Converting a traditional IRA to a Roth IRA sounds straightforward: pay taxes now, enjoy tax-free growth later. But the conversion creates income that ripples through the tax code in unexpected ways. The converted amount gets added to your adjusted gross income for the year, potentially triggering higher tax brackets, Medicare surcharges, and taxation of Social Security benefits that would otherwise remain untaxed.
The Hidden Tax Cascade
When you convert funds from a traditional IRA to a Roth, that amount gets folded into your adjusted gross income. A higher AGI can push you into a higher marginal tax bracket, erode or eliminate deductions that phase out at certain income levels, and trigger a chain of stealth taxes that compound the cost of converting.
For 2026, the 22% bracket for single filers applies to taxable income up to $105,700, at which point the 24% rate kicks in. A retiree who converts a large IRA balance and crosses that line pays extra not just on the converted portion that spills over the threshold, but on all other income pushed into the higher bracket alongside it. That compounding effect can erode much of the long-term benefit the conversion was meant to create.
Medicare IRMAA: The Two-Year Lookback Trap
For anyone on Medicare or approaching eligibility, Roth conversions carry an additional penalty. Medicare Part B and Part D premiums include Income-Related Monthly Adjustment Amounts (IRMAA) for higher earners. The Social Security Administration determines IRMAA based on your modified adjusted gross income from two years prior, which means a conversion made today will show up in your Medicare premiums two years later, long after the tax return is filed and the money is spent.
In 2026, the IRMAA surcharge begins when a single filer’s MAGI exceeds $109,000 (based on their 2024 tax return), or $218,000 for married couples filing jointly. The standard Part B premium is $202.90 per month. Once a single filer crosses the first IRMAA tier, a Part B surcharge of $81.20 per month and a Part D surcharge of $14.50 per month are added on top of the standard premium, costing roughly $1,148 per person per year in additional healthcare expenses. Higher tiers impose progressively steeper charges, with the top tier (above $500,000 for single filers) adding $487.00 per month in Part B surcharges alone.
The IRMAA structure operates on a cliff basis. One dollar over any tier’s boundary triggers the full surcharge for that tier, with no phase-in. A modest Roth conversion that nudges income just past a threshold can therefore generate a disproportionate and persistent annual cost, because the surcharge recurs for the full calendar year your income remains above the line. For Medicare enrollees or those within two years of eligibility, this lookback timing matters as much as the conversion amount itself.
Social Security Taxation Thresholds
Roth conversions also increase the taxation of Social Security benefits, through a two-tier structure that has not been adjusted for inflation since the 1980s and 1990s. Benefits become taxable when provisional income (AGI plus tax-exempt interest plus half of your Social Security benefits) exceeds $25,000 for single filers or $32,000 for married couples filing jointly, at which point up to 50% of benefits can be included in taxable income. Once provisional income surpasses a second tier of $34,000 for singles or $44,000 for joint filers, up to 85% of benefits can be taxed. Because these thresholds are frozen by statute, Social Security cost-of-living adjustments and ordinary investment income push more retirees over the line every year without any change in the law.
For many retirees, a large Roth conversion creates a compounding tax hit: the conversion generates its own tax liability while simultaneously making a larger portion of their Social Security benefits taxable. That double exposure can transform what looks like a modest marginal-rate increase into a significantly higher effective tax bill on the same retirement dollars.
A New Deduction Worth Knowing
One development that can partially offset Roth conversion costs for some retirees is the senior deduction created by the One Big Beautiful Bill Act, signed into law in July 2025. Taxpayers aged 65 and older can claim an additional $6,000 deduction per person (or $12,000 for a couple where both spouses qualify) for tax years 2025 through 2028, available whether they itemize or take the standard deduction. The deduction phases out at a 6% rate once MAGI exceeds $75,000 for singles or $150,000 for joint filers. Because the deduction reduces AGI, it also lowers provisional income, which in turn can reduce or eliminate Social Security taxability for lower- and middle-income retirees. Higher-income retirees who do larger conversions will generally be above the phase-out range and will not benefit.
What to Evaluate First
Before converting, calculate your total AGI with the conversion amount included. Check where that figure lands relative to tax bracket thresholds, IRMAA tiers, and the Social Security provisional income levels. Converting smaller amounts spread across multiple years often reduces the total tax cost compared to a single large conversion that crosses several thresholds at once. For Medicare enrollees, the IRMAA lookback period makes the timing of each conversion as important as the size of it, since an income spike today translates directly into premium surcharges two years from now.
Editor’s note: This article has been updated to reflect 2026 Medicare IRMAA thresholds (single-filer entry at $109,000 MAGI, based on 2024 returns) and the current standard Part B premium of $202.90 per month. The Social Security taxation section was expanded to include the full two-tier provisional income structure ($25,000/$34,000 for singles, $32,000/$44,000 for joint filers), and a new section was added covering the temporary $6,000-per-person senior deduction created by the One Big Beautiful Bill Act (effective 2025 through 2028). The 2026 income tax bracket threshold where the 22% rate ends for single filers ($105,700) was also corrected from the prior draft.
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