Medicare Part A is hospital insurance, and Part B is medical insurance. Together, they form “Original Medicare,” the federal health coverage available to most Americans at age 65. Part A covers care you receive as an inpatient, while Part B covers doctor visits, outpatient services, and preventive care. Most people need both parts working together to have meaningful coverage.
What Part A Covers
Part A is sometimes called hospital insurance because it pays for care when you’re formally admitted as an inpatient. This includes stays at hospitals, critical access hospitals, and skilled nursing facilities. It also covers hospice care for people with terminal illness and certain home health services.
The key distinction is the word “inpatient.” If you’re admitted to a hospital for surgery, recovering from a serious illness, or transferred to a skilled nursing facility for rehabilitation after a hospital stay, Part A is the coverage that kicks in. It does not cover outpatient procedures performed at a hospital, even if you spend hours there. That falls under Part B.
What Part B Covers
Part B handles nearly everything that happens outside a hospital admission. It covers two broad categories: medically necessary services and preventive services. Medically necessary means anything that meets accepted standards of medical practice to diagnose or treat a condition. Preventive services are designed to catch illness early or prevent it entirely.
On the medical side, Part B pays for doctor visits, outpatient procedures, mental health and substance use treatment, ambulance services, durable medical equipment like wheelchairs and hospital beds, and even limited outpatient prescription drugs. If you use an insulin pump covered under Part B’s equipment benefit, your cost for insulin is capped at $35 per month’s supply.
The preventive coverage is extensive and, in most cases, completely free if your provider accepts Medicare’s standard payment terms. Covered preventive services include mammograms, colonoscopies, lung cancer screenings, diabetes screenings, depression screenings, glaucoma tests, bone density measurements, and cardiovascular disease screenings. Vaccines for flu, COVID-19, pneumonia, and hepatitis B are also covered at no cost. Every enrollee gets a one-time “Welcome to Medicare” preventive visit and an annual wellness visit each year after that.
What Original Medicare Does Not Cover
Parts A and B together still leave significant gaps. Routine dental care, including cleanings, fillings, extractions, dentures, and implants, is not covered in most cases. Routine vision exams and hearing aids are also excluded. Long-term custodial care (help with daily activities like bathing and dressing) falls outside Original Medicare’s scope as well. Most outpatient prescription drugs require a separate Part D plan. These gaps are a major reason many people add supplemental coverage, whether through a Medigap policy, a Medicare Advantage plan, or a standalone Part D drug plan.
What Each Part Costs in 2025
Part A is premium-free for most people. If you or your spouse paid Medicare taxes for at least 10 years (40 quarters) of work, you owe nothing monthly for Part A. You do pay a deductible each time you’re admitted to the hospital as an inpatient, and there are daily coinsurance costs for longer stays.
Part B has a standard monthly premium of $185.00 in 2025, which is typically deducted from your Social Security check. Higher earners pay more. The annual deductible for Part B is $257, meaning you pay that amount out of pocket each year before Part B starts sharing costs. After the deductible, you generally pay 20% of the Medicare-approved amount for most services. Preventive services are the exception: those are covered at 100% with no deductible.
When and How to Enroll
Your Initial Enrollment Period lasts seven months. It starts three months before the month you turn 65 and ends three months after. If you’re already receiving Social Security benefits, you’ll be enrolled in Parts A and B automatically. Otherwise, you need to sign up yourself through Social Security.
If you have health coverage through a current employer (your own or a spouse’s), you can delay Part B without penalty. Once that employer coverage ends, you get a Special Enrollment Period to sign up. But if you simply miss your window without qualifying employer coverage, the consequences are real and permanent.
Late Enrollment Penalties
Missing your enrollment window for Part B adds a 10% surcharge to your monthly premium for every full year you were eligible but didn’t sign up. That penalty stays with you for as long as you have Part B. For example, if you waited two full years to enroll, you’d pay a 20% penalty on top of the standard premium for the rest of your life. In 2026 numbers, that turns a $202.90 monthly premium into $243.50, permanently. The longer you wait, the steeper the penalty gets, so enrolling on time is one of the most consequential financial decisions in Medicare.
How Parts A and B Work Together
Think of Part A as your safety net for serious medical events that land you in a hospital bed, and Part B as the coverage that handles your day-to-day medical life: doctor appointments, lab work, screenings, equipment, and outpatient procedures. A single health episode often involves both. You might have surgery covered under Part A, then follow-up visits and physical therapy covered under Part B. Neither part alone provides complete coverage, which is why the vast majority of Medicare beneficiaries carry both.
Parts A and B together are called Original Medicare, but they’re just the foundation. Most people layer on additional coverage to fill the gaps in dental, vision, hearing, and prescription drugs, either through individual supplemental plans or by choosing a Medicare Advantage plan (Part C) that bundles everything together as an alternative to Original Medicare.