How Does Medicare Work

How does Medicare work? It is one of the most important questions Americans ask as they approach age 65 — or when they continue working past traditional retirement age and need to understand how employer coverage interacts with Medicare’s enrollment rules. Medicare is a federal health insurance program primarily for people 65 and older, and also for certain younger individuals with qualifying disabilities or end-stage renal disease. Even though millions of Americans rely on it, Medicare can feel complicated because it has multiple distinct parts, strict enrollment windows, plan choices that can change year to year, and financial penalties for signing up at the wrong time.

At Diversified Insurance Brokers, we help you understand each piece of Medicare, how to coordinate it with your retirement planning, and how to avoid the common and costly enrollment mistakes that create unnecessary penalties and coverage gaps. Our full Medicare Playbook provides a comprehensive planning reference, and our advisors help you apply the rules to your specific situation rather than navigating a confusing system alone. The goal is simple: get the coverage you need, avoid gaps, keep costs predictable, and build a Medicare setup that works in the real world — not just on paper.

Think of Medicare as a framework. The federal government defines the rules and coverage parameters, but your actual healthcare experience depends on which path you choose, when you enroll, how you address the gaps Medicare does not automatically cover, and whether your plan matches how you actually access care. Problems usually come from timing mistakes, choosing a plan that does not fit your provider relationships or prescriptions, or assuming benefits work the same way across different plan types. When Medicare is set up correctly, it can be very strong coverage — and this guide is designed to give you the plain-English foundation you need to make those decisions confidently.

The Four Core Parts of Medicare

Medicare is organized into four distinct parts. Two are administered directly by the federal government under Original Medicare, and two involve private insurance carriers operating under Medicare rules. Your choices across these parts determine how you receive care, what you pay out of pocket, which providers you can access, and how your prescription drugs are covered. Understanding what each part does — and what it does not do — is the foundation of every good Medicare decision.

Medicare Part A: Hospital Insurance

Medicare Part A covers inpatient hospital care, skilled nursing facility care following a qualifying hospital stay, hospice care, and limited home health services. Most people who worked and paid Medicare payroll taxes for at least 40 quarters (10 years) receive Part A with no monthly premium. People with 30 to 39 quarters of work history pay a reduced Part A premium; those with fewer than 30 quarters pay the full premium. Receiving premium-free Part A does not mean hospital care is free — Part A still applies deductibles and cost-sharing that can be significant for extended or repeated inpatient stays.

The Part A benefit period structure is worth understanding clearly. A benefit period begins the day you are admitted as a hospital inpatient and ends when you have been out of the hospital or skilled nursing facility for 60 consecutive days. The Part A deductible applies once per benefit period — not once per year — which means a beneficiary who has two separate hospitalizations in the same calendar year separated by fewer than 60 days pays only one deductible, but a beneficiary with two hospitalizations separated by more than 60 days pays two separate deductibles. For inpatient hospital stays, Part A covers the full cost for days one through 60 (above the deductible), then applies daily coinsurance for days 61 through 90, and lifetime reserve days with higher coinsurance beyond 90 days. Skilled nursing facility coverage applies after a qualifying hospital stay of at least three days and covers the full cost for days one through 20, then applies daily coinsurance for days 21 through 100, with no coverage beyond 100 days. These cost-sharing structures are why many beneficiaries add Medigap coverage or Medicare Advantage to reduce Part A exposure.

Medicare Part B: Medical Insurance

Medicare Part B covers outpatient medical services — doctor and specialist visits, preventive care screenings and vaccines, laboratory tests, diagnostic imaging (X-rays, MRIs, CT scans), durable medical equipment, outpatient surgery, mental health services, and certain medications administered in a clinical setting. Part B has a standard monthly premium that adjusts annually, plus a separate annual deductible and a 20% coinsurance responsibility for most covered services. Critically, Original Medicare Part B has no annual out-of-pocket maximum — meaning the 20% coinsurance applies to every covered service without a ceiling, creating potentially unlimited cost exposure for beneficiaries without supplemental coverage.

Higher-income beneficiaries pay more for Part B due to IRMAA (Income-Related Monthly Adjustment Amount) surcharges, which begin at income above approximately $106,000 for individuals and $212,000 for married couples filing jointly (thresholds adjust annually). IRMAA is determined based on income from two years prior, which means income decisions made at 63 and 64 — including large Roth conversions, business sales, or significant IRA distributions — can affect Medicare Part B premiums at 65 and 66.

Part B enrollment is the most consequential timing decision in Medicare because the late enrollment penalty for Part B is permanent. Delaying Part B without qualifying employer coverage triggers a 10% premium surcharge for each full 12-month period of delay that persists for the entire duration of Medicare enrollment. The rules around when Part B delay is penalty-free — specifically the employer size requirements and the Special Enrollment Period mechanics — are among the most important things to understand before making any Medicare enrollment decision. Our resource on Medicare Part B explained covers the full cost structure and enrollment rules in depth.

Medicare Part C: Medicare Advantage

Medicare Advantage (Part C) plans are offered by private insurance companies approved by CMS. They replace Original Medicare by bundling Part A and Part B coverage — and typically Part D drug coverage — into a single private plan. Medicare Advantage plans must cover everything Original Medicare covers, but they deliver that coverage through a managed care structure: provider networks, copays and coinsurance for most services, prior authorization requirements for many procedures, and annual plan parameters that can change from one year to the next. Most Medicare Advantage plans have $0 monthly premiums beyond the Part B premium and frequently include supplemental benefits — dental, vision, hearing, fitness programs, over-the-counter allowances — that Original Medicare does not cover.

The most important thing to understand about Medicare Advantage is what you are exchanging when you choose it. You exchange Original Medicare’s broad nationwide provider access — any provider who accepts Medicare — for a private plan’s structured managed care arrangement. Sometimes that exchange produces excellent value: strong local networks, bundled convenience, attractive supplemental benefits, and a clear annual out-of-pocket maximum that limits worst-case healthcare spending. Other times the exchange creates friction: preferred specialists outside the network, prior authorizations that delay needed care, or annual plan changes that shift costs or drop providers. The plan is not inherently good or bad — it is a question of fit with your specific providers, prescriptions, and care patterns. Our resource on Medicare Part C explained provides the full framework for evaluating Medicare Advantage options.

Medicare Part D: Prescription Drug Coverage

Medicare Part D covers prescription medications through private insurance carriers. Beneficiaries who choose Original Medicare can enroll in a standalone Part D Prescription Drug Plan (PDP). Beneficiaries who choose Medicare Advantage typically receive drug coverage bundled into their Medicare Advantage plan (MA-PD). Each Part D plan has its own monthly premium, annual deductible, formulary (the specific drugs covered and the tiers at which they are covered), pharmacy network, and cost-sharing structure for different drug tiers.

The 2025 Medicare Part D redesign introduced significant structural changes: a new $2,000 annual out-of-pocket cap on Part D drug costs, the elimination of the coverage gap (the “donut hole”) as a separate phase, and a new monthly payment option for beneficiaries with high catastrophic drug costs. These changes meaningfully improve protection against catastrophic drug expenses for beneficiaries with significant medication needs. However, the variation across plans in how specific medications are tiered, whether prior authorization applies, and which pharmacies are “preferred” still makes annual Part D plan comparison valuable even under the redesigned structure.

Part D carries a late enrollment penalty similar to Part B: 1% of the national base beneficiary premium for each full month of delay without creditable drug coverage, added permanently to the Part D premium for life. Enrolling in a Part D plan even when current medication needs are minimal protects against both the penalty and future drug cost risk as medication needs evolve with age. Our resource on Medicare Part D explained covers the full plan structure, penalty rules, and annual comparison approach.

The Two Structural Paths: Original Medicare vs. Medicare Advantage

Every Medicare beneficiary makes one foundational structural choice: stay on Original Medicare (Parts A and B) and add supplemental coverage, or enroll in a Medicare Advantage plan (Part C) that replaces Original Medicare. This choice shapes virtually every other Medicare decision — how you add drug coverage, how much you pay for healthcare visits, which providers you can see, and how predictable your annual healthcare costs are.

The Original Medicare path keeps the federal government as the primary payer for covered services and relies on private supplemental products to address cost-sharing gaps and drug coverage. Most beneficiaries on this path choose a Medicare Supplement (Medigap) plan to cover Part A and Part B cost-sharing, and a standalone Part D plan for prescriptions. With a comprehensive Medigap plan like Plan G in force, out-of-pocket medical exposure is reduced to the annual Part B deductible — making healthcare costs highly predictable year to year. The tradeoff is higher monthly premiums for the Medigap plan and a separate Part D premium. The significant advantage is nationwide provider access with no network restrictions and no prior authorization requirements for most services.

The Medicare Advantage path replaces Original Medicare with a private plan that manages all coverage through a single carrier. This typically means lower monthly premiums (often $0 beyond Part B), bundled drug coverage, and supplemental benefits like dental and vision. The tradeoff is managed care: network restrictions, prior authorizations, variable copay structures, and annual plan changes that can shift costs and networks significantly from one year to the next. Medicare Advantage plans also impose an annual out-of-pocket maximum that caps worst-case cost exposure — a feature Original Medicare alone does not provide.

For a complete side-by-side comparison of these two paths, our resource on Medicare Advantage vs Medicare Supplement comparison provides detailed analysis across premiums, provider access, cost structure, switching flexibility, and long-term planning considerations.

Compare Plans in Your Area

Use the tool below to compare available Medicare plans based on your ZIP code. Then review results with an advisor to confirm doctor and prescription fit before enrolling.

Medicare Enrollment Periods: Timing Is Everything

Medicare enrollment is governed by strict timing rules. Missing the correct window can create coverage gaps, delayed start dates, and permanent financial penalties. The right enrollment path depends on your age, work status, and whether you have creditable employer coverage — and the consequences of getting the timing wrong can follow you for decades.

Initial Enrollment Period (IEP) — The IEP is a 7-month window: three months before the month you turn 65, your birthday month, and three months after. For most people, this is the first opportunity to enroll in Part A and Part B, and the timing of enrollment within this window determines when coverage begins. Enrolling before or during the birthday month starts coverage on the first of the birthday month; enrolling in the months after introduces delays. Planning the IEP enrollment several months in advance — rather than waiting until the last moment — is strongly advisable to ensure effective dates align with any retirement or employer coverage transition. Our resource on enrolling in Medicare at 65 provides a complete IEP planning guide.

Special Enrollment Period (SEP) — Beneficiaries who continue working at 65 with active coverage through an employer group plan (where the employer has 20 or more employees) may qualify for a penalty-free SEP when they retire or lose employer coverage. The SEP for Part B is 8 months from the date active employment or employer coverage ends — not from the date COBRA begins. COBRA and retiree coverage do not qualify for penalty-free Part B delay. For anyone still working at 65 or covered under a spouse’s employer plan, confirming whether the specific coverage qualifies for SEP protection is the most important planning step before making any enrollment decision. Our resource on Medicare enrollment for people still working explains the employer size rules, creditable coverage requirements, and SEP mechanics in detail.

General Enrollment Period (GEP) — If the IEP is missed without a qualifying SEP, the GEP runs from January 1 through March 31 each year, with coverage beginning July 1. Enrolling during the GEP does not prevent late enrollment penalties — the penalty still applies for the months of delay. The GEP is a fallback option for people who missed their enrollment window, not a planned enrollment strategy. Avoiding the GEP by enrolling during the IEP or a qualifying SEP is always preferable.

Annual Election Period (AEP) — From October 15 through December 7 each year, the AEP allows beneficiaries to change Medicare Advantage plans, switch between Medicare Advantage and Original Medicare, or change Part D plans. Changes made during AEP take effect January 1 of the following year. Annual AEP reviews are valuable because plan parameters change annually — premiums, formularies, networks, and cost-sharing can all shift in ways that make a previously optimal plan less appropriate. Our resource on when Medicare open enrollment is explains the full enrollment calendar.

Avoiding Medicare Late Enrollment Penalties

Medicare late enrollment penalties are among the most avoidable and most consequential financial mistakes in retirement planning. Unlike most penalties, Medicare late enrollment surcharges are not one-time fees — they are permanent ongoing surcharges added to monthly premiums for the duration of Medicare enrollment.

The Part B late enrollment penalty is 10% of the standard Part B premium for each full 12-month period the beneficiary was eligible but not enrolled in Part B without qualifying employer coverage. A beneficiary who goes two full years without Part B enrollment will pay a 20% premium surcharge on Part B for life — a penalty that compounds as Part B premiums increase over time. The Part D late enrollment penalty is 1% of the national base beneficiary premium for each full month of delay without creditable drug coverage, also applied permanently. A beneficiary who goes 24 months without Part D or creditable drug coverage will pay a 24% ongoing drug premium surcharge.

Preventing these penalties requires understanding which coverage qualifies for creditable status, how employer size affects Medicare’s primary/secondary coordination, and what the timing requirements are for each enrollment window. Our dedicated resource on how to avoid Medicare late enrollment penalties explains the penalty calculation, qualifying coverage types, and the documentation required when delay is legitimate.

What Medicare Covers — and What It Does Not

Medicare covers a broad range of medically necessary services, but its coverage gaps are significant and represent some of the largest financial risks in retirement planning. Understanding what Medicare does not cover is as important as understanding what it does.

Long-term custodial care is Medicare’s most significant coverage gap. Medicare does not cover assistance with activities of daily living — bathing, dressing, eating, transferring, toileting — that constitutes the vast majority of what people mean when they discuss nursing home care or home health aide services. Medicare’s skilled nursing facility benefit covers medically necessary skilled care only following a qualifying hospital stay, and only for a limited time. It is not a long-term care solution. The financial exposure from this gap can be enormous: private nursing home rooms cost more than $9,000 per month in many U.S. markets, with no Medicare coverage after the limited skilled care benefit is exhausted. Our resources on whether Medicare covers long-term care and whether Medicare and long-term care insurance are the same explain this distinction in full detail.

Routine dental, vision, and hearing care are not covered by Original Medicare. Routine cleanings, fillings, dentures, eyeglasses, contact lenses, hearing exams, and hearing aids require separate supplemental coverage. Many Medicare Advantage plans include some dental, vision, and hearing benefits, though the depth of those benefits varies considerably and is often more limited than plan marketing suggests. Beneficiaries on the Original Medicare + Medigap path typically purchase separate supplemental dental and vision plans if these benefits are a priority.

International healthcare is covered by Original Medicare only in very narrow circumstances — generally emergency care in border regions of Canada and Mexico under specific conditions. For beneficiaries who travel internationally, Medicare’s international coverage limitation is a meaningful gap. Some Medigap plans (including Plan G) include a foreign travel emergency benefit covering 80% of covered emergency care outside the United States (subject to a lifetime maximum and annual deductible). For travelers who want more comprehensive international coverage, standalone travel medical insurance or dedicated international health coverage provides additional protection. Our resource on emergency travel health insurance explains the options available for filling this gap.

IRMAA: How Income Affects Medicare Premiums

Many beneficiaries are surprised to discover that Medicare Part B and Part D premiums are not flat amounts — they are income-adjusted through IRMAA (Income-Related Monthly Adjustment Amount) surcharges for higher-income beneficiaries. IRMAA begins for individuals with modified adjusted gross income (MAGI) above approximately $106,000 and for married couples above approximately $212,000 (thresholds adjust annually), with surcharges increasing in steps through several income brackets.

The IRMAA feature most likely to surprise beneficiaries is the two-year lookback: IRMAA for 2026 is based on 2024 income. A single high-income year — a large Roth conversion, a business sale, a significant capital gains event — can trigger two years of elevated Medicare premiums even if current retirement income is modest. Coordinating large retirement income decisions with Medicare premium planning is an important dimension of integrated retirement strategy. IRMAA surcharges can be appealed through the Social Security Administration using Form SSA-44 when income decreased significantly due to a qualifying life event — retirement, death of a spouse, or divorce are common qualifying events.

How Medicare Fits Into Retirement Income Planning

Medicare choices have real and ongoing financial consequences in retirement. Part B and Part D premiums become permanent monthly expenses. Medigap premiums add to that total. Medicare Advantage copays create variable annual healthcare costs. IRMAA surcharges can add hundreds of dollars per month for higher-income beneficiaries. When these costs are planned for explicitly — modeled into a retirement income budget — they become manageable and predictable. When they are discovered reactively, they can disrupt retirement spending plans significantly.

Integrating Medicare cost planning with broader retirement income strategy means understanding what your total annual Medicare spending will look like in a routine year and in a high-utilization year, and ensuring your income sources can absorb that spending without stress. Many retirees coordinate predictable Medicare cost exposure with guaranteed income tools that provide consistent cash flow regardless of market conditions. Our resource on safe fixed annuity options explains how guaranteed income products can complement a Medicare-budgeted retirement spending plan. The goal is not to “buy something because Medicare is expensive” — it is to build a retirement plan where healthcare costs are planned for and funded rather than reacted to from volatile assets.

The interaction between Medicare and Social Security also deserves attention in the retirement income planning context. Part B premiums are typically deducted directly from Social Security benefits when the beneficiary is receiving both, which means the timing of Social Security claiming and Medicare enrollment are connected financial decisions. Part B premiums cannot reduce a Social Security benefit below zero — the hold harmless provision protects beneficiaries from net benefit reductions in most years — but the direct deduction mechanism means Social Security income effectively funds Medicare premiums as a built-in expense.