Medicare Parts A, B, C and D Explained Without the Jargon
What each part of Medicare covers, the gap Original Medicare leaves, Advantage versus Medigap, and the deadlines that carry lifetime penalties.
Medicare parts explained in one paragraph: Part A covers hospital stays, Part B covers doctors and outpatient care, Part C (Medicare Advantage) is a private alternative that bundles A and B and usually drugs, and Part D covers prescriptions. Parts A and B together are Original Medicare, and they have no out-of-pocket maximum. Closing that gap with either Medigap or Advantage is the decision that matters. Medicare is four programmes wearing one name, plus a private supplement market layered on top. Here is what each piece does.
Medicare parts explained: what does each one cover?
Part A: hospital insurance. Inpatient hospital stays, limited skilled nursing facility care after a qualifying hospital stay, hospice, and some home health care. Most people pay no premium for Part A because they or a spouse paid Medicare taxes for at least ten years. It has a deductible per benefit period, and coinsurance that kicks in on long stays.
Part B: medical insurance. Doctor visits, outpatient care, preventive services, lab work, durable medical equipment. There is a monthly premium, an annual deductible, and then you pay 20% coinsurance on most services with no cap.
Part C: Medicare Advantage. Not extra coverage. It is an alternative delivery method: a private plan approved by Medicare that provides your Part A and Part B benefits, usually bundles Part D drug coverage, and often adds dental, vision, hearing and a gym benefit. It has a network and an out-of-pocket maximum.
Part D: prescription drugs. Private plans that cover outpatient prescriptions. Each has its own formulary, tiers and pharmacy network.
The gap that defines everything else
Original Medicare (Parts A and B together) has no out-of-pocket maximum. That 20% Part B coinsurance runs indefinitely. On a serious illness costing $400,000, 20% is $80,000, and nothing stops it.
Every other decision in Medicare is really a decision about how to close that hole. There are exactly two ways: buy a Medigap policy, or take a Medicare Advantage plan.
Medigap or Medicare Advantage: which should you choose?
This is the decision that matters, and it is much easier to make at 65 than later.
Medigap (Medicare Supplement)
A private policy that pays the deductibles, copays and coinsurance Original Medicare leaves you. Plans are standardised by letter, so Plan G from one insurer covers exactly the same things as Plan G from another, only the price and the service differ.
- Any provider who accepts Medicare. No networks, no referrals, works anywhere in the country.
- Predictable. With a comprehensive plan, you know roughly what a year costs regardless of what happens medically.
- Higher monthly premium, and you buy a separate Part D plan on top.
- No extras. No dental, vision or hearing.
Medicare Advantage
- Lower or zero additional premium, often with drug coverage and extras included.
- Has an out-of-pocket maximum, which Original Medicare does not.
- Network restrictions. HMO or PPO style, often requiring referrals, and the network can change annually.
- Prior authorisation applies to many services, which Original Medicare largely does not require.
- Costs are variable. Cheap in a healthy year, considerably more expensive in a bad one.
Why the timing matters so much
During your six-month Medigap Open Enrollment Period, which starts when you are 65 and enrolled in Part B, insurers must sell you any Medigap policy they offer at the standard price regardless of your health.
After that window closes, in most states they can medically underwrite you, and decline you, or price you out, based on conditions you have developed since.
The practical consequence: taking Medicare Advantage at 65 and deciding at 72 that you want Medigap instead may not be possible. Going the other way is generally easier. That asymmetry deserves real weight in the initial decision.
When can you enrol in Medicare?
Initial Enrollment Period. Seven months: the three months before the month you turn 65, that month, and the three after. Enrolling in the first three months means coverage starts the month you turn 65.
Special Enrollment Period. If you delayed because you had qualifying employer coverage, you get eight months after that coverage or employment ends to enrol in Part B without penalty.
General Enrollment Period. 1 January to 31 March each year, for people who missed their windows. Penalties usually apply.
Medicare Open Enrollment. 15 October to 7 December each year. Switch between Original Medicare and Advantage, change Advantage plans, or change Part D plans.
Medicare Advantage Open Enrollment. 1 January to 31 March. If you are already in an Advantage plan you may switch to another or return to Original Medicare.
How bad are the late enrollment penalties?
This is the part that costs people the most money for the least reason.
Part B. 10% added to your premium for each full 12-month period you could have enrolled and did not, without qualifying coverage. It applies for as long as you have Part B. Delay three years and you pay 30% more, forever.
Part D. 1% of the national base premium for each month without creditable drug coverage. Also permanent. People with no current prescriptions skip Part D and then discover at 74 that they are paying a lifetime surcharge on a plan they now need.
Still working at 65?
If your employer has 20 or more employees and you have credible coverage, you can usually delay Part B safely and enrol later through the Special Enrollment Period.
If the employer has fewer than 20 employees, Medicare usually becomes the primary payer whether or not you enrolled: meaning your group plan may pay only what it would have paid second, leaving you exposed for the rest.
Get the answer in writing from the employer's benefits administrator. Do not rely on assumptions, and do not rely on the answer a colleague got.
Separately: you cannot contribute to an HSA once enrolled in any part of Medicare. If you are still funding an HSA, that changes the maths on when to enrol.
What does Medicare not cover?
- Long-term custodial care. Help with bathing, dressing, eating. This is the largest uncovered risk in later life, and Medicare does not touch it.
- Most dental, vision and hearing under Original Medicare. Many Advantage plans include limited versions.
- Care outside the United States, with narrow exceptions. Some Medigap plans add limited foreign travel emergency coverage.
How much does Medicare actually cost?
Original Medicare is not free, and the cost has four separate components that people tend to discover one at a time.
Part A premium. Usually zero, if you or a spouse paid Medicare taxes for at least ten years. Fewer than 40 quarters of work history means a monthly premium, which can be substantial.
Part B premium. Everyone pays it, and it is income-related. Higher earners pay an Income-Related Monthly Adjustment Amount on top of the standard premium, assessed on your tax return from two years earlier. That two-year lookback catches people who retired recently: the surcharge is based on your working income, not your retirement income. A life-changing event such as retirement itself can be appealed using Form SSA-44.
Part D premium. Varies by plan, and also carries an income-related surcharge.
Everything Original Medicare does not pay. The Part A deductible per benefit period, the Part B deductible, and 20% coinsurance on most Part B services with no ceiling. This is the largest number on the list and the least predictable.
That last line is why a Medigap policy or a Medicare Advantage plan is not optional in practice. The choice between them is easiest to make during your initial six-month Medigap window, when insurers must sell you a policy regardless of health. Medicare.gov sets out the parts and the enrollment timing.
One planning note that catches people: you cannot contribute to an HSA once enrolled in any part of Medicare, so if you are still funding one, read how an HSA works before setting an enrollment date. The general cost-sharing mechanics are the same ones covered in deductible vs. out-of-pocket maximum.
A sensible order of operations
Approaching 65
- Three months before your 65th birthday month, confirm whether your current coverage lets you delay Part B. Get it in writing.
- If you are stopping HSA contributions, plan the timing: Medicare enrollment ends eligibility, and there is a six-month retroactive rule to be careful with.
- Decide Medigap versus Medicare Advantage while you have guaranteed issue rights. This is the decision that is hard to reverse.
- Enrol in a Part D plan even if you take no medication, unless you have other creditable drug coverage. The penalty is permanent.
- Compare Part D plans on your actual prescriptions using the Medicare Plan Finder, not on premium.
- Re-check your Advantage or Part D plan every autumn. Formularies and networks change annually and your plan can change under you.
The honest summary
With the Medicare parts explained, the choice comes down to this. If you want freedom to see any doctor anywhere, predictable costs, and no prior authorisation fights, and you can afford a higher monthly premium: Original Medicare plus Medigap plus Part D.
If you want a low premium, are comfortable inside a network, and value the extras: Medicare Advantage.
Both are legitimate. What is not legitimate is choosing on the monthly premium alone, because the premium is the one cost that behaves the same in a good year and a bad one, and Medicare, like all insurance, is really about the bad one.
Frequently asked
What is the difference between Medicare Advantage and Medigap?
Medigap supplements Original Medicare by paying the deductibles and coinsurance it leaves you, and you keep access to any provider who accepts Medicare. Medicare Advantage replaces Original Medicare with a private plan that has its own network and out-of-pocket maximum, usually bundling drug coverage and extras. You cannot hold both at the same time.
Do I have to enroll in Medicare at 65 if I am still working?
Not necessarily. If you have credible coverage through an employer with 20 or more employees, you can usually delay Part B without penalty and enrol later through a Special Enrollment Period. If the employer has fewer than 20 employees, Medicare typically becomes primary and delaying can leave you badly exposed. Confirm with the employer's benefits administrator in writing.
What is the Medicare Part B late enrollment penalty?
If you do not enrol when first eligible and do not have qualifying coverage, your Part B premium increases by 10% for each full 12-month period you could have had it. The increase is permanent and applies for as long as you have Part B.
Does Medicare cover long-term care?
No. Medicare covers limited skilled nursing after a qualifying hospital stay, but it does not cover custodial long-term care: help with daily activities such as bathing, dressing and eating. That is paid privately, through long-term care insurance, or through Medicaid once assets are largely spent down.
How much does Medicare cost per month?
Part A is usually free with 40 quarters of work history. Part B carries a standard premium that rises for higher earners through an income-related adjustment based on your tax return from two years earlier. Part D varies by plan and also has an income surcharge. On top of the premiums, Original Medicare leaves a deductible and 20% coinsurance with no cap.
Can I have both Medicare Advantage and Medigap?
No. Medigap supplements Original Medicare, while Medicare Advantage replaces it with a private plan. You hold one or the other. It is generally illegal for an insurer to sell you a Medigap policy while you are enrolled in Medicare Advantage, other than during a valid switching window.
Sources
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