How a Roth Conversion Can Quietly Turn a Tax-Free Stock Sale into a Tax Bill

A Roth conversion may seem like a tax-savvy move, but in the wrong tax year it can quietly turn a retiree’s otherwise tax-free stock sale into a federal bill worth thousands of dollars without pushing them out of a low bracket.

Many retirees who plan to sell appreciated shares in a taxable account are counting on the 0% long-term capital gains rate. A Roth conversion in the same year can eliminate that benefit. Because conversion income is taxed as ordinary income, it fills the bottom of the tax stack first. That can shove gains that would have been taxed at 0% into the 15% bracket instead. The timing issue is especially important late in the year: IRS guidance for the 2024 tax year says a conversion is reported on the return for the year in which it was made. That same guidance also says conversions made in 2018 or later cannot be recharacterized, so once the conversion happens, it cannot be undone.

Conversion Income Takes Priority in the Tax Stack

Under federal tax law, a Roth conversion adds to gross income any amount that would have been taxable if it had not been rolled over. Treasury regulations treat the converted amount as a distribution in the year it leaves the traditional IRA, while any part that represents basis stays out of income. IRS guidance further notes that IRA distributions are taxed as ordinary income, not capital gains.

The stacking rule comes from the capital gains provisions in the tax code. The 0% rate only applies to gains that fit into the space between ordinary taxable income and the top of the 0% bracket. Each dollar of conversion income reduces that room by a dollar.

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

A Sample Couple Loses Most of Its 0% Space

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

1. Ordinary taxable income: $70,000 minus $31,500 equals $38,500.
2. 0% room without a conversion: $96,700 minus $38,500 equals $58,200. The full $50,000 gain fits, so the capital gains tax is $0.
3. Add a $50,000 conversion: ordinary taxable income rises to $88,500.
4. 0% room after the conversion: $96,700 minus $88,500 equals $8,200.
5. Gain shifted into the 15% bracket: $50,000 minus $8,200 equals $41,800. At 15%, that creates $6,270 of capital gains tax.
6. Tax on the conversion: the 2025 joint 12% bracket extends to $96,950, so the entire $50,000 is taxed at 12%, or $6,000.
7. Total extra federal tax: $6,000 plus $6,270 equals $12,270. Spread across the $50,000 conversion, that is an effective 24.54% rate, even though the couple never leaves the 12% bracket. Their taxable income ends at $138,500.

Actual households would also be able to claim age-related deductions, including the senior deduction created by the One, Big, Beautiful Bill. Those deductions reduce taxable income and can shift the break point, but the stacking order remains the same.

Effects Can Also Reach Social Security and Medicare

Treasury says a taxable conversion counts as income for all purposes, including the calculation used to determine how much Social Security is taxable. If more of the couple’s benefits become taxable, ordinary income rises again and even more of the stock gain can be pushed out of the 0% bracket.

Medicare surcharges add another layer. For 2026, the Part B surcharge begins above $218,000 for joint filers and $109,000 for single filers. Above those levels, the monthly premium rises from $202.90 to $284.10, and a Part D surcharge is added as well. In this example, the couple’s gross income of $170,000 remains below the joint threshold, but a larger conversion could move them over it.

Ways to Keep Conversions and Gains Separate

• Use different tax years. Do the conversion in one year and the stock sale in another so each type of income gets its own room under the thresholds.
• Realize gains intentionally during low-income years. In a year without a conversion, the couple has $58,200 of 0% room and could realize gains up to that amount before buying the shares back to increase cost basis.
• Limit the conversion to the gap. Estimate full-year income first, then convert only enough to leave planned gains inside the 0% band.
• Take required distributions first. IRS guidance says amounts that must be distributed for the year cannot be converted. Required minimum distributions add ordinary income before any conversion does.

Who Should Be Most Careful

The numbers matter most for:

• Retirees who depend on tax-free stock sales to pay living expenses.
• Anyone who does not have cash outside the IRA to cover the conversion tax.
• Households near a Medicare surcharge threshold.
• People who expect to be in a lower bracket later.

A conversion is often more attractive when the 0% capital gains band is already filled by other income, when no large stock sale is planned, or when the long-term benefit of tax-free Roth growth outweighs the added capital gains tax. Those years between the last paycheck and the first required distribution are often when the math works best.

Before making a decision, compare both versions of the year, with and without the conversion, using the Qualified Dividends and Capital Gain Tax Worksheet. Then review the 2026 thresholds in Revenue Procedure 2025-32 before carrying out any year-end conversion or sale.