The Hidden $20,000 IRMAA Cliff a Retired Couple Walked Into After One Roth Conversion in the Wrong Year

Linda and Mark retired at 64 with a clean plan: convert a chunk of their traditional IRA to a Roth in their first low-income year, before Social Security and required minimum distributions complicated the picture. They moved $300,000 in one tax year, paid the federal income tax, and felt good about the math. Then the Medicare bill arrived for their 65th birthday year, and the surcharge wiped out most of what they thought they had saved.

A recurring thread on the Bogleheads forum titled Appealing IRMAA Due to Retirement While Roth Converting is full of newly retired couples discovering the same trap: the Roth conversion math was right, but the timing collided with Medicare’s two-year income lookback.

The situation, in one box

1. Couple, age 64, married filing jointly, first year fully retired
2. Base retirement income: $80,000 from pensions and dividends
3. One-time Roth conversion: $300,000
4. Resulting MAGI for the year: $380,000
5. Core decision: Did the conversion trigger an avoidable Medicare premium surcharge?

The higher cost goes beyond the income tax on the conversion itself. It is the Income-Related Monthly Adjustment Amount, IRMAA, which uses your tax return from two years before to set your Medicare Part B and Part D premiums when you turn 65.

How one tax return controls two Medicare years

IRMAA (income-Related Monthly Adjustment Amount) works as a cliff. Cross a bracket by a single dollar, and your premium jumps for the entire year. At a joint MAGI (modified adjusted gross income) of $380,000, Linda and Mark land in the fourth MFJ (married filing jointly) tier for 2026, with a combined Part B and Part D surcharge of roughly $4,800 per spouse per year. For the couple, that is $9,600 a year, and because the high-income tax return follows them through the lookback window, the surcharge applies across two consecutive Medicare premium years.

Total avoidable cost: about $19,200. Had the same $300,000 been spread across three tax years, the couple could have stayed under the MFJ Tier 1 threshold of $218,000 and paid $0 in IRMAA. That is more than a year of property taxes for most households.

The macro backdrop sharpens the sting. Core PCE inflation sits at the 90.9th percentile of its 12-month range, the personal savings rate has slipped to 4% from 6% in early 2024, and University of Michigan consumer sentiment is at 53.3, well below the 60 recessionary threshold. Fixed-income retirees have less margin to absorb a five-figure surprise.

Three paths that change the outcome

1. Front-load conversions before age 63. Because the IRMAA lookback is two years, any conversion completed by the tax year you turn 63 is invisible to your age-65 Medicare premium. If you are 60 to 62 with a large traditional IRA, this is your window.
2. If you must convert in the lookback window, slice it. A $300,000 conversion in one year may push you into Tier 4. The same dollars spread across three years can keep MAGI under $218,000 each year and trigger no surcharge. IRMAA tier management often matters more than tax bracket optimization.
3. File Form SSA-44 for a life-changing event. If retirement genuinely lowered your subsequent income, Social Security allows an IRMAA reconsideration. The underlying work stoppage can qualify. It is worth a properly documented filing.

What to do this week

If you are between 60 and 64 and considering a Roth conversion, pull your projected MAGI before you proceed, then compare it against the current MFJ IRMAA brackets. Treat Roth conversions as more than a pure income-tax decision. The federal tax bill is only part of the cost. Medicare premiums, with their cliff-edge brackets and two-year delay, can quietly add five figures to the real price of a poorly timed conversion. Run the IRMAA math first, then size the conversion to fit the tier you can live with.