If you are doing a Roth conversion in 2026, there is an important problem with the usual “stay under the IRMAA line” advice: the official IRMAA line that will usually matter is not known yet.
A taxable Roth conversion completed in 2026 generally increases your 2026 income. If that income is later used for IRMAA, it would ordinarily affect your 2028 Medicare premiums. Social Security’s own IRMAA tables are organized by premium year and generally use tax-return information from two years earlier.
You cannot know an exact “safe” 2026 Roth conversion amount for 2028 IRMAA today because the official 2028 IRMAA thresholds are not yet available. Estimate a planning range instead: start with projected 2026 IRMAA MAGI, choose an explicitly assumed future premium-year threshold, subtract a safety buffer, then compare several conversion amounts. Treat the threshold assumption as an estimate—not a government number.
Show summary
- The year matters: A 2026 Roth conversion is generally part of 2026 income, while Medicare would ordinarily use that income for 2028 IRMAA.
- Do not use the wrong table: The official 2026 IRMAA thresholds determine 2026 premiums from 2024 income. They are not the exact ceiling for a conversion you make in 2026.
- Use an estimate: Model projected conversion-year IRMAA MAGI against an assumed future premium-year threshold, then subtract a realistic safety buffer.
- Compare more than one amount: Test one conversion below your assumed ceiling and at least one larger amount so IRMAA becomes a priced tradeoff instead of an automatic stop sign.
- IRMAA is only one constraint: Federal and state taxes, taxable Social Security, ACA subsidies, capital gains, NIIT, RMDs, and survivor planning can all change the best conversion amount.
Key Takeaways Ahead
The two-year timing rule changes the calculation
A Roth conversion moves money from a pretax retirement account into a Roth account. The previously untaxed portion of the conversion is generally included in gross income for the year of the conversion, according to IRS Publication 590-A.
Medicare IRMAA works on a different clock. Social Security’s IRMAA sliding-scale tables are tied to the Medicare premium year and generally use MAGI from the tax return two years earlier.
Conversion year: the year the Roth conversion enters your income. Premium year: the Medicare year that may later use that income for IRMAA. A 2026 conversion generally affects 2026 MAGI; if the normal two-year lookback applies, that income would ordinarily be evaluated for 2028 Medicare premiums.
This distinction sounds small. It changes the entire sizing calculation.
Why the current 2026 IRMAA table is not your 2026 conversion ceiling
The official 2026 IRMAA table is useful—but for a different job. It determines 2026 Medicare premiums using 2024 MAGI. For example, the official 2026 first threshold is above $109,000 for many single filers and above $218,000 for married couples filing jointly. The 2026 IRMAA brackets guide owns those current official values.
If you make a Roth conversion during 2026, however, using $109,000 or $218,000 as the exact IRMAA ceiling for that conversion mixes up the income year and the premium year. The future Medicare year that would normally use 2026 income is 2028, and the official 2028 IRMAA threshold table is not available yet.
Wrong: “The 2026 joint IRMAA threshold is $218,000, so I can safely convert up to $218,000 of 2026 MAGI.” Better: “My 2026 income may affect 2028 IRMAA, so I need an explicit future-threshold assumption and a buffer.”
Step 1: Estimate your conversion-year IRMAA MAGI
Start with the year you are actually converting. For IRMAA, MAGI is generally adjusted gross income plus tax-exempt interest. A taxable Roth conversion increases AGI, which is why it can increase the income Medicare later sees.
Before adding the conversion, estimate your baseline from wages, pensions, taxable retirement-account withdrawals, interest, dividends, capital gains, rental or business income, taxable Social Security, tax-exempt interest, and unusual one-time income. The dedicated IRMAA MAGI guide handles that calculation in more depth.
Do not start with “How much do I want to convert?” Start with “What does my income look like before the conversion?”
Step 2: Choose an explicit future-threshold assumption
If the official premium-year table is not available yet, you have an uncertainty problem—not permission to substitute the current table and call it exact.
For planning, choose a clearly labeled assumption for the future threshold. You might use a conservative estimate based on the latest official table and your planning method, but the important part is the label: assumed future threshold. Do not present it as an official CMS or SSA figure until the premium-year values are actually released.
Suppose a married couple projects $170,000 of 2026 IRMAA MAGI before a Roth conversion. For planning only, they decide to test an assumed 2028 first threshold of $230,000. That $230,000 figure is a hypothetical input used to demonstrate the method. It is not an official or projected 2028 threshold.
Step 3: Subtract a safety buffer
A conversion amount that depends on landing within a few dollars of a future threshold is fragile. Your actual year-end income can move because of fund distributions, realized gains, bonuses, interest, taxable Social Security, business income, or another withdrawal.
Use this planning formula:
Assumed future premium-year threshold − projected conversion-year IRMAA MAGI − safety buffer = estimated conversion room.
Using the hypothetical household above, a $230,000 assumed future threshold, $170,000 projected MAGI, and $5,000 buffer would produce $55,000 of estimated room.
That $55,000 is not a recommendation. It is one scenario produced by one threshold assumption. Change the assumed future threshold or the income estimate and the answer changes.
Step 4: Compare three Roth conversion amounts
This is where the exercise becomes useful. Do not ask one model for one magic number. Compare at least three amounts.
Convert below the estimated IRMAA ceiling after the safety buffer. This favors premium-cost certainty if the assumptions are close.
Convert near the estimated ceiling and see how sensitive the result is to income or threshold changes.
Model a larger conversion and price the potential Medicare surcharge instead of assuming crossing a tier must always be avoided.
Recent retirement discussions show why this matters: people are not just asking whether Roth conversions affect IRMAA. They are asking how to choose a ceiling, how much headroom they really have, and which constraint is actually binding. The best models show their work instead of hiding the assumptions behind one “optimal” answer.
IRMAA is a price, not automatically a stop sign
A higher Medicare premium can absolutely make a conversion less attractive. It does not automatically make the conversion wrong.
Once you know the conversion may enter a higher IRMAA tier, the question changes from “Can I avoid the line?” to “Is paying this additional Medicare cost worth the tax, RMD, survivor, or estate benefit I expect from converting more?”
That is the job of the separate Roth conversion and IRMAA tradeoff guide. This page sizes the conversion range. That page decides whether deliberately crossing a tier may be worth it.
I would not let a future Medicare surcharge become the only number in the room. But I also would not pretend it does not matter. Put a dollar value on it, compare it with the rest of the tax picture, and make the tradeoff visible.
Three mistakes that make the estimate unreliable
1. Using the current premium-year table for a later premium year
This is the big one. A 2026 IRMAA table is official for 2026 premiums. It does not become the official 2028 threshold merely because you are making the conversion in 2026.
2. Treating IRMAA as the only ceiling
A larger conversion can affect federal and state tax brackets, taxable Social Security, capital-gain stacking, net investment income tax, deductions, credits, and—before Medicare—Affordable Care Act subsidies. Your IRMAA ceiling may not be the binding constraint.
3. Treating an estimate as precise
The future threshold can change. Your income can change. The year-end tax picture can change. Build the uncertainty into the decision instead of hiding it behind an exact-looking result.
The Roth conversion sizing rule to remember
Match the income year to the premium year first. Then estimate the room.
For a 2026 conversion, start with projected 2026 IRMAA MAGI. If that income would ordinarily be used for 2028 Medicare premiums, use an explicitly assumed 2028 threshold until the official 2028 table exists. Subtract a safety buffer. Then compare multiple conversion amounts rather than treating the first calculated ceiling as “the answer.”
If your real question is when to do the conversions, use the Roth conversion timing-window guide. Timing and sizing are related, but they are not the same decision.
The timing rule and tax treatment were checked against current primary government sources.
Verified that IRMAA tables are premium-year specific and generally use MAGI from the tax return two years earlier.
Verified the official 2026 Part B and Part D IRMAA amounts and the 2026 premium-year figures used as current reference values.
Verified that taxable Roth conversion amounts are generally included in gross income for the year of conversion.
