Yes. A Roth conversion adds to your modified adjusted gross income, and Medicare's IRMAA surcharge uses a two-year lookback — so a 2026 conversion can raise your 2028 Part B and Part D premiums. Because IRMAA is a cliff, crossing a threshold by even $1 triggers the full surcharge for that tier.
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About the Author
Sujit KarkiFinance Researcher & Market Analyst
Independent finance researcher and market analyst with expertise in macroeconomics, equity markets, and personal finance. I help regular investors make better-informed decisions through rigorous, data-driven analysis.
I've written about Roth conversions and about Medicare separately on this site. This is the post where they collide, because the interaction between the two is exactly the kind of thing a good financial plan accounts for and a rushed year-end conversion often doesn't.
Key Takeaways
A Roth conversion adds the full converted amount to your ordinary taxable income in the year you convert — there's no special lower rate for it.
IRMAA, the Medicare income surcharge, uses a two-year lookback, so a 2026 conversion can raise your Part B and Part D premiums starting in 2028.
IRMAA works as a cliff, not a gradual slope — crossing a tier threshold by even $1 triggers the full higher surcharge for that tier, not a prorated amount.
Since the Tax Cuts and Jobs Act eliminated recharacterization, a Roth conversion is permanent — you can't undo it once completed.
Sizing a conversion to 'fill up' a bracket or stay just under an IRMAA threshold is a standard, deliberate strategy — not something to improvise in December.
What actually happened
Nothing new triggered this post — it's the collision of two things I've covered separately finally getting the joint treatment it deserves. Roth conversions get a lot of attention every fall as a year-end tax-planning move: move money from a traditional IRA or 401(k) into a Roth account, pay ordinary income tax on the converted amount now, and let it grow tax-free from that point forward. For people expecting higher tax rates later, or wanting to reduce future required minimum distributions, that trade can make real sense.
What gets less attention is that the income from a conversion doesn't just affect your tax bracket — it also counts toward your Modified Adjusted Gross Income (MAGI) for Medicare's Income-Related Monthly Adjustment Amount, IRMAA. If you're on Medicare or will be within two years, a conversion sized without checking against IRMAA thresholds can trigger a real, multi-year premium increase that easily outweighs the tax benefit the conversion was supposed to deliver.
IRMAA lookback period0Years — 2026 income affects 2028 premiums
Roth conversion reversibility0Permanent since TCJA eliminated recharacterization
The math for one person
Two mechanics matter here, and they compound in a way that catches people off guard. First: IRMAA isn't assessed on your current income — it's assessed on a two-year-old tax return. The premium you pay in 2028 is based on the MAGI reported on your 2026 tax return. That lag means a conversion decision made this year has consequences that don't show up in your monthly budget until two full years later, which makes it easy to convert without connecting the dots at the time.
Second, and more consequential: IRMAA is structured as a cliff, not a smooth slope. Medicare defines a series of MAGI thresholds, and each one has an associated flat surcharge added to your Part B and Part D premiums. Land $1 over a threshold, and you pay the full surcharge for that entire tier — not a prorated amount reflecting how far over you are. A conversion that lands you at $1 over a tier boundary costs the same in surcharge as one that lands you $20,000 over it, which is precisely why sizing the conversion precisely, rather than roughly, matters so much.
Warning
Because a Roth conversion is irreversible under current law, an IRMAA-triggering conversion mistake can't be corrected after the fact the way it could before 2018, when recharacterization let you undo a conversion you regretted. Model the MAGI impact before you convert, not after.
Why converting can still be right
None of this is an argument against Roth conversions — it's an argument for sizing them deliberately. The standard, well-established version of this strategy is often called "filling up the bracket": convert exactly enough to use up the remaining room in your current tax bracket (or to stay just under an IRMAA threshold), and stop there for the year, potentially spreading a larger total conversion across several years instead of doing it all at once.
Approach
Tax outcome
IRMAA outcome
Convert the whole balance in one year
Likely pushes into higher marginal brackets
High risk of crossing one or more IRMAA tiers
Convert to the top of your current bracket, stop there
Efficient — no wasted room in a lower bracket
Can still cross an IRMAA tier if MAGI is already close to one
Convert to just under the nearest IRMAA threshold
May leave some bracket room unused
Avoids the surcharge entirely for that MAGI level
Multi-year conversion plan, sized against both bracket and IRMAA thresholds each year
Most tax-efficient over time
Deliberately avoids cliffs in most years, by design
The years before you or your spouse turn 65 and enroll in Medicare are often the highest-value window for larger conversions, precisely because IRMAA doesn't apply yet — the two-year lookback means conversions done well before Medicare eligibility never trigger a surcharge on your own account at all.
Run your own numbers
Interactive · Roth conversion + IRMAA
Does this conversion push you over an IRMAA cliff?
Placeholder tier data
IRMAA thresholds and surcharges shown are commonly-cited estimates, not confirmed against CMS's official tables. Verify at medicare.gov before relying on this.
MAGI before conversion$130,000
Roth conversion amount$50,000
Marginal tax rate22%
Filing status
Converting $50,000 costs about $0 in tax this year and brings your MAGI to $180,000. That crosses an IRMAA tier — expect roughly $0/yr in added Medicare premium surcharge, starting two years from now.
Conversion cliff estimate — data table
Conversion tax (this year)
$11,000
MAGI after conversion
$180,000
Crosses an IRMAA tier?
Yes
Added surcharge (2 years out, per person)
$2,664/yr
A Roth conversion is irreversible since the TCJA eliminated recharactization. IRMAA applies a two-year MAGI lookback, so 2026 income affects 2028 premiums. Not tax or Medicare advice — verify current thresholds at medicare.gov.
Model your MAGI for the conversion year against current IRMAA thresholds before you convert anything — not after you file, when it's too late to adjust.
If you're within two years of Medicare eligibility, treat IRMAA as a hard constraint on conversion size, not an afterthought to check once the tax math looks good.
Consider spreading a large conversion across multiple years instead of one lump sum, to avoid stacking a big one-time MAGI spike against a single year's thresholds.
Verify current IRMAA thresholds directly at medicare.gov or with a tax professional before finalizing any conversion amount — thresholds adjust periodically and the exact current figures matter.
Remember the two-year lag when budgeting — a 2026 conversion's premium impact shows up in 2028, so don't assume "no change yet" means "no change coming."
Get a second opinion from a fee-only financial planner or CPA for any conversion above a few tens of thousands of dollars — the permanence of the decision justifies the cost of a professional review.
FAQ
Frequently Asked Questions
Yes. A Roth conversion adds to your modified adjusted gross income, and Medicare's IRMAA surcharge uses a two-year lookback — so a 2026 conversion can raise your 2028 Part B and Part D premiums. Because IRMAA is a cliff, crossing a threshold by even $1 triggers the full surcharge for that tier.
Warning
This post's calculator uses commonly-cited IRMAA threshold estimates that are not confirmed against CMS's official current tables — treat them as illustrative, and verify exact figures at medicare.gov before converting. Not tax or Medicare advice.
Sujit Karki||5 min read