The 0% Capital Gains Trap: How a Roth Conversion Can Make a Retiree’s Stock Sales Taxable

Retirees who plan to sell appreciated stock in a taxable account often count on the 0% long-term capital gains rate. A Roth conversion done in the same year can take that rate away. Conversion income is taxed as ordinary income, and ordinary income fills the bottom of the tax stack first. Gains that would have been tax-free get pushed up into the 15% bracket. The timing problem is biggest late in the year: under the IRS publication for tax year 2024, a conversion counts on the return for the year that you converted it. The same publication also bars recharacterizations of conversions made in 2018 or later, so you can’t undo a conversion once it’s done.

Conversion Income Fills the Bottom of the Stack

Under 26 U.S.C. § 408A(d)(3)(A)(i), a conversion puts into gross income any amount that would have been taxable if it hadn’t been rolled over. Treasury applies that rule in 26 CFR 1.408A-4, A-7(a): the converted amount is taxed as a distribution under Section 408(d)(1) and (2) in the year it leaves the traditional IRA, and any portion that is a return of basis stays out of income. IRS guidance adds that IRA distributions are taxed as ordinary income, with no capital gain treatment.

The stacking itself comes from 26 U.S.C. § 1(h)(1)(B). The 0% rate applies only to gains that fit in the space between your ordinary taxable income and the top of the 0% band. Every dollar of conversion income reduces that space by a dollar.

For tax year 2025, the IRS sets the top of the 0% band at $96,700 for married couples filing jointly and $48,350 for single filers. The tax year 2025 standard deduction is $31,500 for joint filers, and it increases to $32,200 for tax year 2026. The capital gains thresholds for 2026 appear in Revenue Procedure 2025-32, so check those figures before applying the example below to a 2026 sale.

One Hypothetical Couple Loses Most of Its 0% Room

Example household: a married couple filing jointly, using tax year 2025 figures. They have $70,000 of ordinary income and plan to sell stock with a $50,000 long-term gain. To keep the math simple, they take only the basic standard deduction.

1. Ordinary taxable income: $70,000 minus $31,500 equals $38,500.
2. Room left at 0%, with no conversion: $96,700 minus $38,500 equals $58,200. The entire $50,000 gain fits, so the capital gains tax is $0.
3. Add a $50,000 conversion: ordinary taxable income becomes $38,500 plus $50,000, or $88,500.
4. Room left at 0% after the conversion: $96,700 minus $88,500 equals $8,200.
5. Gain pushed into the 15% rate: $50,000 minus $8,200 equals $41,800. At 15%, that’s $6,270 of capital gains tax.
6. Tax on the conversion itself: the 2025 joint 12% bracket runs up to $96,950, so all $50,000 is taxed at 12%, or $6,000.
7. Total added federal tax: $6,000 plus $6,270 equals $12,270. Divided by $50,000, that’s an effective 24.54% on the conversion, even though the couple never leaves the 12% bracket. Taxable income ends at $138,500.

Real households would also claim age-based deductions, including the senior deduction added by the One, Big, Beautiful Bill. Those deductions lower taxable income and change where the line falls, but the stacking order stays the same.

Ripple Effects Reach Social Security and Medicare

Treasury says a taxable conversion is included in income for all purposes, including the formula for how much Social Security is taxable (26 CFR 1.408A-4, A-9). If more of the couple’s benefits become taxable, ordinary income increases again and even more gain moves out of the 0% band.

Medicare surcharges are the next layer. The 2026 Part B surcharge starts above $218,000 for joint filers and $109,000 for single filers. Above that line, the monthly premium increases from $202.90 to $284.10, and a Part D surcharge is added on top. The couple’s gross income of $170,000 stays under the joint threshold, but a larger conversion could cross it.

Planning Moves Keep Conversions and Gains Apart

• Use separate years. Convert in one year and sell appreciated stock in another, so each type of income gets its own room under the thresholds.
• Harvest gains on purpose in low-income years. In a year with no conversion, the couple has $58,200 of 0% room. They could realize gains up to that amount and buy the shares back to raise their cost basis higher.
• Size the conversion to fit the gap. Estimate full-year income first, then convert only the amount that leaves your planned gains inside the 0% band.
• Take required distributions first. IRS guidance says you can’t convert amounts that must be distributed for that year. Required minimum distributions add ordinary income before any conversion does.

Who Should Think Twice About Stacking

Running the numbers matters most for these readers:

• Retirees who rely on tax-free stock sales to cover living costs.
• Anyone without cash outside the IRA to pay the conversion tax.
• Households close to a Medicare surcharge threshold.
• People who expect to be in a lower bracket later.

A conversion tends to look better when the 0% band is already used up by other income, when no large sale is planned, or when the long-term value of tax-free Roth growth exceeds the extra capital gains tax. Those quiet years between the last paycheck and the first required distribution are usually where the math works best, which is the whole subject of our free Roth window guide.

Before deciding, run both versions of your year, with and without the conversion, through the Qualified Dividends and Capital Gain Tax Worksheet. Then check the 2026 thresholds in Revenue Procedure 2025-32 before any year-end conversion or sale.

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