She Converts $50,000 to a Roth at 66. It Makes More of Her Social Security Taxable, and Her Real Rate Ends Up Far Higher Than Her Bracket

Retirees usually price a Roth conversion at their tax bracket. If they already receive Social Security, that estimate can be too low. Under Treasury Regulation 1.408A-4, A-7, a converted amount is “includible in gross income as a distribution” in the year it leaves the traditional IRA. A-9 says that it counts “for all purposes,” including “the taxable portion of social security payments under section 86.” Section 408A(d)(3)(C) treats the conversion itself as a taxable distribution (26 U.S.C. § 408A(d)(3)(C)). So every converted dollar can pull more of your benefits into taxable income. Planners call this the tax torpedo.

How Provisional Income Pulls Benefits Into Your Taxable Income

The IRS starts with provisional income: modified adjusted gross income plus one-half of your Social Security benefits (26 U.S.C. § 86(b)(1)). Publication 915 for tax year 2025 sets the base amount at $25,000 if you are single and $32,000 if you are married filing jointly. Above a second threshold, up to 85% of your benefits can be taxable once one-half of benefits plus other income passes $34,000 ($44,000 if you are married filing jointly). Section 86(c) sets these as fixed dollar amounts, $25,000, $32,000, $34,000 and $44,000, with no inflation adjustment, so they apply to tax year 2026 as well.

Between the thresholds, each extra dollar of income says 50 cents of taxable benefits. Above the upper threshold it says 85 cents. In the 12% bracket, that makes a dollar of conversion into 22.2% of tax. In the 22% bracket, it becomes 40.7%.

Ellen Converts $50,000 at 66 and Pays Tax on Far More

Hypothetical: Ellen is 66, single, and receive $40,000 a year in Social Security. She has $20,000 of other taxable income. We use 2026 brackets and the 2026 single standard deduction of $16,100. To keep it simple, we leave out the extra deductions for people 65 and older, which would lower her tax.

Before the Conversion

1. Provisional income: $20,000 + ($40,000 ÷ 2) = $40,000.
2. 85% level: 85% × ($40,000 − $34,000) = $5,100.
3. 50% level: 50% × ($34,000 − $25,000) = $4,500.
4. Taxable benefits: $5,100 + $4,500 = $9,600.
5. Taxable income: $20,000 + $9,600 − $16,100 = $13,500.
6. Tax: 10% on the first $12,400 ($1,240) plus 12% on $1,100 ($132) = $1,372. Her top bracket is 12%.

After a $50,000 Conversion

1. Provisional income: $20,000 + $50,000 + $20,000 = $90,000.
2. Formula result: 85% × ($90,000 − $34,000) + $4,500 = $52,100.
3. Cap: 85% × $40,000 = $34,000, so taxable benefits are $34,000.
4. Says taxable benefits: $34,000 − $9,600 = $24,400.
5. Taxable income: $20,000 + $50,000 + $34,000 − $16,100 = $87,900.
6. Tax: $1,240 + 12% on $38,000 ($4,560) + 22% on income above $50,400, which is $37,500 ($8,250), for a total of $14,050.

Ellen converted $50,000 but says $74,400 to her taxable income. Her tax rose by $12,678, which works out to 25.4% of the conversion. That is more than double the 12% bracket she started in.

Three Ways Retirees Get Around the Torpedo

Convert before taking. Provisional income only affects how much of your benefits gets taxed once you receive benefits. Years after retirement but before you claim can be the best time to convert (we sized up that low-tax gap between your last paycheck and your first RMD in a free Roth Window guide).

Size conversions to stay under a threshold. If your provisional income sits below the base amount, a lower conversion that stays under it leaves your benefits untaxed. You can also stay below the 85% threshold and limit how much each dollar says.

Convert enough to clear the zone. Once taxable benefits reach the 85% cap, more income can’t pull in any more benefits. Ellen hits that cap after about $28,706 of conversion. The rest of her $50,000 is taxed at just her bracket rate. Some planners convert in one large year so later years avoid the zone entirely.

Who Should Think Twice Before Converting

Conversions tend to make less sense for retirees who:

• would have to pay the tax out of the IRA itself
• expect a lower tax rate later
• plan to leave the account to charity
• need the money within five years

A conversion makes more sense when your future required withdrawals or your heirs’ tax rates look higher than what you pay now, including the torpedo.

Medicare Surcharges Are the Next Thing to Check

Conversion income can also raise your Medicare premiums. In 2026, single taxpayers face Part B surcharges at MAGI over $109,000, which lifts the premium from $202.90 to $284.10. Ellen’s adjusted gross income after the conversion is $104,000, under that line. The 2027 schedule is still pending. Benefits are also going up: the 2027 cost-of-living adjustment is 3.6%, while the Section 86 thresholds stay fixed. Over time, that can push more retirees into the torpedo zone. Before you convert, run the Social Security worksheet with and without the conversion.

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