The Retirement Math Most People Get Wrong About Medicare Costs

There is a number floating around retirement planning circles that tends to stop people cold. Financial services firms that track retirement savings data consistently find that Americans dramatically underestimate what healthcare will cost them after they leave the workforce. The gap between what people expect to spend on medical care in retirement and what they actually spend is not a rounding error — it is often hundreds of thousands of dollars. For Medicare beneficiaries, understanding exactly why that gap exists, and what tools are available to close it, is not just useful financial planning. It is essential self-defense.

Original Medicare — the federal program consisting of Part A (hospital insurance) and Part B (medical insurance) — was never designed to be a complete healthcare solution. Part A covers inpatient hospital stays, skilled nursing facility care, hospice, and some home health services. Most people do not pay a premium for Part A if they or their spouse worked and paid Medicare taxes for at least 10 years. Part B, which covers outpatient care, doctor visits, preventive services, and durable medical equipment, carries a standard monthly premium of $185.00 in 2025, though higher-income beneficiaries pay more through what is called Income-Related Monthly Adjustment Amounts, or IRMAA. In 2025, IRMAA surcharges begin for individuals with modified adjusted gross income above $106,000 and for married couples filing jointly above $212,000.

Here is the structural problem that catches so many new retirees off guard: Original Medicare has no annual out-of-pocket maximum. Under employer-sponsored insurance, you are accustomed to hitting a ceiling — say $5,000 or $7,000 — after which your insurer covers 100% of costs for the rest of the year. Original Medicare does not work that way. After you meet the Part B deductible ($257 in 2025), Medicare pays 80% of approved costs and you are responsible for the remaining 20% indefinitely. If you are hospitalized, the Part A deductible is $1,676 per benefit period in 2025 — and critically, that is per benefit period, not per year. If you are discharged and readmitted more than 60 days later, you owe that deductible again. For a serious illness requiring multiple hospitalizations, those costs can compound quickly.

This is precisely the vulnerability that Medigap policies — also called Medicare Supplement Insurance — were designed to address. Medigap plans are sold by private insurers and are standardized by the federal government into lettered plan types (Plan G, Plan N, Plan K, and others). Because they are standardized, a Plan G from one insurer covers exactly the same benefits as a Plan G from a competitor — the only difference is the premium. Plan G is currently the most comprehensive option available to beneficiaries who became eligible for Medicare after January 1, 2020 (Plan F, which covered the Part B deductible, is no longer available to new enrollees). In 2025, Plan G premiums vary widely by age, location, and insurer — ranging from roughly $100 per month in lower-cost states to $300 or more per month in higher-cost markets — but in exchange, beneficiaries face virtually no cost-sharing beyond the annual Part B deductible once the plan kicks in. For someone managing a chronic condition or anticipating significant medical needs, that predictability has real financial value.

The alternative path — Medicare Advantage, also known as Part C — bundles Part A, Part B, and usually Part D drug coverage into a single private plan. Medicare Advantage plans are required by law to cap out-of-pocket costs; in 2025, that cap is set at $9,350 for in-network services and $14,000 for combined in- and out-of-network costs. Many plans advertise $0 premiums and extra benefits like dental, vision, and hearing coverage that Original Medicare does not provide. These features are genuinely attractive, particularly for beneficiaries on fixed incomes. However, Medicare Advantage plans operate through networks — HMOs require you to use in-network providers and get referrals for specialists, while PPOs offer more flexibility at higher cost-sharing. If you travel frequently, spend winters in a different state, or have established relationships with specialists who are not in a plan's network, Medicare Advantage may create access problems that offset its financial advantages.

The decision between these two paths — Original Medicare plus Medigap versus Medicare Advantage — is where the retirement healthcare math gets personal. Consider two hypothetical 65-year-olds. The first is relatively healthy, takes no specialty medications, lives in a metropolitan area with robust Medicare Advantage networks, and is comfortable with managed care. For this person, a $0-premium Medicare Advantage plan with a $9,350 out-of-pocket maximum may represent excellent value, especially if they use the dental and vision benefits. The second person has a complex chronic condition, sees multiple specialists at an academic medical center, and wants the freedom to seek care anywhere in the country without prior authorization. For this person, Original Medicare with Plan G — despite the monthly premium — may provide both better access and more predictable costs over time. The right answer is not universal; it depends on your specific health profile, your providers, and your financial cushion.

Enrollment timing matters enormously and mistakes can be costly. When you first become eligible for Medicare at 65, you have a 7-month Initial Enrollment Period — the three months before your birthday month, your birthday month itself, and the three months after. If you enroll in Part B during this window and want a Medigap plan, federal law guarantees you the right to buy any Medigap plan sold in your state without medical underwriting — meaning insurers cannot charge you more or deny you coverage based on pre-existing conditions. This is called guaranteed issue. Miss this window and apply for Medigap later, and in most states, insurers can subject you to medical underwriting and either charge higher premiums or deny coverage entirely. The exceptions are states with their own guaranteed issue protections, including New York and Connecticut, which require year-round guaranteed issue for Medigap, and states with birthday rules — California, Idaho, Illinois, Kentucky, Louisiana, Maine, Maryland, Missouri, Nevada, New Jersey, Oklahoma, and Oregon — which give beneficiaries a 30-day window around their birthday each year to switch Medigap plans without underwriting.

For beneficiaries already enrolled in Medicare Advantage who want to switch, the Annual Enrollment Period runs October 15 through December 7 each year, with changes taking effect January 1. There is also an Open Enrollment Period from January 1 through March 31, during which Medicare Advantage enrollees can switch to a different Advantage plan or return to Original Medicare — but returning to Original Medicare during this window does not automatically restore Medigap guaranteed issue rights in most states, so beneficiaries making this switch should contact their State Health Insurance Assistance Program (SHIP) counselor before acting. SHIP provides free, unbiased counseling in every state; find your local program at shiphelp.org.

The broader financial planning lesson embedded in all of this is that healthcare costs in retirement are not a fixed, predictable line item — they are a variable that can swing dramatically based on your health, your coverage choices, and the decisions you make at enrollment. Fidelity's 2024 estimate of $315,000 in lifetime healthcare costs for a 65-year-old couple assumes enrollment in Medicare and average health — it is not a worst-case scenario. For beneficiaries who develop serious conditions, require long-term care, or face repeated hospitalizations without adequate supplemental coverage, out-of-pocket costs can far exceed that figure. Building a realistic healthcare budget for retirement means accounting for premiums (Part B, Part D, and Medigap or Advantage), potential out-of-pocket costs under your specific plan structure, and the possibility that your health needs will change over time. Reviewing your Medicare coverage annually during the Annual Enrollment Period — not just when you first sign up — is one of the most concrete steps you can take to make sure your coverage still fits your life.