Medicare Part A is hospital insurance, covering inpatient stays and facility-based care. Part B is medical insurance, covering doctor visits, outpatient services, and preventive care. Together they form Original Medicare, but they work differently in terms of what they pay for, what they cost you, and how you enroll.
What Part A Covers
Part A kicks in when you’re admitted as an inpatient. It helps pay for care in hospitals, critical access hospitals, and skilled nursing facilities. It also covers hospice care and some home health care. Think of Part A as covering situations where you’re staying somewhere overnight for medical treatment: the room, the meals, the nursing care, and the drugs administered during your stay.
Skilled nursing facility coverage comes with a notable requirement. Medicare only covers a skilled nursing stay if you first had a qualifying inpatient hospital stay of at least three consecutive days (not counting the day you’re discharged). If you meet that requirement, days 1 through 20 in the facility are fully covered after your deductible. Days 21 through 100 require a daily copayment. After day 100, Medicare stops covering skilled nursing entirely.
Hospice care under Part A covers comfort-focused treatment for people with a terminal illness. This includes pain management, counseling, and related services, with little to no out-of-pocket cost for most covered services.
What Part B Covers
Part B covers two broad categories: medically necessary services and preventive services. Medically necessary services include doctor visits, outpatient procedures, lab tests, diagnostic imaging, physical therapy, and durable medical equipment like wheelchairs, walkers, and hospital beds. If you’re getting surgery that doesn’t require an overnight hospital admission, that’s Part B territory.
Preventive services are a standout feature of Part B. Screenings, vaccines, and yearly wellness visits are covered, and you pay nothing for most preventive services as long as your provider accepts Medicare’s payment terms. This includes things like flu shots, certain cancer screenings, and cardiovascular checks.
A useful way to remember the split: if you’re sleeping in a hospital bed, Part A is likely paying. If you’re walking into a clinic or doctor’s office and going home the same day, it’s Part B.
How They Cost You Differently
Part A and Part B have completely different cost structures, which is one of the biggest practical differences between them.
Most people pay no monthly premium for Part A. If you or your spouse paid Medicare taxes for at least 10 years (40 quarters of work), Part A is premium-free. You still owe a deductible each time you’re admitted to the hospital for a new benefit period, but there’s no recurring monthly bill.
Part B always has a monthly premium. For 2025, the standard Part B premium is $185.00 per month. Higher-income enrollees pay more based on their tax returns from two years prior. Part B also has an annual deductible of $257 in 2025. After meeting that deductible, you typically pay 20% of the Medicare-approved amount for most services, with no annual cap on out-of-pocket spending. That uncapped 20% is a major reason many people buy supplemental coverage.
Enrollment Rules and Timing
Your Initial Enrollment Period for Medicare lasts seven months: it starts three months before the month you turn 65 and ends three months after that birthday month. During this window, you can sign up for both Part A and Part B. Most people who are already receiving Social Security benefits get enrolled automatically.
Part A enrollment is straightforward for most people since it’s free. There’s little reason to delay. Part B is where timing matters more, because you’re committing to a monthly premium. If you have employer-based coverage through your own job or a spouse’s job, you can typically delay Part B without penalty. But if you don’t have qualifying coverage and you miss your enrollment window, the consequences are real.
What Happens If You Enroll Late
Missing your enrollment window for Part B triggers a late enrollment penalty that most people carry for life. The penalty adds 10% to your monthly premium for every full 12-month period you could have signed up but didn’t. So if you waited three years without qualifying coverage, your Part B premium would be 30% higher than the standard rate, permanently.
Beyond the financial penalty, you may also have to wait for the next General Enrollment Period (January through March each year) to sign up, with coverage not starting until July. That gap could leave you uninsured for months.
Part A penalties work similarly if you’re among the smaller group who must pay a Part A premium, though this affects far fewer people.
What Neither Part Covers
Original Medicare, even with both Part A and Part B, has significant gaps. Routine dental care like cleanings, fillings, extractions, dentures, and implants is not covered in most cases. Routine vision exams, eyeglasses, and hearing aids are also excluded. Long-term custodial care (help with daily activities like bathing or dressing without a skilled medical need) falls outside Medicare’s scope as well.
These gaps are why many people either enroll in a Medicare Advantage plan (Part C), which often bundles dental and vision benefits, or purchase a standalone Medigap policy to help cover the cost-sharing that Part A and Part B leave behind. Prescription drug coverage requires a separate Part D plan unless it’s included in a Medicare Advantage plan.
Part A vs. Part B at a Glance
- Part A: Covers inpatient hospital stays, skilled nursing (after a 3-day hospital stay), hospice, and some home health. Usually premium-free. Has a per-benefit-period hospital deductible.
- Part B: Covers doctor visits, outpatient care, preventive services, lab work, and medical equipment. Costs $185/month in 2025 with a $257 annual deductible, then 20% coinsurance with no annual cap.
Both parts are optional in the sense that you can decline them, but declining Part B when you don’t have other coverage creates a penalty that follows you indefinitely. For most people turning 65, enrolling in both parts during that initial seven-month window is the simplest path to avoiding unnecessary costs later.