A single year-end Roth move could lift tax costs above 30%

For many retirees, the year-end Roth conversion has long seemed straightforward: move just enough money into a Roth IRA to fill a favorable tax bracket, pay the tax, and let the money grow tax-free later.

But that familiar strategy can carry a much bigger price tag under the One Big Beautiful Bill Act (OBBBA).

A Roth conversion that appears efficient on a tax-bracket chart may also shrink a valuable senior deduction, increase the share of Social Security benefits subject to tax, and lead to higher Medicare premiums later on.

The catch is that those costs do not always appear where retirees expect them. By the time they do, the conversion cannot be undone.

Before making a year-end move, retirees should ask whether filling a tax bracket is still the best Roth conversion strategy or whether it could leave them paying more than planned.

How the senior deduction changes the Roth conversion math

The OBBBA created a temporary $6,000 deduction for each eligible taxpayer age 65 and older, according to the Internal Revenue Service (IRS), applying to tax years 2025 through 2028.

For married couples in which both spouses qualify, that adds up to a $12,000 deduction. Paired with the $32,200 standard deduction, it can produce meaningful tax relief. But the benefit begins to phase out once modified adjusted gross income (MAGI) rises above $150,000 on a joint return, shrinking by 6% above that level, Fidelity reported.

For a couple where both spouses qualify, the combined $12,000 deduction disappears entirely at a MAGI of $250,000. In one example, a $100,000 conversion that lifts a couple from $150,000 to $250,000 wipes out the full deduction and increases the taxable amount by $112,000.

Fidelity estimated that losing the deduction alone would add $2,856 in federal income taxes beyond the tax created by the conversion itself.

The IRS also noted that couples with MAGI below $150,000 can preserve the full $47,500 in deductions, including the standard deduction, an added $1,650 for each qualifying spouse based on age, and the senior deduction.

Medicare IRMAA can add surcharges two years later

Medicare’s Income-Related Monthly Adjustment Amount, or IRMAA, uses income from two years earlier to determine premiums, Medicare confirmed.

Kiplinger described the surcharge as a cliff because crossing an income threshold by even one dollar can trigger the full tier’s surcharge for the entire year.

A conversion in one year can therefore affect Medicare costs two years later, and the Social Security Administration (SSA) has confirmed that a voluntary Roth conversion is not considered a life-changing event for an appeal.

For joint filers whose MAGI exceeded $218,000, Part B premiums rise by $81.20 per person per month and Part D premiums by $14.50, according to the Centers for Medicare & Medicaid Services. That works out to about $2,297 a year for a couple, while higher tiers rise much more sharply, with the top bracket adding $487 per person per month in Part B alone.

A retiree who crosses even the first IRMAA threshold while converting in the 24% bracket is effectively paying well above 24% on that income. Fidelity said the combined effect of the bracket tax, the lost senior deduction, and the Medicare surcharge can push the effective rate past 30%.

Social Security taxation can intensify the impact

Social Security benefits become partially taxable once combined income passes thresholds that have not changed for decades.

The IRS says combined income is adjusted gross income plus nontaxable interest plus half of benefits. For married couples filing jointly, up to 85% of benefits become taxable once that figure exceeds $44,000.

Because Roth conversion income counts toward adjusted gross income, a large conversion can push retirees into a higher Social Security taxation tier.

For example, a couple receiving $40,000 in annual Social Security benefits could move from the 50% inclusion tier to the 85% tier, causing as much as $14,000 in previously sheltered benefits to become taxable.

Tom Callahan, a certified financial planner at Range Financial Advisors, warned that avoiding conversions just to sidestep early-retirement surcharges can create bigger problems later.

The Roth conversion you skip this year to avoid IRMAA in early retirement can reappear later as a larger Medicare surcharge and a larger RMD, because the pretax IRA kept growing.

Fidelity’s analysis stresses that the size of the conversion matters more than a simple yes-or-no decision. The effective tax rate changes at different income levels, so the numbers need to be reviewed carefully.

What retirees should consider before converting

Retirees planning a conversion before year-end should estimate MAGI at several conversion amounts to see where each hidden cost begins, Fidelity recommended.

Those nearing the senior deduction or state and local tax (SALT) deduction phaseout thresholds may want to spread conversions over multiple years to better manage the tax burden.

A Roth conversion completed before age 63 generally will not affect Medicare premiums through the usual two-year IRMAA lookback, Q3 Advisors reported, since most people do not enroll in Medicare until age 65. Still, enrollment timing and personal circumstances can change how that rule applies.

Callahan advised retirees to evaluate the full retirement picture instead of focusing on one year’s Medicare premium. That includes future required minimum distributions (RMDs) at ages 75 and 80, the tax hit for a surviving spouse filing as single, and the taxes heirs may face on inherited retirement accounts.

The aim, he said, is to reduce taxes over a lifetime rather than avoid a surcharge in just one year.

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