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. So she moved $150,000 from the traditional IRA into a Roth, paid the federal tax, and moved on.

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: the dollars inside grow tax-free, future RMDs shrink, and heirs inherit a cleaner asset. The point is that the Medicare piece is real money and 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. A voluntary conversion clears none of those criteria, so the surcharge is final once the return is filed.

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 chunk, 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, so the prior urgency of “lock in low rates before they expire” has faded. At the same time, OBBBA introduced a new senior deduction for taxpayers 65 and older that begins to phase out above $75,000 in income for single filers. A sizable Roth conversion can erode that deduction, adding yet another cost that a simple tax-bracket analysis will miss.

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 levels the following year, the surcharge drops back with it in the year after that.

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 was a year ago. 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 are real 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 and knows the bracket thresholds 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 context on the One Big Beautiful Bill Act (signed July 4, 2025), which permanently extended TCJA tax rates and introduced a new senior deduction that phases out above $75,000 for single filers 65 and older, a variable that can further complicate Roth conversion sizing. The 2026 IRMAA Tier 4 surcharge figures ($446.30 per month on Part B and $83.30 per month on Part D for single filers with MAGI between $205,000 and $500,000) and the total all-in conversion cost of approximately $42,355 were confirmed against CMS-published data and remain unchanged.

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