Medicare is usually described as four letters, and that is where the confusion starts. Parts A, B, C and D are not four levels of the same coverage, and they are not all run the same way. Two are the federal program itself, one is a private-plan alternative to it, and one is drug coverage you add. Here are the Medicare parts explained in plain English: what each one covers, how they combine into the two main ways to get Medicare, and when you can sign up so you avoid gaps and late penalties.
Part A: hospital insurance
Part A helps cover inpatient care in a hospital, care in a skilled nursing facility, hospice care, and some home health care. Think of it as the coverage for when you are admitted, not for when you visit a doctor's office.
Most people pay no monthly premium for Part A because they or a spouse paid Medicare taxes while working, generally for at least 10 years. People who do not qualify that way can usually buy Part A for a monthly premium. Premium-free does not mean cost-free: Part A has a deductible for hospital stays and cost sharing for longer stays, and those amounts are adjusted every year.
Part B: medical insurance
Part B covers services from doctors and other health care providers, outpatient care, durable medical equipment such as walkers and wheelchairs, some home health care, and many preventive services like screenings and vaccines.
Part B has a standard monthly premium that almost everyone pays, and people with higher incomes pay more. There is also a yearly deductible, after which you generally pay a share of the cost for most services. Because these figures change every year, confirm the current amounts on Medicare.gov or with a licensed agent rather than relying on an older chart.
Together, Part A and Part B are called Original Medicare. It lets you see any doctor or hospital in the country that accepts Medicare, generally without referrals. It also has no yearly cap on what you pay out of pocket, which is the gap a Medigap policy is designed to fill.
Part C: Medicare Advantage
Part C, better known as Medicare Advantage, is not extra coverage on top of Parts A and B. It is a different way to receive them. Private insurance companies approved by Medicare offer these plans, which bundle your Part A and Part B benefits and usually include Part D drug coverage. Many also add benefits Original Medicare does not cover, such as some dental, vision or hearing care.
The trade-off is structure. Medicare Advantage plans usually have provider networks and may require referrals or prior authorization for some services. They also have a yearly limit on what you pay out of pocket for covered care, something Original Medicare does not offer on its own. You still have Part A and Part B when you join one, and you generally keep paying your Part B premium.
Part D: prescription drug coverage
Part D helps pay for prescription drugs, and private insurers approved by Medicare offer it. You get it one of two ways: a stand-alone drug plan that works alongside Original Medicare, or a Medicare Advantage plan that includes drug coverage.
Every Part D plan has its own list of covered drugs and its own tiers, so the same prescription can cost very different amounts from one plan to the next. Our guide to how a formulary works explains how to check your drugs before you choose. Under the Inflation Reduction Act, Part D now has a yearly cap on out-of-pocket drug costs, and once you reach it you pay nothing more for covered drugs that year. The cap is indexed and changes annually.
One warning: if you go without Part D or other creditable drug coverage for a stretch after you first become eligible, you may owe a late enrollment penalty for as long as you have Part D.
How the parts fit together: two paths
In practice, almost everyone ends up on one of two paths:
- Original Medicare: Part A and Part B, usually plus a stand-alone Part D plan, and often a Medigap (Medicare Supplement) policy to help with deductibles and coinsurance.
- Medicare Advantage: a single private plan that delivers Part A and Part B benefits and usually Part D, often with extra benefits and a network.
Medigap is not a lettered part of Medicare. It is private insurance that works only with Original Medicare, and it is illegal for someone to sell you a Medigap policy while you have a Medicare Advantage plan. Our side-by-side comparison of Medicare Advantage and Original Medicare walks through which path tends to fit which situation.
When you can sign up for each part
Timing matters as much as the plan you pick. The main windows are:
- Initial Enrollment Period: a seven-month window around your 65th birthday, starting three months before the month you turn 65 and ending three months after it. This is your first chance to sign up for Part A and Part B.
- Special Enrollment Period for people still working: if you or your spouse have job-based coverage when you turn 65, you can generally delay Part B and sign up within eight months after that employment or coverage ends.
- General Enrollment Period: January 1 to March 31 each year, for people who missed their earlier window. Signing up late can mean a Part B late enrollment penalty that lasts as long as you have Part B, and it grows the longer you wait.
- Medicare Annual Enrollment Period: October 15 to December 7 each year, when people who already have Medicare can switch between Original Medicare and Medicare Advantage or change drug plans. Our guide to the Medicare Annual Enrollment Period covers what you can change.
- Medigap Open Enrollment Period: a one-time, six-month window that starts the first month you are 65 or older and enrolled in Part B. During it, you can buy any Medigap policy sold in your state, and the insurer cannot turn you down because of health problems.
That last window is the one people most often miss. Once it closes, federal law generally no longer guarantees you a Medigap policy, and an insurer may ask health questions, charge more, or decline you, apart from specific situations such as losing certain other coverage. Some states add their own protections. Many people under 65 also qualify for Medicare because of a disability, end-stage renal disease or ALS; their enrollment timing and Medigap options can differ.
Where to get help choosing
This article is general information, not personalized advice, and costs and rules can change from year to year. Medicare's own resources are free: you can compare every plan in your area at Medicare.gov, call 1-800-MEDICARE (1-800-633-4227), or get free one-on-one counseling from your State Health Insurance Assistance Program (SHIP).
A licensed agent can also compare the plans they represent against your doctors, prescriptions and budget. If you would like help, you can request a free quote, and it costs nothing to ask.
We do not offer every plan available in your area. Currently we represent 4 organizations which offer 1,172 products in your area. Please contact Medicare.gov, 1-800-MEDICARE, or your local State Health Insurance Program (SHIP) to get information on all of your options.