A colorful dimensional stage shows the same Social Security benefit beside a Roth conversion and tax-free interest, illustrating how other income can change the benefit’s taxable amount.

Why a Roth Conversion or “Tax‑Free” Interest Can Raise Social Security Taxes

Ross Marino |
Categories
Social Security
Retirement Income
Tax Planning
Roth Conversions
IRA

Your Social Security deposit can look unchanged from one month to the next. Then a Roth conversion is completed, or municipal-bond interest appears on a year-end statement.

When the tax return is prepared, more of the benefit may be taxable even though the monthly payment never moved.

The surprise is not a new Social Security tax rate. It is the way other income can move more of the benefit into taxable income.

A Roth conversion is not itself tax-free. To the extent it moves previously untaxed money, the converted amount is generally included in gross income for that year.[1] The tax-free feature generally comes later, when a Roth IRA distribution is qualified.[2]

Municipal-bond interest reaches the formula another way. The interest may be exempt from federal income tax, but it is still added when taxable Social Security is calculated.[3]

How does the combined-income calculation work?

The IRS does not look at Social Security by itself. The calculation commonly called combined income adds three parts:

  • Adjusted gross income before taxable Social Security.
  • Tax-exempt interest, including municipal-bond interest.
  • One-half of annual Social Security benefits.

A transaction does not have to change the monthly benefit to affect the result. A pension can begin. Wages can continue. A capital gain can be realized. A Roth conversion can be completed.

For a married couple filing jointly, both spouses' income and benefits are considered in the calculation. That remains true when only one spouse receives Social Security.[4]

What does “up to 85 percent” actually mean?

For the filing statuses most retirees use, the federal thresholds work in two steps:

  • For single filers, combined income from $25,000 through $34,000 can make up to 50 percent of benefits taxable. If combined income exceeds $34,000, up to 85 percent can be taxable.
  • For married couples filing jointly, combined income from $32,000 through $44,000 can make up to 50 percent of benefits taxable. If combined income exceeds $44,000, up to 85 percent can be taxable.

These percentages do not create a 50 percent or 85 percent tax rate. They describe how much of the benefit may be included in taxable income. The household's regular federal tax rules then determine the tax due.[5]

A temporary federal deduction is available to some taxpayers age 65 or older through 2028. It may reduce the final tax bill. It did not change the combined-income formula or thresholds.[5][6]

The thresholds are fixed and are not indexed for inflation or wage growth. As benefits or other income rise, more households can be included in the calculation without making a dramatic financial change.[6]

Dovetail Principle: Financial Decisions Need to Fit Together.

Income decisions often arrive one at a time. The tax result does not keep them separate.

A Roth conversion may serve a useful long-term purpose. Municipal bonds may still fit. The planning job is to see what each choice changes before judging it alone.

Place the choices in the same annual income picture. Then the effect on Social Security can become visible before year-end.

What can a Roth conversion change in a simple example?

Suppose a married couple receives $24,000 of Social Security and has $30,000 of other income. The illustration assumes no tax-exempt interest. It also excludes special adjustments.

  • Before a conversion, combined income is $42,000. Under the federal worksheet, $5,000 of Social Security would be included in taxable income.
  • After a fully taxable $10,000 Roth conversion, combined income is $52,000. The taxable portion of Social Security would rise to $12,800.

The conversion adds $10,000 of taxable income. It also causes another $7,800 in Social Security to be included in taxable income in this simplified example.[6]

That does not reveal the couple's final tax bill. Deductions and the rest of the return still matter. The example shows why the conversion amount and taxable Social Security should be reviewed together.

Where does “tax-free” interest fit?

Municipal-bond interest may not appear in adjusted gross income as taxable interest. The Social Security calculation adds it back as tax-exempt interest.

That does not mean a municipal bond was the wrong choice. It means its federal tax exemption answers one question, while the Social Security formula asks another.

What should be visible before year-end?

A useful annual income review can place these amounts in one projection:

  • The taxable portion of a planned Roth conversion.
  • IRA withdrawals, pension income, and wages.
  • Realized gains and other investment income.
  • Tax-exempt interest.

The review can show whether a decision changes the taxable portion of Social Security. It can also show whether withholding or estimated payments need attention.

Social Security allows federal withholding at 7, 10, 12, or 22 percent of the monthly benefit.[7] Quarterly estimated payments may also be used.[8] These payments can reduce a year-end balance due. The combined-income calculation does not change.

Make the overlap visible before the return does

The useful question is not simply whether Social Security will be taxable. It is the other income decision that is changing the answer this year.

A Roth conversion may still make sense. Municipal-bond interest may still fit. The point is to review the overlap before the decision is treated as a stand-alone move.

For a broader look at how tax choices connect with retirement income, visit Dovetail's Retirement Tax Planning page.

When the full income picture is visible during the year, there is more room to discuss what should happen now and what may be better reviewed later.

Related Reading: Can a Roth Conversion Affect Health Coverage Costs? That article looks at how the same conversion income may affect health coverage costs.

About the author

Ross Marino, CFP®, CeFT®, is the Founder & CEO of Dovetail Financial and creator of Human-First Financial Guidance®. He helps people nearing or living in retirement connect their lives and wealth so that financial decisions become clearer, more personal, and easier to navigate.

Read More Articles

Notes

  1. Publication 590-A (2025), Contributions to Individual Retirement Arrangements (IRAs), Internal Revenue Service, 2025.
  2. Grow Your Retirement Savings Tax-Free With a Roth IRA, Fidelity.
  3. Tax-Free Municipal Bonds? Not Always, Charles Schwab.
  4. Topic No. 423, Social Security and Equivalent Railroad Retirement Benefits, Internal Revenue Service, updated June 3, 2026.
  5. Taxes on Social Security Are Based on Your Income, AARP, updated June 15, 2026.
  6. Taxation of Social Security Benefits and the Senior Deduction in P.L. 119-21: In Brief, Congressional Research Service, revised August 1, 2025.
  7. Request to Withhold Taxes, Social Security Administration.
  8. Taxes on Social Security Benefits: 6 Things to Know for 2026, Kiplinger, updated July 9, 2026.

Disclosure

Disclosure: This content is provided by Dovetail Financial Group LLC (“Dovetail Financial”) for informational and educational purposes only. It is not intended as, and should not be construed as, individualized investment, tax, legal, or accounting advice; a recommendation to buy or sell any security; or a recommendation to adopt any investment strategy. Because each person’s situation is unique, readers should consult their own financial, tax, and legal professionals before taking action based on this content.

Information contained herein is believed to be reliable, but its accuracy or completeness is not guaranteed. Any opinions expressed are current as of the date of publication and are subject to change without notice. All investing involves risk, including the possible loss of principal. Asset allocation and diversification do not guarantee profits or protect against losses in declining markets. Past performance is not a guarantee of future results. Dovetail Financial Group LLC is a registered investment adviser. Registration does not imply a certain level of skill or training. Additional information about Dovetail Financial Group LLC, including Form ADV Part 2A and Form CRS, is available at adviserinfo.sec.gov. © 2026 Dovetail Financial Group LLC. All rights reserved.