Medicare Part A vs Part B: Which Pays for What

Part A vs Part B is one of the first comparisons new enrollees need to understand. Original Medicare has two parts, and each pays for a different kind of care. Part A is hospital insurance. Part B is medical insurance.

The line between them is not always obvious. For example, two people can spend the same night in the same hospital bed and get billed in very different ways. The deciding factor is often a single word on their chart. Knowing how the part a vs part b split works helps beneficiaries and their families avoid surprise bills, plan for out-of-pocket costs, and choose the right supplemental coverage.

What Part A and Part B Each Cover

Part A covers care you receive as a formally admitted inpatient. That includes semi-private rooms, meals, nursing care, and drugs given during an inpatient hospital stay. It also covers skilled nursing facility (SNF) care after a qualifying hospital stay. Hospice care falls under Part A, along with some home health services. Inpatient care in a psychiatric hospital is covered too, with a 190-day lifetime limit.

Part B covers medically necessary outpatient services and preventive care. Think doctor visits, lab tests, X-rays, outpatient surgery, and emergency room visits that don’t lead to admission. It also pays for durable medical equipment such as walkers, wheelchairs, and oxygen. Ambulance rides, outpatient mental health care, and many preventive screenings are included. Certain drugs a provider administers, like chemotherapy infusions, are also covered under Part B.

Here is where the part a vs part b question gets tricky. Your doctor’s services during a hospital stay are billed under Part B, not Part A. As a result, even a fully covered inpatient stay can generate Part B charges. In most cases, those charges come from physicians, surgeons, anesthesiologists, and radiologists who treat you in the hospital.

Part A vs Part B: A Side-by-Side Breakdown

The table below compares how each part handles common services. It also shows how cost-sharing typically works. Exact deductibles and premiums change every year, so always confirm current figures on Medicare.gov.

Part A cost-sharing is built around the “benefit period.” A benefit period starts the day you’re admitted. It ends once you’ve gone 60 days in a row without inpatient or SNF care. You can have several benefit periods in one year. Each one carries a new deductible. After 90 days in a single benefit period, you can tap 60 lifetime reserve days. Once those are used, they never renew.

Part B works differently. You pay one annual deductible. After that, Medicare typically pays 80% of the approved amount. You pay the remaining 20%. However, Original Medicare has no yearly out-of-pocket maximum. That 20% can add up fast during cancer treatment or dialysis. Understanding part a vs part b cost-sharing is a key reason many people buy a Medigap policy.

The Observation Status Trap and Other Gray Areas

Being in a hospital does not automatically mean Part A pays. If the hospital keeps you under “observation status,” you are technically an outpatient. Part B covers those services instead. This matters a great deal for SNF coverage. Part A only pays for skilled nursing care after a qualifying stay of at least 3 consecutive days as an inpatient. Observation days don’t count toward that 3-day rule.

Hospitals must give you a Medicare Outpatient Observation Notice (MOON) if you’re under observation for more than 24 hours. Read it carefully. Ask your doctor directly whether you’ve been admitted. Certain beneficiaries now have appeal rights when a hospital changes their status from inpatient to observation. Your local SHIP program can explain how that process works.

Home health care is another gray area. Both parts can cover it. Generally, Part A covers the first 100 visits following a qualifying hospital or SNF stay. Part B covers home health that doesn’t follow a hospital stay, plus any visits beyond that first 100. Typically, you owe nothing for covered home health visits under either part. Durable medical equipment used at home, though, still carries 20% coinsurance.

How to Enroll and Protect Yourself From Gaps

Your Initial Enrollment Period lasts 7 months. It starts 3 months before the month you turn 65 and ends 3 months after. If you already get Social Security benefits, the Social Security Administration (SSA) enrolls you in both parts automatically. Otherwise, you sign up through SSA online, by phone, or at a local office.

Many people delay Part B because they have job-based coverage. That’s allowed if the coverage comes from current employment at a large enough employer. You then get an 8-month Special Enrollment Period after that job or coverage ends. Be careful, however. COBRA and retiree plans don’t count as current employment coverage. Missing your window triggers a Part B late penalty. It adds 10% to your premium for each full 12-month period you went without coverage. That penalty usually lasts for life.

Part A penalties work differently. Most people get premium-free Part A after paying Medicare taxes for 40 quarters, or about 10 years. Those who must buy Part A and enroll late face a 10% surcharge. It lasts for twice the number of years they delayed. When weighing part a vs part b enrollment timing, remember that Part A is free for most people. Few have a reason to delay it. One exception involves people still contributing to a Health Savings Account (HSA), since Part A enrollment ends HSA eligibility.

Finally, consider how you’ll fill the gaps. Neither part covers most dental, vision, or hearing care. Neither covers routine outpatient prescriptions; that’s Part D. You can add a Medigap plan from carriers like Mutual of Omaha, Aetna, Cigna, or AARP/UnitedHealthcare. Alternatively, a Medicare Advantage plan from Humana, Blue Cross, or other insurers bundles Part A and Part B coverage. Advantage plans also include an annual out-of-pocket cap. Compare options during Open Enrollment, which runs October 15 through December 7. Your state’s free SHIP counselors can walk you through plans without a sales pitch.

Frequently Asked Questions

Is the ER covered by Part A or Part B?

An emergency room visit is typically covered by Part B. That changes if you’re formally admitted as an inpatient. In that case, outpatient services from the 3 days before admission are generally bundled into the Part A stay.

Do I need both Part A and Part B?

In most situations, yes. The part a vs part b split means each fills a separate role. For example, Part A won’t pay for doctor visits, and Part B won’t pay for your hospital room. You also need both to join a Medicare Advantage plan or buy Medigap.

Who pays for my doctor while I’m in the hospital?

Part B pays your doctors’ bills, even during an inpatient stay. Part A covers the hospital’s charges for your room, nursing, and supplies. This is one of the most common part a vs part b surprises on a Medicare Summary Notice.

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Official Sources & Resources

For verified information on Medicare regulations and consumer protection:

• Medicare.gov (Official Site): medicare.gov
• CMS (Centers for Medicare & Medicaid Services): cms.gov
• NAIC (National Association of Insurance Commissioners): naic.org
• KFF Medicare Research: kff.org/medicare
• Social Security Administration: ssa.gov

Content last reviewed October 2026. If you notice any outdated information, please contact us.

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