound care, IV medications — for a limited time. It does not cover custodial care: help with bathing, dressing, toileting, or eating. That's a crucial distinction. If you need long-term assisted living or nursing home care without a skilled component, Medicare pays nothing. What Medicare Doesn't Cover maps out all four parts' blind spots in one place.

No Out-of-Pocket Maximum in Original Medicare

This is often overlooked. A typical private insurance plan caps what you pay each year. Original Medicare (Parts A and B together) has no such limit. A catastrophic illness could expose you to Part B's 20% coinsurance on enormous bills with no ceiling. Medigap plans exist specifically to address this vulnerability.

For a broader look at how coverage gaps affect care decisions — including the rehab vs. habilitation distinction that affects many post-hospital patients — see Rehabilitative vs. Habilitative Services.

Why Most Beneficiaries Need Both — and How They Work Together

Parts A and B aren't alternatives to each other — they're complements. Medicare designed them as a paired system, and the vast majority of beneficiaries enroll in both from the start for a simple reason: life doesn't divide neatly into hospital care and outpatient care.

Consider a realistic scenario: you develop chest pain, call your doctor, and end up admitted through the emergency room. Your ER visit — before admission — is Part B. Your inpatient hospital stay is Part A. When you're discharged to a skilled nursing facility, that's Part A again. When you follow up with your cardiologist's office two weeks later, that's Part B. All of this can happen within a single episode of illness.

Without Part A, your hospital bill would be entirely out of pocket. Without Part B, your doctors' fees, outpatient diagnostics, and facility-based procedures would be uncovered. The two parts aren't redundant — they're sequential, each picking up where the other stops.

To see how Parts C and D fit into this picture and complete the coverage system, read How the Four Medicare Parts Work Together.

One practical implication: if you're comparing Medicare plan types and wondering whether Original Medicare (A+B) or Medicare Advantage (Part C) makes more sense for you, the comparison still begins here. Every Medicare Advantage plan is required by law to provide at least the same coverage as Parts A and B. Knowing what A and B include tells you the minimum benefit floor for any Medicare plan you consider.

If you're interested in how the plan-type decision maps to familiar concepts like network restrictions and referral requirements, our HMO vs. PPO comparison provides useful context — many Medicare Advantage plans operate on HMO or PPO structures.