Medicare Part A covers hospital and inpatient care, while Part B covers doctor visits, outpatient services, and preventive care. Together, they form what’s called Original Medicare, the federal health insurance program available to most Americans at age 65. Understanding what each part pays for, what it costs, and how enrollment works can save you hundreds or even thousands of dollars in unexpected bills.
What Part A Covers
Part A is hospital insurance. It helps pay for inpatient care in hospitals, critical access hospitals, and skilled nursing facilities. It also covers hospice care and some home health care services. The key word is “inpatient.” If you’re formally admitted to a hospital and staying overnight, Part A is the piece of Medicare picking up most of that bill.
Skilled nursing facility care is covered only under specific conditions, typically after a qualifying hospital stay of at least three days. Part A doesn’t cover long-term care or custodial care, like help with bathing or dressing, which catches many people off guard.
What Part B Covers
Part B is medical insurance, covering two broad categories: medically necessary services and preventive services. Medically necessary services include anything that meets accepted standards of medical practice to diagnose or treat a condition. Preventive services focus on catching illness early or preventing it altogether.
Specific Part B coverage includes:
- Doctor visits and outpatient care
- Ambulance services
- Durable medical equipment like wheelchairs, walkers, and hospital beds
- Mental health and substance use disorder services
- Oxygen equipment and accessories
- Limited outpatient prescription drugs
- Clinical research participation
One valuable detail: you pay nothing for most preventive services (things like flu shots, cancer screenings, and annual wellness visits) as long as your provider accepts Medicare assignment. That means the provider agrees to accept Medicare’s approved amount as full payment.
What Original Medicare Does Not Cover
Even with both Part A and Part B, significant gaps remain. Original Medicare does not cover most dental care, including cleanings, fillings, tooth extractions, and dentures. It also excludes eye exams for prescription glasses, hearing aids and the exams needed to fit them, long-term care, cosmetic surgery, massage therapy, and routine physical exams (though it does cover an annual wellness visit, which is a different thing).
These gaps are why many people add supplemental coverage through a Medigap policy, a Medicare Advantage plan (Part C), or standalone dental and vision plans.
What Part A Costs
Most people pay $0 in monthly premiums for Part A because they or a spouse paid Medicare taxes for at least 10 years while working. If you don’t meet that threshold, you’ll pay a monthly premium that varies based on how many quarters of work history you have.
Even with premium-free Part A, you still face out-of-pocket costs when you use it. Each time you’re admitted to the hospital, you pay a deductible for that benefit period. After a certain number of days, daily coinsurance kicks in, and those amounts increase the longer you stay. Skilled nursing facility care follows a similar structure, with full coverage for the first 20 days after a qualifying stay and daily coinsurance after that.
What Part B Costs
Part B has a standard monthly premium of $185.00 in 2025, up from $174.70 in 2024. You also pay an annual deductible of $257 before Part B starts sharing costs. After meeting that deductible, you typically pay 20% of the Medicare-approved amount for most services, with no cap on out-of-pocket spending.
That 20% coinsurance with no annual maximum is one of the biggest financial risks in Original Medicare. A major surgery or cancer treatment can leave you responsible for tens of thousands of dollars, which is another reason supplemental coverage matters.
Higher Premiums for Higher Earners
If your income exceeds certain thresholds, you’ll pay more for Part B through what’s called an Income Related Monthly Adjustment Amount (IRMAA). Social Security looks at your tax return from two years prior to determine your bracket.
For 2026, single filers with income above $109,000 start paying surcharges. The Part B premium scales up from $284.10 per month at the first tier to $689.90 at the highest tier (income of $500,000 or more). For married couples filing jointly, the surcharges begin above $218,000 in income, topping out at $689.90 per month for income of $750,000 or more. If you’re married filing separately, the brackets are less forgiving: income above $109,000 jumps you straight to $649.20 per month.
When and How to Enroll
Your Initial Enrollment Period is a 7-month window that starts 3 months before the month you turn 65 and ends 3 months after it. Signing up during the first three months of this window gets your coverage started sooner. Waiting until the last few months delays your start date.
If you’re still working and covered by an employer plan when you turn 65, the enrollment rules depend on your employer’s size. At companies with 20 or more employees, the employer plan pays first and Medicare pays second, so you can generally delay enrolling in Part B without penalty. At companies with fewer than 20 employees, Medicare pays first. In that case, delaying Part B enrollment could leave you underinsured and trigger a penalty later.
Late Enrollment Penalties
Missing your enrollment window for Part B carries a lasting financial consequence. Your monthly premium increases by 10% for every full 12-month period you could have had Part B but didn’t sign up. This penalty doesn’t go away. It’s added to your premium for as long as you have Part B.
For example, if you delayed enrollment by two full years without qualifying for a special enrollment period, you’d pay a 20% surcharge on top of the standard premium for the rest of your time on Medicare. At 2025 rates, that’s roughly an extra $37 per month, every month, indefinitely. The penalty is designed to discourage people from waiting until they get sick to sign up, and it’s one of the most common and costly mistakes people make with Medicare.
Part A vs. Part B at a Glance
- Part A (Hospital Insurance): Inpatient hospital stays, skilled nursing facility care, hospice, some home health. Usually premium-free.
- Part B (Medical Insurance): Doctor visits, outpatient care, preventive services, medical equipment, ambulance services. Standard premium of $185/month in 2025 with a $257 annual deductible.
Together, Part A and Part B form the foundation of Medicare coverage. Most people layer additional coverage on top, whether through a Medicare Advantage plan that bundles everything together, a Medigap supplement that covers the cost-sharing gaps, or a Part D plan for prescription drugs. The right combination depends on your health needs, your budget, and how much financial risk you’re comfortable carrying.