The $19,200 Medicare Surprise That Hits Retirees After One Roth Conversion in the Wrong Year

Picture a couple in their first year of retirement. Both are 64, both stopped working in January, and they finally have the breathing room to do the Roth conversion their planner has been suggesting for years. They move $300,000 from a traditional IRA into a Roth in one clean transaction, pay the federal tax bill, and feel good about locking in tax-free growth. Eighteen months later, the Social Security Administration sends a letter saying their Medicare Part B premium will jump several hundred dollars per month, per spouse. They never saw it coming.

This is the IRMAA (Income-Related Monthly Adjustment Amount) trap, and it punishes the exact people who do everything else right.

The Setup in One Page

• Ages: Both spouses are 64, enrolling in Medicare next year
• Base retirement income: $80,000 (pensions, dividends, a little Social Security)
• The decision: A one-shot $300,000 Roth conversion
• Resulting MAGI for the conversion year: $380,000
• What they missed: The two-year IRMAA lookback

IRMAA is the surcharge Medicare adds to Part B and Part D premiums when modified adjusted gross income exceeds certain thresholds. The catch is the lookback: premiums in 2028 are priced off the 2026 tax return. The year you do the conversion is the year that haunts you, and you do not see the bill until two birthdays later.

Why a Single Number Drives the Whole Outcome

IRMAA works as a staircase with cliffs. One dollar over a threshold moves both spouses into the next tier for the full year. For married filing jointly in 2026, the tiers look like this: $218,000 or less means no surcharge; $218,001 to $274,000 costs roughly $2,297 per couple; $274,001 to $342,000 costs about $5,772; $342,001 to $410,000 costs about $9,240; $410,001 to $749,999 costs about $14,000 per couple per year.

At $380,000 MAGI, our couple lands squarely in the $342,001 to $410,000 tier. Their combined Part B and Part D surcharge runs about $9,240 for the affected premium year, on top of the federal income tax they already paid on the conversion. A roughly $9,000 annual hit can compound into a $19,200 bill before they ever touch the Roth money.

Had the same $300,000 conversion been split across three calendar years at $100,000 each, MAGI in each year would have been roughly $180,000, comfortably below the $218,000 Tier 0 ceiling. The IRMAA cost would have been zero. The federal tax bill barely changes. The surcharge was an unforced error.

Three Paths That Actually Move the Needle

1. Front-load conversions before age 63. The lookback means any conversion done in the year you turn 63 or earlier never touches a Medicare premium. Couples who retire in their late 50s or early 60s have a genuine window that most people waste.
2. Split conversions across multiple years inside the window. If you must convert after 63, size each year deliberately so MAGI lands just under the next IRMAA cliff. Crossing a tier by $500 costs the same as crossing by $50,000.
3. File Form SSA-44 if a real-life-changing event applies. Stopping work qualifies; a Roth conversion alone does not. If retirement genuinely dropped ongoing income, you can ask SSA to use a more recent estimate instead of the conversion year.

What to Do Before You Sign the Conversion Paperwork

Pull the current IRMAA tables, add your expected base income, and write down the maximum conversion that keeps MAGI below the next cliff. With the 10-year Treasury at 4.46% and CPI running near the 90th percentile of the past year, interest and Social Security COLA increases are quietly pushing more retirees into IRMAA territory without any conversion at all. The single most expensive mistake here is treating Roth conversions as a tax-only decision. The tax is the easy part. The Medicare premium is the part nobody warns you about.