Roth Conversion and IRMAA: Stop at the Tier or the Tax Bracket?

A Roth conversion should usually stop at the top of a Medicare IRMAA tier, not at the top of a tax bracket, when the tier line falls inside the bracket. IRMAA works like a cliff: one dollar of income over a tier line raises a married couple's Medicare premiums by up to $3,472.80 for the year. That makes the last slice of a bracket-filling conversion the most expensive money you convert. This guide from the TaxesSaved CPA team, part of our retirement tax planning work for households with $1 million or more in IRAs and 401(k)s, shows how to compare the two lines with 2026 numbers.

Does a Roth conversion count toward IRMAA?

Yes. Every dollar you convert is added to your adjusted gross income, and Social Security uses modified adjusted gross income (MAGI), which is your AGI plus tax-exempt interest, to set Medicare IRMAA surcharges (SSA, MAGI definition). Premiums are set from your tax return two years earlier, so a 2026 conversion sets your 2028 Part B and Part D premiums. That lookback starts mattering in the year you turn 63.

Municipal bond interest counts toward MAGI even though it isn't taxed, and so does the taxable part of Social Security. A conversion done in 2026 by a 63-year-old couple sets their first year of Medicare premiums at 65.

What are the 2026 IRMAA tiers and what does each one cost?

For 2026, the standard Part B premium is $202.90 a month, and surcharges start above $218,000 of MAGI for married couples filing jointly and $109,000 for single filers (SSA IRMAA tables). The yearly surcharge below is the extra Part B premium plus the Part D add-on, for 12 months:

• Standard: up to $218,000 joint / $109,000 single. No surcharge.

• Tier 1: up to $274,000 joint / $137,000 single. $1,148.40 a year per person; $2,296.80 for a couple.

• Tier 2: up to $342,000 joint / $171,000 single. $2,884.80 per person; $5,769.60 for a couple.

• Tier 3: up to $410,000 joint / $205,000 single. $4,620.00 per person; $9,240.00 for a couple.

• Tier 4: below $750,000 joint / $500,000 single. $6,355.20 per person; $12,710.40 for a couple.

• Tier 5: $750,000 joint / $500,000 single and above. $6,936.00 per person; $13,872.00 for a couple.

These figures assume both spouses are enrolled in Part B and Part D. The thresholds that will apply to 2028 premiums aren't published yet, so this article uses the 2026 thresholds as a stand-in.

How much does crossing an IRMAA tier cost?

Crossing a tier line by one dollar costs the full step to the next tier for the whole year. For a married couple, the steps are $2,296.80 into Tier 1, $3,472.80 into Tier 2, $3,470.40 into Tier 3, $3,470.40 into Tier 4 and $1,161.60 into Tier 5. For a single filer, each step is half that.

The step costs the same whether you go over by $1 or use the whole tier. Spread across a full Tier 2 or Tier 3 band ($68,000 wide for a couple), the $3,470 step adds about 5.1 percentage points to the tax rate on that slice. Spread across $1, it is a $3,470 charge on a dollar. So the rule is simple: if you cross a tier line, use the tier.

Should a Roth conversion stop at the IRMAA tier or the tax bracket?

Stop at whichever line gives the lowest cost per dollar on the last slice you convert, and that is often a tier line. Take a married couple, both 63, with $150,000 of MAGI before converting and the $32,200 standard deduction. Their taxable income is $117,800, in the 22% bracket, which ends at $211,400 (IRS Rev. Proc. 2025-32). Here is what each stopping point costs, counting the 2028 IRMAA surcharge as part of the conversion's price:

• Convert $68,000 and stop at $218,000 of MAGI: $14,960 of federal tax, no IRMAA. Every dollar costs 22%.

• Convert $93,600 to fill the 22% bracket: $20,592 of tax plus $2,296.80 of Tier 1 surcharge. The extra $25,600 above the IRMAA line costs 31.0%.

• Convert $124,000 to the top of Tier 1 ($274,000 of MAGI): $27,888 of tax plus the same $2,296.80. The $56,000 above the IRMAA line costs 27.2%.

• Convert $124,001: the couple lands in Tier 2, and one dollar costs $3,472.80 more in premiums.

• Convert $192,000 to the top of Tier 2 ($342,000): the next $68,000 costs 29.1% (24% tax plus the Tier 2 step).

Filling the 22% bracket is the most expensive choice on the list, because it pays the whole Tier 1 step on a $25,600 sliver. This couple's real choice is between $68,000 (stay under the first IRMAA line) and $124,000 or more (use all of Tier 1), and the answer depends on the rate they expect to pay on that money later.

When is it worth crossing an IRMAA tier?

Crossing a tier is worth it when the cost per dollar of the full slice, tax plus surcharge, is lower than the rate you expect to pay on the same dollars later. IRMAA is a one-year cost: each year's premium is set by one year's MAGI. Required minimum distributions, Social Security and a surviving spouse's single-filer brackets are what raise the later rate.

Take a married couple, both 64, with about $3.5 million in traditional IRAs and $230,000 of MAGI before converting, which already puts them in Tier 1. Their slices look like this:

• Convert $112,000, to the top of Tier 2 ($342,000): $26,608 of tax plus a $3,472.80 step. About 26.9% per dollar.

• The next $68,000, to the top of Tier 3 ($410,000): 24% tax plus a $3,470.40 step. About 29.1%.

• The next $25,750, to the top of the 24% bracket ($435,750 of MAGI): 24% tax plus the $3,470.40 step into Tier 4. About 37.5%.

• Past that, the 32% bracket runs to $544,650 of MAGI, still in Tier 4, at 32% with no new step.

The 37.5% slice costs more than the 32% bracket above it. For this household the sensible stopping points are $410,000 of MAGI, or well past it: converting all the way from $410,000 to $544,650 averages about 33.0%. With $3.5 million still tax-deferred, RMDs alone are likely to keep them in the upper tiers later, which is why a multi-year plan sometimes accepts a high-tier year now to shrink the balance faster.

How much room should you leave below an IRMAA threshold?

Leave a cushion of several thousand dollars below the tier line, because MAGI is final only on December 31 and the 2028 thresholds aren't known yet. Mutual funds pay most capital gain distributions in December, dividends and interest arrive late in the year, and tax-exempt interest counts toward MAGI. A conversion sized to the dollar in early December can cross the line by the end of the month.

Run the numbers again after the December distributions are known and before you submit the conversion, and plan the fourth-quarter estimated tax payment due January 15, 2027. More on the year-end steps is in Roth Conversion Deadline 2026: What to Do Before December 31.

Can you appeal IRMAA after a Roth conversion?

Generally not because of the conversion itself. Form SSA-44 lets you ask Social Security to use a more recent year's income after a life-changing event that reduced your household income: marriage, divorce, the death of a spouse, loss of income such as retirement, or an employer settlement payment (SSA). A Roth conversion you chose to make isn't on that list. Retirement is, so the year you stop working matters: a request based on the lower income after retirement can lower premiums that were set by a high-income working year.

How TaxesSaved sizes a Roth conversion around IRMAA

These examples come from a question clients bring to TaxesSaved planning meetings: a conversion amount was sized to stay under an IRMAA tier, and they want to know whether the tier or the bracket should decide. The TaxesSaved CPA team, with Big Four backgrounds, answers it inside a written multi-year Roth conversion plan that sets each year's stopping point against both lines, puts the plan in place, and prepares the returns that report it, for clients in all 50 states.

For more real situations, read Roth Conversion Case Studies: 6 Real Retirement Tax Plans From 2026 and Before a Large Roth Conversion: 10 Questions Our Clients Ask.

To see what your IRA or 401(k) could cost you in taxes and Medicare surcharges, run the free Retirement Tax Calculator.

Examples are composites built from questions raised in TaxesSaved planning meetings in 2026. Ages, balances and other details are changed and rounded, and no example describes a single household. Figures are federal only, assume the standard deduction and no other income changes, and use IRS Revenue Procedure 2025-32 and SSA's 2026 IRMAA tables, as of November 4, 2026. This article is for education only and isn't tax, legal or investment advice. Tax rules change and every situation is different, so talk with a qualified tax professional before you act.