How Medicare Works: A Beginner's Guide to Parts A-D

What Is Medicare, and Where Did It Come From?

Medicare is a federal health insurance program signed into law in 1965, created because nearly half of Americans over 65 had no health insurance at the time and most couldn't afford private coverage. Today it's run by the Centers for Medicare & Medicaid Services (CMS), and funded by the 1.45% Medicare payroll tax (matched by employers), monthly premiums, general federal revenue, and cost-sharing at the time of service. You've been paying into this system your entire working life, understanding what you're entitled to just means using what you've already paid for.

Who Qualifies for Medicare?

Most Americans qualify at age 65, regardless of retirement status. People under 65 who've received Social Security Disability Insurance (SSDI) for 24 months automatically qualify, and that waiting period is waived entirely for ALS (Lou Gehrig's disease). Anyone, at any age, with End-Stage Renal Disease requiring dialysis or a transplant also qualifies. To get premium-free Part A, you or your spouse generally need at least 40 quarters (10 years) of work paying Medicare taxes; if you fall short, you can still buy into Part A.

Part A: Hospital Insurance

Part A covers inpatient hospital care, skilled nursing facility care (under specific conditions), hospice, and some home health care. It does not cover doctor visits, most outpatient care, most prescriptions, or long-term custodial care, the daily-living assistance many people mistakenly assume Medicare will pay for.

Cost: Part A is premium-free for most Americans, since you've already paid for it through decades of payroll taxes. Cost-sharing is organized around a "benefit period," which starts at admission and ends once you've been out of the hospital 60 consecutive days; a readmission after that resets the clock and the deductible. The source lesson behind this guide cites a 2024 Part A deductible of $1,632 per benefit period, these figures adjust yearly, so confirm current numbers at Medicare.gov.

Skilled nursing facility coverage only applies after a qualifying hospital stay of at least three consecutive days, admission to a certified facility within 30 days, a doctor's certification of need, and a need for skilled, not just custodial, care.

Part B: Medical Insurance

Part B covers what Part A doesn't: doctor visits, outpatient procedures, many preventive services, durable medical equipment, mental health care, outpatient therapy, and ambulance services. It excludes most prescription drugs, routine dental and vision care, hearing aids, long-term care, and most care outside the U.S.

Part B has a standard monthly premium (higher for top earners through Income-Related Monthly Adjustment Amounts, or IRMAA, based on income from two years prior) and an annual deductible, after which you typically pay 20% of Medicare-approved costs, with no out-of-pocket maximum in Original Medicare alone. That uncapped 20% is one of the biggest financial exposures in the whole program, and the main reason supplemental coverage matters.

The upside: a long list of preventive services, the Annual Wellness Visit, mammograms, cancer and diabetes screenings, bone density tests, and flu, pneumonia, and COVID-19 vaccines among them, are covered at no cost through a participating provider. Use them.

Part C: Medicare Advantage

Medicare Advantage isn't a separate type of coverage the way A, B, and D are, it's an alternative way to receive your Medicare benefits, through a private insurer approved by Medicare instead of the government directly. You must already have Part A and Part B to join, and you'll keep paying your Part B premium; the plan may add its own premium or charge $0 beyond that.

By law, these plans must cover everything Original Medicare covers, and most add benefits it doesn't: prescription drug coverage, dental, vision, hearing, fitness memberships, and transportation to appointments. Plan types include HMOs (network-restricted, lower premiums), PPOs (more flexibility, higher cost out-of-network), PFFS plans, and Special Needs Plans for people with chronic conditions or dual Medicare/Medicaid eligibility.

The biggest advantage is a built-in annual out-of-pocket maximum, something Original Medicare alone doesn't offer, once you hit it, the plan covers 100% of in-network costs for the rest of the year. The tradeoffs are network restrictions, prior authorization requirements, and plans that can change their benefits and networks every year.

Part D: Prescription Drug Coverage

Part D is Medicare's prescription drug benefit, available either as a standalone Prescription Drug Plan alongside Original Medicare, or bundled into a Medicare Advantage plan (MA-PD). Each plan has a formulary, covered drugs sorted into tiers, from low-copay generics up to high-cost specialty drugs, plus a deductible, an initial coverage phase where costs are shared, and a catastrophic phase where, once you cross a spending threshold, you pay $0 for covered drugs the rest of the year.

The Inflation Reduction Act eliminated the old Part D coverage gap (the "donut hole") starting in 2025 and capped annual out-of-pocket drug costs, one of the biggest improvements to Part D since it launched. When choosing a plan, don't shop on premium alone, the Medicare Plan Finder at Medicare.gov lets you enter your actual medications and compare total annual cost, not just the sticker price.

Medicare Supplement Insurance (Medigap): Filling the Gaps

Medigap is private insurance that fills the holes in Original Medicare, deductibles, copayments, and coinsurance, and works alongside it, not instead of it. Depending on the plan, it can cover Part A and B coinsurance, the Part A deductible, skilled nursing coinsurance, and foreign travel emergency care. Medigap plans are standardized by federal law, so Plan G from any insurer covers the same benefits; only the premium differs, letting you shop purely on price.

Plan G is the most comprehensive option for new enrollees, covering everything except the annual Part B deductible. Plan N has a lower premium but adds small office and ER copays. Plans K and L trade higher cost-sharing for lower premiums, with built-in out-of-pocket caps. Medigap does not cover prescription drugs, dental, vision, hearing, or long-term care.

Medigap vs. Medicare Advantage: The Central Decision

Most people approaching 65 face one core choice: Original Medicare plus Medigap and Part D, or Medicare Advantage. Neither is universally better, it depends on your health, finances, geography, and tolerance for network restrictions. One asymmetry to know before choosing: switching from Medicare Advantage back to Medigap later isn't guaranteed. After your initial enrollment window, Medigap insurers in most states can medically underwrite, meaning higher premiums or denial based on health history, a reason to weigh this before choosing Medicare Advantage, not after.

When to Enroll in Medicare: Windows and Deadlines

Initial Enrollment Period (IEP): A seven-month window, three months before your birthday month, your birthday month, and three months after. Enroll before your birthday month for the smoothest coverage start.

Special Enrollment Period (SEP): If you or your spouse are still working at 65 with coverage from an employer with 20+ employees, you can delay Medicare penalty-free and get an 8-month SEP once employment or coverage ends. COBRA and marketplace coverage don't count as employer coverage, the SEP clock starts when the actual employer coverage ends, not when COBRA runs out. At smaller employers, Medicare usually becomes primary, so enrolling at 65 is typically necessary even while working.

General Enrollment Period (GEP): If you miss your IEP without a qualifying SEP, you can enroll January 1 – March 31 each year, with coverage starting July 1, but late enrollment penalties will likely apply.

Annual Enrollment Period (AEP): Every October 15 – December 7, you can switch between Original Medicare and Medicare Advantage, or change plans, effective January 1. A second window, the Medicare Advantage Open Enrollment Period (January 1 – March 31), lets those already in Medicare Advantage switch plans or move back to Original Medicare.

Medicare Late Enrollment Penalties, and How to Avoid Them

Missing your window doesn't just cost time, in some cases, it costs you for life.

Part B: 10% added to your premium for every full 12-month period you were eligible but not enrolled, permanently, for as long as you have Part B.

Part D: A percentage of the national base premium added for every month without Part D or other "creditable" drug coverage, also permanent. Employer, TRICARE, or VA coverage that's at least as good as Part D counts as creditable, letting you delay without penalty.

Part A: For those who pay a Part A premium, a 10% penalty applies for twice the number of years delayed.

Know your window, mark your calendar three months before your 65th birthday, and ask before it closes if you're unsure whether an SEP applies to you.

What Medicare Actually Costs

The figures below reflect the numbers cited in the source lesson for this guide (2024). Medicare premiums, deductibles, and thresholds are adjusted annually, so verify current amounts at Medicare.gov before budgeting.

Because IRMAA surcharges are based on income from two years prior, a large Roth conversion or capital gains event at 63 or 64 can raise premiums at 65. If your income has since dropped due to retirement or another qualifying event, you can appeal using Form SSA-44. Two more notes: Medicare premiums can be tax-deductible as a medical expense (self-employed enrollees may deduct them above the line), and once you enroll you can no longer contribute to an HSA, though you can still spend existing HSA funds tax-free on Medicare costs, so maxing contributions beforehand pays off.

Help Paying for Medicare: Savings Programs and Extra Help

If your income and assets are limited, Medicare Savings Programs (state-administered, with four tiers, QMB, SLMB, QI, and QDWI, covering different combinations of premiums, deductibles, and coinsurance) can significantly reduce costs; apply through your state Medicaid office. Extra Help, the Low Income Subsidy, does the same for Part D and can save thousands a year, anyone already on Medicaid, SSI, or a Medicare Savings Program automatically qualifies. Many states also run pharmaceutical assistance programs that coordinate with Part D. These programs are widely underused, so it's worth a few minutes to check eligibility.

Common Medicare Mistakes to Avoid

Missing the Initial Enrollment Period, the costliest mistake, with permanent penalties on Part B and D.

Assuming Medicare is free, premiums, deductibles, and uncapped Part B coinsurance add up.

Choosing a plan on premium alone instead of comparing total annual cost with your actual medications.

Not verifying your doctors are in-network before enrolling in Medicare Advantage, networks change yearly.

Dropping Medigap for Medicare Advantage, then struggling to switch back due to medical underwriting.

Assuming COBRA extends your enrollment window, it doesn't.

Skipping Extra Help or Medicare Savings Program applications when you'd likely qualify.

Not reviewing coverage every Annual Enrollment Period, even if you plan to stay put.