When Roth Conversions Backfire: 7 Hidden Costs to Check Before You Convert

A Roth conversion moves eligible pre-tax retirement money into a Roth account. You generally pay ordinary income tax on the previously untaxed amount in the conversion year; qualified Roth withdrawals can be tax-free later. That trade may make sense if you expect higher future tax rates, want more tax flexibility, or have years of relatively low income ahead.

The trouble starts when someone looks only at the federal tax bracket. The IRS, Social Security, and health insurance programs do not all measure income in the same way. One conversion may change several calculations at once. Here are the seven costs worth checking before you move the money.

1. Medicare IRMAA: a small income increase can cause a big premium jump

Medicare's income-related monthly adjustment amount (IRMAA) increases Part B and Part D costs for beneficiaries above certain income thresholds. Unlike a progressive income tax bracket, moving just above an IRMAA threshold can put your premium into the next tier.

Social Security generally bases a year's IRMAA on your tax return from two years earlier. For example, the 2026 premiums shown below generally use 2024 MAGI—not income from a conversion completed in 2026. A taxable 2026 conversion may affect 2028 premiums, using thresholds and premiums applicable then.

An illustrative 2024-to-2026 example: Imagine a married couple whose 2024 MAGI would have been $217,500 without a conversion. A $1,000 fully taxable conversion would raise it to $218,500—above the threshold used for 2026. If both spouses are enrolled in Parts B and D for all 12 months of 2026, the first IRMAA tier would add approximately $2,296.80 in combined annual premiums compared with no IRMAA, before the income tax cost. Real costs depend on enrollment months and coverage.

2. Higher marginal tax brackets can make part of the conversion expensive

A large conversion can stack taxable income on top of wages, pensions, interest, and other distributions. Only the dollars that enter a higher bracket face that bracket's rate—not your entire income—but the extra tax can still undermine the benefit of converting.

As one benchmark, the 2026 federal 24% bracket begins above $105,700 of taxable income for single filers and $211,400 for married couples filing jointly. These are taxable-income thresholds, not AGI or IRMAA thresholds. State income taxes can add another cost. See the IRS 2026 tables.

Instead of asking “Can I convert $100,000?”, ask: “How much can I convert before the next marginal dollar becomes unusually expensive?” The answer depends on other income, deductions, filing status, and the rest of the tax return.

3. The Social Security tax torpedo

If you already receive Social Security, converting pre-tax retirement money can increase your combined income and cause a larger portion of your benefits to be taxable. Depending on total income, up to 85% of benefits can be included in taxable income. That does not mean your benefits are taxed at an 85% rate.

For this calculation, combined income generally includes adjusted gross income, tax-exempt interest, and half of Social Security benefits. A conversion can therefore raise both income and the taxable portion of your benefits. See IRS Publication 915.

4. ACA health insurance credits can shrink—or disappear

Retiring before Medicare often means buying health insurance through the Affordable Care Act Marketplace. A taxable Roth conversion can increase the household income used to determine your premium tax credit. That may raise the amount you pay for coverage even if your other income is unchanged.

A major 2026 rule matters here: The temporary expansion of Marketplace premium tax credits ended after 2025. For 2026, households above 400% of the federal poverty line generally are not eligible for the credit, subject to the program's rules and exceptions. Also, the previous repayment caps for excess advance premium tax credits no longer apply for 2026. A conversion that pushes projected income unexpectedly higher may create a substantial reconciliation bill at tax time. Check IRS Premium Tax Credit Q&A and IRS Publication 505 (2026).

This is one reason a conversion that looks affordable before age 65 may be less attractive in a year when you rely on Marketplace subsidies.

5. Income-tested deductions and investment taxes can change

Conversions can affect deductions and taxes tied to modified adjusted gross income (MAGI). In tax years 2025 through 2028, for instance, eligible people age 65 or older may claim an additional senior deduction of up to $6,000 per person; it starts to phase out above MAGI of $75,000 for individuals or $150,000 for joint filers. A conversion may reduce the deduction available to you. IRS guidance.

There is also the 3.8% net investment income tax (NIIT). An IRA conversion itself is generally not net investment income, but it can increase MAGI enough to expose eligible investment income to the tax. The relevant MAGI thresholds include $200,000 for single filers and $250,000 for joint filers. This effect matters most when investment income is already present. IRS NIIT guidance.

6. Paying the tax from retirement money can weaken the strategy

A conversion does not require you to pay the federal tax bill out of a separate checking account. But when possible, using outside cash to cover the tax can keep more money in the Roth account to potentially grow tax-free.

If you ask the custodian to withhold taxes out of a traditional IRA distribution, that withheld amount usually does not enter the Roth account. You have converted less money than you might have expected. And if you are younger than 59½, the taxable portion distributed but not converted may also face the 10% additional early-distribution tax unless an exception applies. A properly completed conversion itself generally avoids that additional 10% tax. See IRS Publication 590-A.

Cash-flow planning matters beyond taxes, too. You may need estimated payments or adjusted withholding, and the funds used for the tax bill will no longer be available for emergencies or other investments.

7. You generally cannot undo the conversion after the market moves

Roth conversions made after 2017 cannot be recharacterized back to a traditional IRA. If you convert while your account value is high and it falls later, you generally cannot reverse the conversion to erase the tax result. This is different from recharacterizing certain ordinary IRA contributions. IRS IRA FAQs.

There are also withdrawal rules worth checking. Converted amounts can have their own five-year rules for the 10% additional tax if withdrawn early, separate from the five-year requirement for tax-free Roth earnings. And if you have reached the age when required minimum distributions (RMDs) apply, your RMD itself cannot be converted; that requirement must be handled separately. See IRS Publication 590-B and 590-A.

Wait—does every dollar converted become taxable income?

No, and this point is often overlooked. If your traditional IRA contains only pre-tax dollars, converting it generally creates ordinary taxable income for the amount converted. But if you previously made nondeductible contributions, you may have after-tax basis in your traditional IRAs. The nontaxable portion is calculated under the IRS pro-rata rules, typically taking into account relevant traditional, SEP, and SIMPLE IRA balances rather than treating one account as isolated.

For that reason, do not assume a $40,000 conversion always adds $40,000 to AGI. Check your prior Forms 8606 and ask a tax professional to calculate the taxable share when basis is involved. You can still face a meaningful income increase—but the correct starting number is the taxable portion, not automatically the entire amount. IRS Form 8606 instructions.

A practical checklist before converting

1. Estimate the taxable amount, including any nondeductible IRA basis and the pro-rata rule.
2. Run a full-year tax projection. Include wages, pensions, capital gains, interest, Social Security, other distributions, and state taxes.
3. Check your health insurance. Model Medicare IRMAA for the appropriate future year or Marketplace tax-credit eligibility for your current coverage year.
4. Test a smaller conversion. Compare the cost of partial conversions over several years with a single large conversion. A smaller amount is not always better, but it gives you more control over thresholds.
5. Plan how to pay the tax. Evaluate outside cash and whether estimated tax payments will be necessary.
6. Confirm timing and account rules. Check your custodian's year-end processing deadline, RMD obligations, and Roth withdrawal rules.
7. Compare future benefits against today's full cost. Project tax rates and withdrawals under more than one scenario, rather than assuming tax rates will only rise.

When a Roth conversion may still be worth it

A conversion isn't a mistake just because it generates a tax bill. Some retirees have lower-income years between leaving work and claiming Social Security or taking RMDs. Others want a mix of traditional and Roth accounts so they can manage taxable withdrawals later. Depending on assumptions about future tax rates and time horizon, paying tax now can still be worthwhile.

The more useful question is not “Are Roth conversions good or bad?” It's “What amount, in what year, produces the best likely after-tax outcome for my situation?” Often that means evaluating partial conversions and not crossing an expensive threshold without a good reason.

Frequently asked questions

Primary sources and fact-check notes

Reviewed against official U.S. government guidance on November 1, 2026. Figures below are 2026-specific when marked; future-year income thresholds and premiums can change.

1. Social Security Administration — 2026 Medicare premiums and IRMAA thresholds.
2. Social Security Administration — Request to lower IRMAA.
3. IRS Publication 590-A — Conversions, RMD restrictions and tax treatment.
4. IRS Form 8606 Instructions — After-tax basis and conversions.
5. IRS Publication 915 — Taxation of Social Security benefits.
6. IRS — Premium Tax Credit eligibility and 2026 rules.
7. IRS — Net Investment Income Tax guidance.
8. IRS — Recharacterization and conversion FAQs.