Cross an IRMAA threshold by one dollar and Medicare bills you for the entire tier, all year, for each spouse. That is the whole reason Roth conversions and Medicare premiums collide: the surcharge is a cliff, not a slope. A conversion that fits perfectly inside your tax bracket can still trip the wire, because the wire is somewhere else entirely.
How Roth conversions trigger IRMAA
The mechanics run in three steps. First, every dollar you move from a traditional IRA to a Roth counts as ordinary taxable income in the conversion year; IRS Publication 590-A confirms this. Second, Medicare sets your premiums from your modified adjusted gross income two years earlier, so a conversion done in 2026 first shows up in your 2028 premiums. Third, the Income-Related Monthly Adjustment Amount raises your Part B and Part D premiums in tiers, and crossing a threshold by a single dollar triggers the full surcharge for that tier for the entire year. The surcharge applies per person on Medicare, so both spouses pay it individually.
For 2026, the surcharge kicks in when MAGI crosses $109,000 for single filers or $218,000 for married couples filing jointly, per CMS data. The first-tier Part B surcharge is $81.20 per person per month. Take the concrete case: a married couple with $190,000 of base MAGI converts $28,000 and lands at $218,000. No IRMAA. Convert $30,000 instead and land at $220,000, and the first-tier surcharge applies to both spouses: $81.20 a month, times 12 months, times two people, or about $1,949 a year in extra Part B premiums alone. That extra $2,000 of conversion bought roughly $1,949 of Medicare cost, before counting the federal tax on it. This is why planners say to convert up to the line, not beyond it. There is no undo button on December 31, and since the Tax Cuts and Jobs Act, conversions completed in 2018 or later cannot be recharacterized.
| Filing status | 2026 IRMAA threshold (MAGI) | First-tier Part B surcharge |
|---|---|---|
| Single | $109,000 | $81.20/month per person |
| Married filing jointly | $218,000 | $81.20/month per person |
The damage scales fast. In 2025, married couples with 2023 MAGI above $212,000 paid IRMAA, and at roughly $400,000 of MAGI, Part B alone reached about $592 per person per month. Both spouses on Medicare at that tier means more than $14,000 a year in premiums purely from crossing income lines. One analysis of a couple whose conversion pushed them into the first tier found the pair paying an extra $5,828 a year, and calculated that covering it required about $7,770 in pre-tax IRA withdrawals, money that at a 7.5% return could have grown past $47,000 over 25 years. A conversion in the 24% federal bracket can effectively cost closer to 29% once the stealth tax is included.
The conversion framework that respects the cliff
The IRMAA-aware process runs like this: project your base income for the year, identify your marginal tax bracket ceiling, calculate MAGI rather than just taxable income, overlay the IRMAA thresholds, and decide which tier you are willing to land in two years from now. Then convert up to the line. This pairs naturally with the bracket-filling math for sizing each year's conversion, and it sits next to the other stealth cost we have modeled, the ACA subsidy trap.
One honest caveat before you lock in numbers: the official 2028 thresholds are not published yet. Treating the 2026 figures as exact 2028 ceilings creates false precision. Use them as the planning line, keep a buffer under it, and revisit when CMS publishes.
My take: IRMAA is the stealth tax that turns a good conversion into a mediocre one, and the most common damage happens to people doing the responsible thing, converting in the 22% to 24% brackets to get ahead of required minimum distributions. But I think the "never cross a tier" crowd overcorrects. A temporary $2,000 surcharge can be cheap if the conversion saves $20,000 in lifetime RMD taxes and survivor-bracket exposure. IRMAA is a cost to model, not a stop sign. The unforgivable version is the accident: crossing a tier by $2,000 because nobody checked the line before year end.
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Frequently asked questions
Does a Roth conversion raise my Medicare premiums?
It can. The conversion counts as ordinary income, raising your MAGI. If that pushes you over an IRMAA threshold, your Part B and Part D premiums rise two years later.
What are the 2026 IRMAA thresholds?
$109,000 MAGI for single filers, $218,000 for married couples filing jointly, per CMS. First-tier Part B surcharge: $81.20 per person per month.
How long until IRMAA hits after a conversion?
Two years. A 2026 conversion first affects 2028 premiums, and the official 2028 thresholds are not out yet, so plan with a buffer.
Is the surcharge per person?
Yes. Each spouse on Medicare pays it. First-tier for both spouses runs about $1,949 a year in extra Part B premiums.
Should I ever intentionally cross a tier?
Sometimes. A planned crossing that front-loads conversions and slashes future RMDs can beat staying under. What you want to avoid is the accidental crossing. See our conversion ladder mistakes and converting while working part time for the other ways conversions go sideways.
Sources: IRS Publication 590-A (conversion income rules); CMS, 2026 IRMAA thresholds; TheStreet, "Roth conversions can trigger Medicare IRMAA surcharge"; Morningstar/MarketWatch, "No do-overs: How one extra dollar on your Roth conversion triggers a tax bill"; Root Financial, "The Roth Conversion Mistake that Could Double Your Medicare Costs". Figures verified October 2026.