Medicare: What It Actually Covers, and Why a Roth Conversion Two Years Ago Can Raise This Year's Premium
Turning 65 comes with an assumption that healthcare costs are largely solved by Medicare, but the program has real coverage gaps, real premiums, and a income-based surcharge that reaches back two full tax years. Understanding the pieces before you enroll, and before you realize a large capital gain, avoids expensive surprises later.
The core principle
Medicare is the US federal health insurance program, generally available starting at age 65, split into distinct parts that each cover something different. Part A covers inpatient hospital stays and is usually premium-free if you or a spouse paid Medicare payroll taxes for at least ten years of work history. Part B covers outpatient care, doctor visits, and other medical services, and carries a monthly premium that every enrollee pays regardless of work history. Part D covers prescription drugs, sold through private insurers under federal rules. Part C, commonly called Medicare Advantage, bundles hospital, medical, and often drug coverage into a single private plan as an alternative to original Medicare plus a separate drug plan.
The base Part B premium is the same for most enrollees, but it is not the number every enrollee actually pays. Income-Related Monthly Adjustment Amount, or IRMAA, adds a surcharge to both Part B and Part D premiums for higher earners, applied in tiers based on modified adjusted gross income. Critically, IRMAA is not based on this year's income; it is based on the tax return filed two years earlier, which means income you report today can affect a Medicare premium you will not pay until two years from now, well after that income has already been spent or reinvested.
The coverage gap that surprises people most is what original Medicare simply does not pay for at all: long-term custodial care such as an extended nursing home stay, and in most cases dental, vision, and hearing services. Original Medicare also has no annual out-of-pocket maximum on its own, which is why most enrollees pair it with a supplemental Medigap policy, or choose Medicare Advantage instead, which typically does include an out-of-pocket cap in exchange for a narrower provider network.
It is also worth understanding what happens for lower-income enrollees, since Medicare's cost structure is not only about surcharges at the top. Medicare Savings Programs and Extra Help, both administered at the state or federal level depending on the specific program, can substantially reduce or eliminate Part B premiums and prescription drug costs for enrollees with limited income and assets. These programs are worth checking for directly rather than assuming they do not apply, since the qualifying income and asset thresholds are higher than many people assume, and a retiree living primarily on Social Security with modest savings may qualify for meaningful assistance they are not currently receiving simply because they never applied.
How the math works
Example 1: budgeting the real annual cost, not just the premium. A retiree pays a Part B premium of roughly $185 a month, a Part D premium of roughly $45 a month, and a Medigap supplemental premium of roughly $170 a month. Total monthly premium cost is $185 + $45 + $170 = $400, or $400 x 12 = $4,800 a year, before counting any deductibles, coinsurance, or costs for services Medicare does not cover at all, such as routine dental work. The premium alone, in this example, already runs close to $5,000 a year for a single person, a figure many people underestimate when they picture Medicare as a largely free benefit.
Example 2: how a single high-income year moves a future premium. Suppose the standard Part B premium is $185 a month, and a couple's modified adjusted gross income for the relevant lookback year crosses into an IRMAA tier that adds a surcharge of $70 per person per month to Part B, plus a smaller Part D surcharge of $35 per person per month. For two people, the added annual cost is 2 x ($70 + $35) x 12 = 2 x $105 x 12 = $2,520 for that one year of Medicare, triggered entirely by a single prior tax return crossing an income threshold, such as a large Roth conversion completed two years before. Had they spread the same total conversion across two smaller tax years instead of one large one, they might have stayed under the threshold in both years and avoided the surcharge entirely.
How it shows up in real portfolios
The most common scenario involves someone still working past age 65 who assumes they can simply delay Medicare enrollment without consequence, since they have employer coverage. Delaying Part B is only penalty-free if the employer coverage counts as creditable coverage under Medicare's rules, generally meaning group coverage from an employer with a sufficient number of employees. Someone who delays based on a smaller employer's plan, or a retiree health plan that does not qualify, can face a permanent late enrollment penalty added to their premium for the rest of their life once they do enroll.
A second scenario, common among newly retired high earners, involves someone who completes a large Roth conversion or realizes a significant capital gain in the two years immediately before enrolling in Medicare, without realizing the IRMAA lookback will apply that income to a premium tier well after the conversion is complete. A retiree who understands this timing in advance can often complete large one-time conversions earlier, well before the two-year IRMAA lookback window begins, avoiding a surcharge on income that no longer reflects their actual retirement spending level.
A third scenario involves a retiree budgeting for out-of-pocket healthcare costs and discovering, often only after a hospitalization, how significant the gap is between what original Medicare covers and what a serious illness actually costs without a Medigap policy or Medicare Advantage plan's out-of-pocket cap in place. This is a common driver behind separately planning for long-term care costs, which fall almost entirely outside what Medicare pays for.
A fourth scenario involves a married couple where one spouse turns 65 several years before the other, which means the household is managing two entirely separate enrollment timelines and two separate sets of IRMAA calculations at once, since Medicare eligibility and premiums are determined individually rather than as a joint household benefit the way some tax provisions work. A couple in this position often needs to think about health coverage for the younger, not-yet-eligible spouse separately, whether through a former employer's retiree plan, COBRA, or a marketplace policy, while simultaneously managing the older spouse's Medicare enrollment and premium planning, two coverage problems running on different clocks within the same household budget.
Actionable breakdown
- Enroll on time to avoid permanent penalties:
- Confirm whether your current employer coverage is creditable.
- Mark your initial enrollment window well before age 65.
- Choose a coverage structure deliberately:
- Original Medicare plus Medigap: broader provider choice, higher premium.
- Medicare Advantage: lower premium, narrower network, built-in out-of-pocket cap.
- Plan around IRMAA's two-year lookback:
- Time large Roth conversions well before enrollment if possible.
- Spread large one-time income events across multiple years.
- Budget separately for dental, vision, hearing, and long-term care.
Common pitfalls
- Delaying Part B enrollment based on employer coverage that does not actually qualify as creditable, triggering a permanent late penalty.
- Realizing a large capital gain or Roth conversion without checking whether it will land inside the IRMAA two-year lookback window.
- Assuming Medicare covers long-term custodial care, dental, vision, or hearing, none of which original Medicare pays for in most cases.
- Underestimating the annual cost of premiums, deductibles, and coinsurance combined, rather than budgeting for the base Part B premium alone.
Related concepts
For the income figure that determines IRMAA tiers, see modified adjusted gross income (MAGI) and net investment income tax. For the strategy most likely to trigger a surprise surcharge, see conversion and Roth IRA. For broader retirement income planning, see the guides on social security, withdrawal strategies, and estate planning.
The bottom line
Medicare covers a great deal but not everything, and its costs depend heavily on decisions and income events from two years earlier, so planning enrollment timing and large one-time income together avoids the most common and most expensive surprises.