The $42,355 Price Tag Nobody Mentions When Converting to a Roth IRA

The Quiet Surprise in the Mailbox

A 64-year-old single retiree with an $800,000 traditional IRA ran the numbers in 2024 and decided to get ahead of future required minimum distributions. The logic was sound: convert a chunk now while tax rates feel manageable, then shrink the balance that will eventually be forced out. She moved $150,000 from the traditional IRA into a Roth, paid the federal tax, and considered the matter settled.

Two years later, a Medicare letter arrived carrying an extra $1,116 for the remainder of 2026. The charge was the Income Related Monthly Adjustment Amount, the surcharge Medicare tacks on when your income from two years earlier crosses certain thresholds. She had modeled the conversion tax carefully. Medicare, watching from the back row, was nowhere in that model.

How a 2024 Decision Reaches Into 2026

Medicare sets your premiums by looking at your tax return from two years prior. For 2026, that reference year is 2024. In this retiree’s case, ordinary income looked unremarkable before the conversion: $40,000 in Social Security plus $20,000 in dividends produced a modified adjusted gross income of roughly $60,000, well below any surcharge line.

The conversion changed everything. Adding $150,000 of converted IRA funds pushed her 2024 MAGI to about $210,000. For a single filer in 2026, that figure falls inside IRMAA Tier 4, which covers $205,000 to $500,000. The 2026 surcharge at that tier runs $446.30 a month on Part B and $83.30 a month on Part D, for a combined $529.60 a month above the standard premium. Over a full year, that adds $6,355 in extra Medicare cost. The $1,116 on her letter reflected the partial-year impact once the new premium rate took effect.

The Real Price Tag on the Conversion

The conversion tax was the obvious line item. At a 24% marginal federal rate, converting $150,000 translated to roughly $36,000 in federal income tax for 2024. Add the full-year IRMAA hit, and the all-in cost of moving that money climbs to approximately $42,355. The Roth still carries long-term value: dollars inside grow tax-free, future RMDs shrink, and heirs inherit a cleaner asset. The Medicare piece is real money, though, and it rarely appears in the spreadsheet retirees use to decide whether to convert.

A Roth conversion is not an event Medicare allows you to appeal. The Social Security Administration does accept IRMAA appeals for qualifying life events such as retirement, divorce, or death of a spouse, but a voluntary conversion clears none of those criteria. Once the return is filed, the surcharge is final.

What Drives the Outcome Most

Conversion sizing is the single biggest lever. Crossing an IRMAA bracket by one dollar triggers the full surcharge at that tier for the entire year, so the goal is to fill the room under the next threshold without spilling over it. For many single retirees, that means converting in annual slices of roughly $40,000 to $50,000 rather than one large lump sum, reaching the same long-term tax benefit without the Medicare surprise.

One wrinkle added by the One Big Beautiful Bill Act, signed into law on July 4, 2025, is worth keeping in mind. The legislation permanently extended the Tax Cuts and Jobs Act tax rates, which softens the old urgency of locking in low rates before expiration. At the same time, the OBBBA introduced a new senior deduction worth up to $6,000 for taxpayers 65 and older. That deduction begins to phase out at $75,000 in MAGI for single filers and disappears entirely at $175,000, which means the retiree in our example, with a $210,000 MAGI after the conversion, would receive no benefit from it at all. A sizable Roth conversion can erase that deduction, adding yet another cost that a simple tax-bracket analysis will miss. The deduction is also temporary, available only for tax years 2025 through 2028, so the planning window is finite.

Two other tactics can do most of the remaining work:

1. Pay the conversion tax from a taxable brokerage account, not from the IRA itself. Withholding the tax out of the converted amount shrinks the Roth and, for anyone under 59 and a half, can create a penalty. Paying from outside the account keeps the full $150,000 compounding inside the Roth.
2. If possible, complete conversions before age 63. Because Medicare uses a two-year lookback, income recognized at 62 never appears in an IRMAA calculation. Retirees who front-load conversions in their early 60s often sidestep the problem entirely.

It is also worth noting that IRMAA is recalculated every year based on the most recent lookback return. A one-time conversion spike does not lock you into a higher surcharge permanently. If income returns to normal in the year after a conversion, the surcharge drops back accordingly the following year. For many retirees, that means the $6,355 hit is a one-time cost, not a recurring one.

What to Carry Forward

Roth conversions are often the right call, especially for retirees facing a large traditional IRA balance. With TCJA rates now made permanent, the tax-rate argument for urgency is weaker than it once was. The stronger case for converting rests on long-term RMD management: a smaller traditional IRA balance at age 73 means lower forced distributions, less exposure to future bracket creep, and a cleaner asset for heirs. Those benefits hold regardless of the rate environment.

The lesson this retiree learned is that the income tax on the conversion is only part of the cost. Medicare adds its own line item two years later, calculated on the same income, and once the return is filed there is no way to argue it away. Before finalizing a conversion, pull up the current IRMAA brackets, sketch your projected MAGI, and ask whether two smaller conversions spread across two tax years get you to the same long-term place without tripping a surcharge tier. A tax professional who works regularly with Medicare-eligible clients can often save more than the consultation costs, particularly now that the post-OBBBA senior deduction phase-out has added a new variable to the math.

Editor’s note: This pass added the $6,000 value of the OBBBA senior deduction, its $175,000 complete phase-out threshold for single filers, and the clarification that the deduction is temporary (tax years 2025 through 2028). Context was also added noting that the retiree in the example, with a post-conversion MAGI of $210,000, would lose the deduction entirely, illustrating a second hidden cost of large conversions.

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