How a Roth Conversion at 64 Added $1,148 to a Retiree’s Medicare Premiums Two Years Later

A Smart Tax Move That Backfired

A 64-year-old single retiree pulls in about $60,000 a year from a pension and modest 401(k) withdrawals. Her advisor points out that her income sits comfortably inside the 22% federal bracket, and a Roth conversion looks like a sensible move before required minimum distributions (RMDs) kick in at age 73. She converts $60,000 from her traditional IRA. The federal income tax bill is painful but manageable. Then, two years later, a letter from Social Security lands in her mailbox: her Medicare premiums have jumped by roughly $95.70 a month for the entire year. That works out to about $1,148 she never budgeted for, and it has almost nothing to do with her income at that moment.

This situation comes up constantly on retirement planning forums. Someone runs the conversion math, writes the tax check, and walks away feeling good about the long-term plan. Then they get blindsided by an Income-Related Monthly Adjustment Amount (IRMAA) surcharge two years later. The online calculator never flagged it, and by the time the bill arrives, there is nothing to be done.

The One Detail That Drives the Outcome

Medicare premiums are income-tested, but the measurement clock runs on a two-year delay. Your modified adjusted gross income (MAGI) from 2026 determines what you pay for Part B and Part D in 2028. A 2026 conversion could affect your 2028 Medicare premiums because the entire converted amount flows into MAGI in the year the money moves.

For a single filer, the first IRMAA tier in 2026 kicks in at MAGI above $109,000. Without the conversion, this retiree’s MAGI sits at $60,000, comfortably below that line, and she pays the standard Part B premium of $202.90 a month with no Part D surcharge. Stack the $60,000 conversion on top and her MAGI climbs to $120,000. That puts her squarely inside the first tier for single filers, where the Part B adjustment runs $81.20 a month and Part D tacks on another $14.50. Combined, that extra $95.70 a month adds up to $1,148 across the year.

The IRMAA cliff works as a step function, not a sliding scale. One dollar over $109,000 triggers the full Tier 1 surcharge. The retiree pays the elevated premium for one year, tied to her age-64 income, and then it drops back to baseline once her MAGI normalizes. The $60,000 conversion ultimately costs her the income tax up front plus that one-year, $1,148 Medicare hit she almost certainly never saw coming on a planning spreadsheet.

How It Connects to the Rest of the Picture

A large conversion can push more of her Social Security benefits into taxable territory, since provisional income climbs as conversion dollars flow into MAGI. If she had planned to start Social Security at 65 or 66, the same conversion year could produce a meaningfully larger tax bill on those benefits.

The interaction with future RMDs is what made the strategy appealing in the first place. Shrinking the traditional IRA today means smaller forced withdrawals later, which can keep her under IRMAA thresholds throughout her 70s. The long-term math still favors the conversion in many cases. The problem is execution: the conversion amount lands in MAGI all at once, and that single-year spike is what trips the surcharge.

One additional wrinkle worth knowing: the One Big Beautiful Bill Act, signed in July 2025, introduced a temporary $6,000 senior bonus deduction for filers age 65 and older, available through 2028. For a 64-year-old retiree doing a conversion today, that deduction becomes available the following year, but it phases out once MAGI exceeds $75,000 for single filers. A large conversion can inadvertently eliminate it, adding yet another hidden cost that rarely appears on a standard planning worksheet.

What to Think Through Before You Convert

Two ideas tend to matter more than the rest:

1. Stage the conversion across multiple years. Splitting $60,000 into three $20,000 increments keeps MAGI at roughly $80,000 each year, well under the $109,000 single-filer IRMAA threshold. The total tax bill comes out similar, but no surcharge is triggered. Converting only enough to fill the top of your current bracket without any spillover is the cleanest version of this approach.
2. Finish big conversions before age 63 if you can. Because of the two-year lookback, conversions completed at 62 or earlier fall outside Medicare’s lookback window before age 65 enrollment begins. After that, every high-income conversion year becomes a potential premium surprise two years down the road.

One more point worth understanding: the SSA-44 life event form lets you appeal an IRMAA surcharge after a qualifying life-changing event such as retirement, marriage, or the death of a spouse. A voluntary Roth conversion does not qualify. Once the surcharge year is locked in, there is no clean way out.

Conversions remain one of the most powerful tools in long-term tax planning. The math rewards patience over enthusiasm, and one conversation with someone who understands how IRMAA, Social Security taxation, and bracket management interact can more than pay for itself.

Editor’s note: This article has been updated to reflect verified 2026 figures, including the standard Medicare Part B premium of $202.90 per month, the Tier 1 IRMAA Part B surcharge of $81.20 and Part D surcharge of $14.50 per month (totaling $1,148 annually rather than the previously stated $1,116), and context on the OBBBA’s new $6,000 senior bonus deduction that can also be affected by large Roth conversions.

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